reorderbuffer.c

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/*-------------------------------------------------------------------------
 *
 * reorderbuffer.c
 *    PostgreSQL logical replay/reorder buffer management
 *
 *
 * Copyright (c) 2012-2020, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *    src/backend/replication/reorderbuffer.c
 *
 * NOTES
 *    This module gets handed individual pieces of transactions in the order
 *    they are written to the WAL and is responsible to reassemble them into
 *    toplevel transaction sized pieces. When a transaction is completely
 *    reassembled - signalled by reading the transaction commit record - it
 *    will then call the output plugin (cf. ReorderBufferCommit()) with the
 *    individual changes. The output plugins rely on snapshots built by
 *    snapbuild.c which hands them to us.
 *
 *    Transactions and subtransactions/savepoints in postgres are not
 *    immediately linked to each other from outside the performing
 *    backend. Only at commit/abort (or special xact_assignment records) they
 *    are linked together. Which means that we will have to splice together a
 *    toplevel transaction from its subtransactions. To do that efficiently we
 *    build a binary heap indexed by the smallest current lsn of the individual
 *    subtransactions' changestreams. As the individual streams are inherently
 *    ordered by LSN - since that is where we build them from - the transaction
 *    can easily be reassembled by always using the subtransaction with the
 *    smallest current LSN from the heap.
 *
 *    In order to cope with large transactions - which can be several times as
 *    big as the available memory - this module supports spooling the contents
 *    of a large transactions to disk. When the transaction is replayed the
 *    contents of individual (sub-)transactions will be read from disk in
 *    chunks.
 *
 *    This module also has to deal with reassembling toast records from the
 *    individual chunks stored in WAL. When a new (or initial) version of a
 *    tuple is stored in WAL it will always be preceded by the toast chunks
 *    emitted for the columns stored out of line. Within a single toplevel
 *    transaction there will be no other data carrying records between a row's
 *    toast chunks and the row data itself. See ReorderBufferToast* for
 *    details.
 *
 *    ReorderBuffer uses two special memory context types - SlabContext for
 *    allocations of fixed-length structures (changes and transactions), and
 *    GenerationContext for the variable-length transaction data (allocated
 *    and freed in groups with similar lifespan).
 *
 *    To limit the amount of memory used by decoded changes, we track memory
 *    used at the reorder buffer level (i.e. total amount of memory), and for
 *    each transaction. When the total amount of used memory exceeds the
 *    limit, the transaction consuming the most memory is then serialized to
 *    disk.
 *
 *    Only decoded changes are evicted from memory (spilled to disk), not the
 *    transaction records. The number of toplevel transactions is limited,
 *    but a transaction with many subtransactions may still consume significant
 *    amounts of memory. The transaction records are fairly small, though, and
 *    are not included in the memory limit.
 *
 *    The current eviction algorithm is very simple - the transaction is
 *    picked merely by size, while it might be useful to also consider age
 *    (LSN) of the changes for example. With the new Generational memory
 *    allocator, evicting the oldest changes would make it more likely the
 *    memory gets actually freed.
 *
 *    We still rely on max_changes_in_memory when loading serialized changes
 *    back into memory. At that point we can't use the memory limit directly
 *    as we load the subxacts independently. One option do deal with this
 *    would be to count the subxacts, and allow each to allocate 1/N of the
 *    memory limit. That however does not seem very appealing, because with
 *    many subtransactions it may easily cause trashing (short cycles of
 *    deserializing and applying very few changes). We probably should give
 *    a bit more memory to the oldest subtransactions, because it's likely
 *    the source for the next sequence of changes.
 *
 * -------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>
#include <sys/stat.h>

#include "access/detoast.h"
#include "access/heapam.h"
#include "access/rewriteheap.h"
#include "access/transam.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "catalog/catalog.h"
#include "lib/binaryheap.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "replication/logical.h"
#include "replication/reorderbuffer.h"
#include "replication/slot.h"
#include "replication/snapbuild.h"  /* just for SnapBuildSnapDecRefcount */
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/sinval.h"
#include "utils/builtins.h"
#include "utils/combocid.h"
#include "utils/memdebug.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/relfilenodemap.h"


/* entry for a hash table we use to map from xid to our transaction state */
typedef struct ReorderBufferTXNByIdEnt
{
    TransactionId xid;
    ReorderBufferTXN *txn;
} ReorderBufferTXNByIdEnt;

/* data structures for (relfilenode, ctid) => (cmin, cmax) mapping */
typedef struct ReorderBufferTupleCidKey
{
    RelFileNode relnode;
    ItemPointerData tid;
} ReorderBufferTupleCidKey;

typedef struct ReorderBufferTupleCidEnt
{
    ReorderBufferTupleCidKey key;
    CommandId   cmin;
    CommandId   cmax;
    CommandId   combocid;       /* just for debugging */
} ReorderBufferTupleCidEnt;

/* Virtual file descriptor with file offset tracking */
typedef struct TXNEntryFile
{
    File        vfd;            /* -1 when the file is closed */
    off_t       curOffset;      /* offset for next write or read. Reset to 0
                                 * when vfd is opened. */
} TXNEntryFile;

/* k-way in-order change iteration support structures */
typedef struct ReorderBufferIterTXNEntry
{
    XLogRecPtr  lsn;
    ReorderBufferChange *change;
    ReorderBufferTXN *txn;
    TXNEntryFile file;
    XLogSegNo   segno;
} ReorderBufferIterTXNEntry;

typedef struct ReorderBufferIterTXNState
{
    binaryheap *heap;
    Size        nr_txns;
    dlist_head  old_change;
    ReorderBufferIterTXNEntry entries[FLEXIBLE_ARRAY_MEMBER];
} ReorderBufferIterTXNState;

/* toast datastructures */
typedef struct ReorderBufferToastEnt
{
    Oid         chunk_id;       /* toast_table.chunk_id */
    int32       last_chunk_seq; /* toast_table.chunk_seq of the last chunk we
                                 * have seen */
    Size        num_chunks;     /* number of chunks we've already seen */
    Size        size;           /* combined size of chunks seen */
    dlist_head  chunks;         /* linked list of chunks */
    struct varlena *reconstructed;  /* reconstructed varlena now pointed to in
                                     * main tup */
} ReorderBufferToastEnt;

/* Disk serialization support datastructures */
typedef struct ReorderBufferDiskChange
{
    Size        size;
    ReorderBufferChange change;
    /* data follows */
} ReorderBufferDiskChange;

/*
 * Maximum number of changes kept in memory, per transaction. After that,
 * changes are spooled to disk.
 *
 * The current value should be sufficient to decode the entire transaction
 * without hitting disk in OLTP workloads, while starting to spool to disk in
 * other workloads reasonably fast.
 *
 * At some point in the future it probably makes sense to have a more elaborate
 * resource management here, but it's not entirely clear what that would look
 * like.
 */
int         logical_decoding_work_mem;
static const Size max_changes_in_memory = 4096; /* XXX for restore only */

/* ---------------------------------------
 * primary reorderbuffer support routines
 * ---------------------------------------
 */
static ReorderBufferTXN *ReorderBufferGetTXN(ReorderBuffer *rb);
static void ReorderBufferReturnTXN(ReorderBuffer *rb, ReorderBufferTXN *txn);
static ReorderBufferTXN *ReorderBufferTXNByXid(ReorderBuffer *rb,
                                               TransactionId xid, bool create, bool *is_new,
                                               XLogRecPtr lsn, bool create_as_top);
static void ReorderBufferTransferSnapToParent(ReorderBufferTXN *txn,
                                              ReorderBufferTXN *subtxn);

static void AssertTXNLsnOrder(ReorderBuffer *rb);

/* ---------------------------------------
 * support functions for lsn-order iterating over the ->changes of a
 * transaction and its subtransactions
 *
 * used for iteration over the k-way heap merge of a transaction and its
 * subtransactions
 * ---------------------------------------
 */
static void ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn,
                                     ReorderBufferIterTXNState *volatile *iter_state);
static ReorderBufferChange *ReorderBufferIterTXNNext(ReorderBuffer *rb, ReorderBufferIterTXNState *state);
static void ReorderBufferIterTXNFinish(ReorderBuffer *rb,
                                       ReorderBufferIterTXNState *state);
static void ReorderBufferExecuteInvalidations(ReorderBuffer *rb, ReorderBufferTXN *txn);

/*
 * ---------------------------------------
 * Disk serialization support functions
 * ---------------------------------------
 */
static void ReorderBufferCheckMemoryLimit(ReorderBuffer *rb);
static void ReorderBufferSerializeTXN(ReorderBuffer *rb, ReorderBufferTXN *txn);
static void ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
                                         int fd, ReorderBufferChange *change);
static Size ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
                                        TXNEntryFile *file, XLogSegNo *segno);
static void ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
                                       char *change);
static void ReorderBufferRestoreCleanup(ReorderBuffer *rb, ReorderBufferTXN *txn);
static void ReorderBufferCleanupSerializedTXNs(const char *slotname);
static void ReorderBufferSerializedPath(char *path, ReplicationSlot *slot,
                                        TransactionId xid, XLogSegNo segno);

static void ReorderBufferFreeSnap(ReorderBuffer *rb, Snapshot snap);
static Snapshot ReorderBufferCopySnap(ReorderBuffer *rb, Snapshot orig_snap,
                                      ReorderBufferTXN *txn, CommandId cid);

/* ---------------------------------------
 * toast reassembly support
 * ---------------------------------------
 */
static void ReorderBufferToastInitHash(ReorderBuffer *rb, ReorderBufferTXN *txn);
static void ReorderBufferToastReset(ReorderBuffer *rb, ReorderBufferTXN *txn);
static void ReorderBufferToastReplace(ReorderBuffer *rb, ReorderBufferTXN *txn,
                                      Relation relation, ReorderBufferChange *change);
static void ReorderBufferToastAppendChunk(ReorderBuffer *rb, ReorderBufferTXN *txn,
                                          Relation relation, ReorderBufferChange *change);

/*
 * ---------------------------------------
 * memory accounting
 * ---------------------------------------
 */
static Size ReorderBufferChangeSize(ReorderBufferChange *change);
static void ReorderBufferChangeMemoryUpdate(ReorderBuffer *rb,
                                            ReorderBufferChange *change, bool addition);

/*
 * Allocate a new ReorderBuffer and clean out any old serialized state from
 * prior ReorderBuffer instances for the same slot.
 */
ReorderBuffer *
ReorderBufferAllocate(void)
{
    ReorderBuffer *buffer;
    HASHCTL     hash_ctl;
    MemoryContext new_ctx;

    Assert(MyReplicationSlot != NULL);

    /* allocate memory in own context, to have better accountability */
    new_ctx = AllocSetContextCreate(CurrentMemoryContext,
                                    "ReorderBuffer",
                                    ALLOCSET_DEFAULT_SIZES);

    buffer =
        (ReorderBuffer *) MemoryContextAlloc(new_ctx, sizeof(ReorderBuffer));

    memset(&hash_ctl, 0, sizeof(hash_ctl));

    buffer->context = new_ctx;

    buffer->change_context = SlabContextCreate(new_ctx,
                                               "Change",
                                               SLAB_DEFAULT_BLOCK_SIZE,
                                               sizeof(ReorderBufferChange));

    buffer->txn_context = SlabContextCreate(new_ctx,
                                            "TXN",
                                            SLAB_DEFAULT_BLOCK_SIZE,
                                            sizeof(ReorderBufferTXN));

    buffer->tup_context = GenerationContextCreate(new_ctx,
                                                  "Tuples",
                                                  SLAB_LARGE_BLOCK_SIZE);

    hash_ctl.keysize = sizeof(TransactionId);
    hash_ctl.entrysize = sizeof(ReorderBufferTXNByIdEnt);
    hash_ctl.hcxt = buffer->context;

    buffer->by_txn = hash_create("ReorderBufferByXid", 1000, &hash_ctl,
                                 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

    buffer->by_txn_last_xid = InvalidTransactionId;
    buffer->by_txn_last_txn = NULL;

    buffer->outbuf = NULL;
    buffer->outbufsize = 0;
    buffer->size = 0;

    buffer->spillCount = 0;
    buffer->spillTxns = 0;
    buffer->spillBytes = 0;

    buffer->current_restart_decoding_lsn = InvalidXLogRecPtr;

    dlist_init(&buffer->toplevel_by_lsn);
    dlist_init(&buffer->txns_by_base_snapshot_lsn);

    /*
     * Ensure there's no stale data from prior uses of this slot, in case some
     * prior exit avoided calling ReorderBufferFree. Failure to do this can
     * produce duplicated txns, and it's very cheap if there's nothing there.
     */
    ReorderBufferCleanupSerializedTXNs(NameStr(MyReplicationSlot->data.name));

    return buffer;
}

/*
 * Free a ReorderBuffer
 */
void
ReorderBufferFree(ReorderBuffer *rb)
{
    MemoryContext context = rb->context;

    /*
     * We free separately allocated data by entirely scrapping reorderbuffer's
     * memory context.
     */
    MemoryContextDelete(context);

    /* Free disk space used by unconsumed reorder buffers */
    ReorderBufferCleanupSerializedTXNs(NameStr(MyReplicationSlot->data.name));
}

/*
 * Get an unused, possibly preallocated, ReorderBufferTXN.
 */
static ReorderBufferTXN *
ReorderBufferGetTXN(ReorderBuffer *rb)
{
    ReorderBufferTXN *txn;

    txn = (ReorderBufferTXN *)
        MemoryContextAlloc(rb->txn_context, sizeof(ReorderBufferTXN));

    memset(txn, 0, sizeof(ReorderBufferTXN));

    dlist_init(&txn->changes);
    dlist_init(&txn->tuplecids);
    dlist_init(&txn->subtxns);

    return txn;
}

/*
 * Free a ReorderBufferTXN.
 */
static void
ReorderBufferReturnTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
{
    /* clean the lookup cache if we were cached (quite likely) */
    if (rb->by_txn_last_xid == txn->xid)
    {
        rb->by_txn_last_xid = InvalidTransactionId;
        rb->by_txn_last_txn = NULL;
    }

    /* free data that's contained */

    if (txn->tuplecid_hash != NULL)
    {
        hash_destroy(txn->tuplecid_hash);
        txn->tuplecid_hash = NULL;
    }

    if (txn->invalidations)
    {
        pfree(txn->invalidations);
        txn->invalidations = NULL;
    }

    pfree(txn);
}

/*
 * Get an fresh ReorderBufferChange.
 */
ReorderBufferChange *
ReorderBufferGetChange(ReorderBuffer *rb)
{
    ReorderBufferChange *change;

    change = (ReorderBufferChange *)
        MemoryContextAlloc(rb->change_context, sizeof(ReorderBufferChange));

    memset(change, 0, sizeof(ReorderBufferChange));
    return change;
}

/*
 * Free an ReorderBufferChange.
 */
void
ReorderBufferReturnChange(ReorderBuffer *rb, ReorderBufferChange *change)
{
    /* update memory accounting info */
    ReorderBufferChangeMemoryUpdate(rb, change, false);

    /* free contained data */
    switch (change->action)
    {
        case REORDER_BUFFER_CHANGE_INSERT:
        case REORDER_BUFFER_CHANGE_UPDATE:
        case REORDER_BUFFER_CHANGE_DELETE:
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_INSERT:
            if (change->data.tp.newtuple)
            {
                ReorderBufferReturnTupleBuf(rb, change->data.tp.newtuple);
                change->data.tp.newtuple = NULL;
            }

            if (change->data.tp.oldtuple)
            {
                ReorderBufferReturnTupleBuf(rb, change->data.tp.oldtuple);
                change->data.tp.oldtuple = NULL;
            }
            break;
        case REORDER_BUFFER_CHANGE_MESSAGE:
            if (change->data.msg.prefix != NULL)
                pfree(change->data.msg.prefix);
            change->data.msg.prefix = NULL;
            if (change->data.msg.message != NULL)
                pfree(change->data.msg.message);
            change->data.msg.message = NULL;
            break;
        case REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT:
            if (change->data.snapshot)
            {
                ReorderBufferFreeSnap(rb, change->data.snapshot);
                change->data.snapshot = NULL;
            }
            break;
            /* no data in addition to the struct itself */
        case REORDER_BUFFER_CHANGE_TRUNCATE:
            if (change->data.truncate.relids != NULL)
            {
                ReorderBufferReturnRelids(rb, change->data.truncate.relids);
                change->data.truncate.relids = NULL;
            }
            break;
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
        case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
        case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
            break;
    }

    pfree(change);
}

/*
 * Get a fresh ReorderBufferTupleBuf fitting at least a tuple of size
 * tuple_len (excluding header overhead).
 */
ReorderBufferTupleBuf *
ReorderBufferGetTupleBuf(ReorderBuffer *rb, Size tuple_len)
{
    ReorderBufferTupleBuf *tuple;
    Size        alloc_len;

    alloc_len = tuple_len + SizeofHeapTupleHeader;

    tuple = (ReorderBufferTupleBuf *)
        MemoryContextAlloc(rb->tup_context,
                           sizeof(ReorderBufferTupleBuf) +
                           MAXIMUM_ALIGNOF + alloc_len);
    tuple->alloc_tuple_size = alloc_len;
    tuple->tuple.t_data = ReorderBufferTupleBufData(tuple);

    return tuple;
}

/*
 * Free an ReorderBufferTupleBuf.
 */
void
ReorderBufferReturnTupleBuf(ReorderBuffer *rb, ReorderBufferTupleBuf *tuple)
{
    pfree(tuple);
}

/*
 * Get an array for relids of truncated relations.
 *
 * We use the global memory context (for the whole reorder buffer), because
 * none of the existing ones seems like a good match (some are SLAB, so we
 * can't use those, and tup_context is meant for tuple data, not relids). We
 * could add yet another context, but it seems like an overkill - TRUNCATE is
 * not particularly common operation, so it does not seem worth it.
 */
Oid *
ReorderBufferGetRelids(ReorderBuffer *rb, int nrelids)
{
    Oid        *relids;
    Size        alloc_len;

    alloc_len = sizeof(Oid) * nrelids;

    relids = (Oid *) MemoryContextAlloc(rb->context, alloc_len);

    return relids;
}

/*
 * Free an array of relids.
 */
void
ReorderBufferReturnRelids(ReorderBuffer *rb, Oid *relids)
{
    pfree(relids);
}

/*
 * Return the ReorderBufferTXN from the given buffer, specified by Xid.
 * If create is true, and a transaction doesn't already exist, create it
 * (with the given LSN, and as top transaction if that's specified);
 * when this happens, is_new is set to true.
 */
static ReorderBufferTXN *
ReorderBufferTXNByXid(ReorderBuffer *rb, TransactionId xid, bool create,
                      bool *is_new, XLogRecPtr lsn, bool create_as_top)
{
    ReorderBufferTXN *txn;
    ReorderBufferTXNByIdEnt *ent;
    bool        found;

    Assert(TransactionIdIsValid(xid));

    /*
     * Check the one-entry lookup cache first
     */
    if (TransactionIdIsValid(rb->by_txn_last_xid) &&
        rb->by_txn_last_xid == xid)
    {
        txn = rb->by_txn_last_txn;

        if (txn != NULL)
        {
            /* found it, and it's valid */
            if (is_new)
                *is_new = false;
            return txn;
        }

        /*
         * cached as non-existent, and asked not to create? Then nothing else
         * to do.
         */
        if (!create)
            return NULL;
        /* otherwise fall through to create it */
    }

    /*
     * If the cache wasn't hit or it yielded an "does-not-exist" and we want
     * to create an entry.
     */

    /* search the lookup table */
    ent = (ReorderBufferTXNByIdEnt *)
        hash_search(rb->by_txn,
                    (void *) &xid,
                    create ? HASH_ENTER : HASH_FIND,
                    &found);
    if (found)
        txn = ent->txn;
    else if (create)
    {
        /* initialize the new entry, if creation was requested */
        Assert(ent != NULL);
        Assert(lsn != InvalidXLogRecPtr);

        ent->txn = ReorderBufferGetTXN(rb);
        ent->txn->xid = xid;
        txn = ent->txn;
        txn->first_lsn = lsn;
        txn->restart_decoding_lsn = rb->current_restart_decoding_lsn;

        if (create_as_top)
        {
            dlist_push_tail(&rb->toplevel_by_lsn, &txn->node);
            AssertTXNLsnOrder(rb);
        }
    }
    else
        txn = NULL;             /* not found and not asked to create */

    /* update cache */
    rb->by_txn_last_xid = xid;
    rb->by_txn_last_txn = txn;

    if (is_new)
        *is_new = !found;

    Assert(!create || txn != NULL);
    return txn;
}

/*
 * Queue a change into a transaction so it can be replayed upon commit.
 */
void
ReorderBufferQueueChange(ReorderBuffer *rb, TransactionId xid, XLogRecPtr lsn,
                         ReorderBufferChange *change)
{
    ReorderBufferTXN *txn;

    txn = ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);

    change->lsn = lsn;
    change->txn = txn;

    Assert(InvalidXLogRecPtr != lsn);
    dlist_push_tail(&txn->changes, &change->node);
    txn->nentries++;
    txn->nentries_mem++;

    /* update memory accounting information */
    ReorderBufferChangeMemoryUpdate(rb, change, true);

    /* check the memory limits and evict something if needed */
    ReorderBufferCheckMemoryLimit(rb);
}

/*
 * Queue message into a transaction so it can be processed upon commit.
 */
void
ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
                          Snapshot snapshot, XLogRecPtr lsn,
                          bool transactional, const char *prefix,
                          Size message_size, const char *message)
{
    if (transactional)
    {
        MemoryContext oldcontext;
        ReorderBufferChange *change;

        Assert(xid != InvalidTransactionId);

        oldcontext = MemoryContextSwitchTo(rb->context);

        change = ReorderBufferGetChange(rb);
        change->action = REORDER_BUFFER_CHANGE_MESSAGE;
        change->data.msg.prefix = pstrdup(prefix);
        change->data.msg.message_size = message_size;
        change->data.msg.message = palloc(message_size);
        memcpy(change->data.msg.message, message, message_size);

        ReorderBufferQueueChange(rb, xid, lsn, change);

        MemoryContextSwitchTo(oldcontext);
    }
    else
    {
        ReorderBufferTXN *txn = NULL;
        volatile Snapshot snapshot_now = snapshot;

        if (xid != InvalidTransactionId)
            txn = ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);

        /* setup snapshot to allow catalog access */
        SetupHistoricSnapshot(snapshot_now, NULL);
        PG_TRY();
        {
            rb->message(rb, txn, lsn, false, prefix, message_size, message);

            TeardownHistoricSnapshot(false);
        }
        PG_CATCH();
        {
            TeardownHistoricSnapshot(true);
            PG_RE_THROW();
        }
        PG_END_TRY();
    }
}

/*
 * AssertTXNLsnOrder
 *      Verify LSN ordering of transaction lists in the reorderbuffer
 *
 * Other LSN-related invariants are checked too.
 *
 * No-op if assertions are not in use.
 */
static void
AssertTXNLsnOrder(ReorderBuffer *rb)
{
#ifdef USE_ASSERT_CHECKING
    dlist_iter  iter;
    XLogRecPtr  prev_first_lsn = InvalidXLogRecPtr;
    XLogRecPtr  prev_base_snap_lsn = InvalidXLogRecPtr;

    dlist_foreach(iter, &rb->toplevel_by_lsn)
    {
        ReorderBufferTXN *cur_txn = dlist_container(ReorderBufferTXN, node,
                                                    iter.cur);

        /* start LSN must be set */
        Assert(cur_txn->first_lsn != InvalidXLogRecPtr);

        /* If there is an end LSN, it must be higher than start LSN */
        if (cur_txn->end_lsn != InvalidXLogRecPtr)
            Assert(cur_txn->first_lsn <= cur_txn->end_lsn);

        /* Current initial LSN must be strictly higher than previous */
        if (prev_first_lsn != InvalidXLogRecPtr)
            Assert(prev_first_lsn < cur_txn->first_lsn);

        /* known-as-subtxn txns must not be listed */
        Assert(!rbtxn_is_known_subxact(cur_txn));

        prev_first_lsn = cur_txn->first_lsn;
    }

    dlist_foreach(iter, &rb->txns_by_base_snapshot_lsn)
    {
        ReorderBufferTXN *cur_txn = dlist_container(ReorderBufferTXN,
                                                    base_snapshot_node,
                                                    iter.cur);

        /* base snapshot (and its LSN) must be set */
        Assert(cur_txn->base_snapshot != NULL);
        Assert(cur_txn->base_snapshot_lsn != InvalidXLogRecPtr);

        /* current LSN must be strictly higher than previous */
        if (prev_base_snap_lsn != InvalidXLogRecPtr)
            Assert(prev_base_snap_lsn < cur_txn->base_snapshot_lsn);

        /* known-as-subtxn txns must not be listed */
        Assert(!rbtxn_is_known_subxact(cur_txn));

        prev_base_snap_lsn = cur_txn->base_snapshot_lsn;
    }
#endif
}

/*
 * ReorderBufferGetOldestTXN
 *      Return oldest transaction in reorderbuffer
 */
ReorderBufferTXN *
ReorderBufferGetOldestTXN(ReorderBuffer *rb)
{
    ReorderBufferTXN *txn;

    AssertTXNLsnOrder(rb);

    if (dlist_is_empty(&rb->toplevel_by_lsn))
        return NULL;

    txn = dlist_head_element(ReorderBufferTXN, node, &rb->toplevel_by_lsn);

    Assert(!rbtxn_is_known_subxact(txn));
    Assert(txn->first_lsn != InvalidXLogRecPtr);
    return txn;
}

/*
 * ReorderBufferGetOldestXmin
 *      Return oldest Xmin in reorderbuffer
 *
 * Returns oldest possibly running Xid from the point of view of snapshots
 * used in the transactions kept by reorderbuffer, or InvalidTransactionId if
 * there are none.
 *
 * Since snapshots are assigned monotonically, this equals the Xmin of the
 * base snapshot with minimal base_snapshot_lsn.
 */
TransactionId
ReorderBufferGetOldestXmin(ReorderBuffer *rb)
{
    ReorderBufferTXN *txn;

    AssertTXNLsnOrder(rb);

    if (dlist_is_empty(&rb->txns_by_base_snapshot_lsn))
        return InvalidTransactionId;

    txn = dlist_head_element(ReorderBufferTXN, base_snapshot_node,
                             &rb->txns_by_base_snapshot_lsn);
    return txn->base_snapshot->xmin;
}

void
ReorderBufferSetRestartPoint(ReorderBuffer *rb, XLogRecPtr ptr)
{
    rb->current_restart_decoding_lsn = ptr;
}

/*
 * ReorderBufferAssignChild
 *
 * Make note that we know that subxid is a subtransaction of xid, seen as of
 * the given lsn.
 */
void
ReorderBufferAssignChild(ReorderBuffer *rb, TransactionId xid,
                         TransactionId subxid, XLogRecPtr lsn)
{
    ReorderBufferTXN *txn;
    ReorderBufferTXN *subtxn;
    bool        new_top;
    bool        new_sub;

    txn = ReorderBufferTXNByXid(rb, xid, true, &new_top, lsn, true);
    subtxn = ReorderBufferTXNByXid(rb, subxid, true, &new_sub, lsn, false);

    if (new_top && !new_sub)
        elog(ERROR, "subtransaction logged without previous top-level txn record");

    if (!new_sub)
    {
        if (rbtxn_is_known_subxact(subtxn))
        {
            /* already associated, nothing to do */
            return;
        }
        else
        {
            /*
             * We already saw this transaction, but initially added it to the
             * list of top-level txns.  Now that we know it's not top-level,
             * remove it from there.
             */
            dlist_delete(&subtxn->node);
        }
    }

    subtxn->txn_flags |= RBTXN_IS_SUBXACT;
    subtxn->toplevel_xid = xid;
    Assert(subtxn->nsubtxns == 0);

    /* add to subtransaction list */
    dlist_push_tail(&txn->subtxns, &subtxn->node);
    txn->nsubtxns++;

    /* Possibly transfer the subtxn's snapshot to its top-level txn. */
    ReorderBufferTransferSnapToParent(txn, subtxn);

    /* Verify LSN-ordering invariant */
    AssertTXNLsnOrder(rb);
}

/*
 * ReorderBufferTransferSnapToParent
 *      Transfer base snapshot from subtxn to top-level txn, if needed
 *
 * This is done if the top-level txn doesn't have a base snapshot, or if the
 * subtxn's base snapshot has an earlier LSN than the top-level txn's base
 * snapshot's LSN.  This can happen if there are no changes in the toplevel
 * txn but there are some in the subtxn, or the first change in subtxn has
 * earlier LSN than first change in the top-level txn and we learned about
 * their kinship only now.
 *
 * The subtransaction's snapshot is cleared regardless of the transfer
 * happening, since it's not needed anymore in either case.
 *
 * We do this as soon as we become aware of their kinship, to avoid queueing
 * extra snapshots to txns known-as-subtxns -- only top-level txns will
 * receive further snapshots.
 */
static void
ReorderBufferTransferSnapToParent(ReorderBufferTXN *txn,
                                  ReorderBufferTXN *subtxn)
{
    Assert(subtxn->toplevel_xid == txn->xid);

    if (subtxn->base_snapshot != NULL)
    {
        if (txn->base_snapshot == NULL ||
            subtxn->base_snapshot_lsn < txn->base_snapshot_lsn)
        {
            /*
             * If the toplevel transaction already has a base snapshot but
             * it's newer than the subxact's, purge it.
             */
            if (txn->base_snapshot != NULL)
            {
                SnapBuildSnapDecRefcount(txn->base_snapshot);
                dlist_delete(&txn->base_snapshot_node);
            }

            /*
             * The snapshot is now the top transaction's; transfer it, and
             * adjust the list position of the top transaction in the list by
             * moving it to where the subtransaction is.
             */
            txn->base_snapshot = subtxn->base_snapshot;
            txn->base_snapshot_lsn = subtxn->base_snapshot_lsn;
            dlist_insert_before(&subtxn->base_snapshot_node,
                                &txn->base_snapshot_node);

            /*
             * The subtransaction doesn't have a snapshot anymore (so it
             * mustn't be in the list.)
             */
            subtxn->base_snapshot = NULL;
            subtxn->base_snapshot_lsn = InvalidXLogRecPtr;
            dlist_delete(&subtxn->base_snapshot_node);
        }
        else
        {
            /* Base snap of toplevel is fine, so subxact's is not needed */
            SnapBuildSnapDecRefcount(subtxn->base_snapshot);
            dlist_delete(&subtxn->base_snapshot_node);
            subtxn->base_snapshot = NULL;
            subtxn->base_snapshot_lsn = InvalidXLogRecPtr;
        }
    }
}

/*
 * Associate a subtransaction with its toplevel transaction at commit
 * time. There may be no further changes added after this.
 */
void
ReorderBufferCommitChild(ReorderBuffer *rb, TransactionId xid,
                         TransactionId subxid, XLogRecPtr commit_lsn,
                         XLogRecPtr end_lsn)
{
    ReorderBufferTXN *subtxn;

    subtxn = ReorderBufferTXNByXid(rb, subxid, false, NULL,
                                   InvalidXLogRecPtr, false);

    /*
     * No need to do anything if that subtxn didn't contain any changes
     */
    if (!subtxn)
        return;

    subtxn->final_lsn = commit_lsn;
    subtxn->end_lsn = end_lsn;

    /*
     * Assign this subxact as a child of the toplevel xact (no-op if already
     * done.)
     */
    ReorderBufferAssignChild(rb, xid, subxid, InvalidXLogRecPtr);
}


/*
 * Support for efficiently iterating over a transaction's and its
 * subtransactions' changes.
 *
 * We do by doing a k-way merge between transactions/subtransactions. For that
 * we model the current heads of the different transactions as a binary heap
 * so we easily know which (sub-)transaction has the change with the smallest
 * lsn next.
 *
 * We assume the changes in individual transactions are already sorted by LSN.
 */

/*
 * Binary heap comparison function.
 */
static int
ReorderBufferIterCompare(Datum a, Datum b, void *arg)
{
    ReorderBufferIterTXNState *state = (ReorderBufferIterTXNState *) arg;
    XLogRecPtr  pos_a = state->entries[DatumGetInt32(a)].lsn;
    XLogRecPtr  pos_b = state->entries[DatumGetInt32(b)].lsn;

    if (pos_a < pos_b)
        return 1;
    else if (pos_a == pos_b)
        return 0;
    return -1;
}

/*
 * Allocate & initialize an iterator which iterates in lsn order over a
 * transaction and all its subtransactions.
 *
 * Note: The iterator state is returned through iter_state parameter rather
 * than the function's return value.  This is because the state gets cleaned up
 * in a PG_CATCH block in the caller, so we want to make sure the caller gets
 * back the state even if this function throws an exception.
 */
static void
ReorderBufferIterTXNInit(ReorderBuffer *rb, ReorderBufferTXN *txn,
                         ReorderBufferIterTXNState *volatile *iter_state)
{
    Size        nr_txns = 0;
    ReorderBufferIterTXNState *state;
    dlist_iter  cur_txn_i;
    int32       off;

    *iter_state = NULL;

    /*
     * Calculate the size of our heap: one element for every transaction that
     * contains changes.  (Besides the transactions already in the reorder
     * buffer, we count the one we were directly passed.)
     */
    if (txn->nentries > 0)
        nr_txns++;

    dlist_foreach(cur_txn_i, &txn->subtxns)
    {
        ReorderBufferTXN *cur_txn;

        cur_txn = dlist_container(ReorderBufferTXN, node, cur_txn_i.cur);

        if (cur_txn->nentries > 0)
            nr_txns++;
    }

    /*
     * TODO: Consider adding fastpath for the rather common nr_txns=1 case, no
     * need to allocate/build a heap then.
     */

    /* allocate iteration state */
    state = (ReorderBufferIterTXNState *)
        MemoryContextAllocZero(rb->context,
                               sizeof(ReorderBufferIterTXNState) +
                               sizeof(ReorderBufferIterTXNEntry) * nr_txns);

    state->nr_txns = nr_txns;
    dlist_init(&state->old_change);

    for (off = 0; off < state->nr_txns; off++)
    {
        state->entries[off].file.vfd = -1;
        state->entries[off].segno = 0;
    }

    /* allocate heap */
    state->heap = binaryheap_allocate(state->nr_txns,
                                      ReorderBufferIterCompare,
                                      state);

    /* Now that the state fields are initialized, it is safe to return it. */
    *iter_state = state;

    /*
     * Now insert items into the binary heap, in an unordered fashion.  (We
     * will run a heap assembly step at the end; this is more efficient.)
     */

    off = 0;

    /* add toplevel transaction if it contains changes */
    if (txn->nentries > 0)
    {
        ReorderBufferChange *cur_change;

        if (rbtxn_is_serialized(txn))
        {
            /* serialize remaining changes */
            ReorderBufferSerializeTXN(rb, txn);
            ReorderBufferRestoreChanges(rb, txn, &state->entries[off].file,
                                        &state->entries[off].segno);
        }

        cur_change = dlist_head_element(ReorderBufferChange, node,
                                        &txn->changes);

        state->entries[off].lsn = cur_change->lsn;
        state->entries[off].change = cur_change;
        state->entries[off].txn = txn;

        binaryheap_add_unordered(state->heap, Int32GetDatum(off++));
    }

    /* add subtransactions if they contain changes */
    dlist_foreach(cur_txn_i, &txn->subtxns)
    {
        ReorderBufferTXN *cur_txn;

        cur_txn = dlist_container(ReorderBufferTXN, node, cur_txn_i.cur);

        if (cur_txn->nentries > 0)
        {
            ReorderBufferChange *cur_change;

            if (rbtxn_is_serialized(cur_txn))
            {
                /* serialize remaining changes */
                ReorderBufferSerializeTXN(rb, cur_txn);
                ReorderBufferRestoreChanges(rb, cur_txn,
                                            &state->entries[off].file,
                                            &state->entries[off].segno);
            }
            cur_change = dlist_head_element(ReorderBufferChange, node,
                                            &cur_txn->changes);

            state->entries[off].lsn = cur_change->lsn;
            state->entries[off].change = cur_change;
            state->entries[off].txn = cur_txn;

            binaryheap_add_unordered(state->heap, Int32GetDatum(off++));
        }
    }

    /* assemble a valid binary heap */
    binaryheap_build(state->heap);
}

/*
 * Return the next change when iterating over a transaction and its
 * subtransactions.
 *
 * Returns NULL when no further changes exist.
 */
static ReorderBufferChange *
ReorderBufferIterTXNNext(ReorderBuffer *rb, ReorderBufferIterTXNState *state)
{
    ReorderBufferChange *change;
    ReorderBufferIterTXNEntry *entry;
    int32       off;

    /* nothing there anymore */
    if (state->heap->bh_size == 0)
        return NULL;

    off = DatumGetInt32(binaryheap_first(state->heap));
    entry = &state->entries[off];

    /* free memory we might have "leaked" in the previous *Next call */
    if (!dlist_is_empty(&state->old_change))
    {
        change = dlist_container(ReorderBufferChange, node,
                                 dlist_pop_head_node(&state->old_change));
        ReorderBufferReturnChange(rb, change);
        Assert(dlist_is_empty(&state->old_change));
    }

    change = entry->change;

    /*
     * update heap with information about which transaction has the next
     * relevant change in LSN order
     */

    /* there are in-memory changes */
    if (dlist_has_next(&entry->txn->changes, &entry->change->node))
    {
        dlist_node *next = dlist_next_node(&entry->txn->changes, &change->node);
        ReorderBufferChange *next_change =
        dlist_container(ReorderBufferChange, node, next);

        /* txn stays the same */
        state->entries[off].lsn = next_change->lsn;
        state->entries[off].change = next_change;

        binaryheap_replace_first(state->heap, Int32GetDatum(off));
        return change;
    }

    /* try to load changes from disk */
    if (entry->txn->nentries != entry->txn->nentries_mem)
    {
        /*
         * Ugly: restoring changes will reuse *Change records, thus delete the
         * current one from the per-tx list and only free in the next call.
         */
        dlist_delete(&change->node);
        dlist_push_tail(&state->old_change, &change->node);

        if (ReorderBufferRestoreChanges(rb, entry->txn, &entry->file,
                                        &state->entries[off].segno))
        {
            /* successfully restored changes from disk */
            ReorderBufferChange *next_change =
            dlist_head_element(ReorderBufferChange, node,
                               &entry->txn->changes);

            elog(DEBUG2, "restored %u/%u changes from disk",
                 (uint32) entry->txn->nentries_mem,
                 (uint32) entry->txn->nentries);

            Assert(entry->txn->nentries_mem);
            /* txn stays the same */
            state->entries[off].lsn = next_change->lsn;
            state->entries[off].change = next_change;
            binaryheap_replace_first(state->heap, Int32GetDatum(off));

            return change;
        }
    }

    /* ok, no changes there anymore, remove */
    binaryheap_remove_first(state->heap);

    return change;
}

/*
 * Deallocate the iterator
 */
static void
ReorderBufferIterTXNFinish(ReorderBuffer *rb,
                           ReorderBufferIterTXNState *state)
{
    int32       off;

    for (off = 0; off < state->nr_txns; off++)
    {
        if (state->entries[off].file.vfd != -1)
            FileClose(state->entries[off].file.vfd);
    }

    /* free memory we might have "leaked" in the last *Next call */
    if (!dlist_is_empty(&state->old_change))
    {
        ReorderBufferChange *change;

        change = dlist_container(ReorderBufferChange, node,
                                 dlist_pop_head_node(&state->old_change));
        ReorderBufferReturnChange(rb, change);
        Assert(dlist_is_empty(&state->old_change));
    }

    binaryheap_free(state->heap);
    pfree(state);
}

/*
 * Cleanup the contents of a transaction, usually after the transaction
 * committed or aborted.
 */
static void
ReorderBufferCleanupTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
{
    bool        found;
    dlist_mutable_iter iter;

    /* cleanup subtransactions & their changes */
    dlist_foreach_modify(iter, &txn->subtxns)
    {
        ReorderBufferTXN *subtxn;

        subtxn = dlist_container(ReorderBufferTXN, node, iter.cur);

        /*
         * Subtransactions are always associated to the toplevel TXN, even if
         * they originally were happening inside another subtxn, so we won't
         * ever recurse more than one level deep here.
         */
        Assert(rbtxn_is_known_subxact(subtxn));
        Assert(subtxn->nsubtxns == 0);

        ReorderBufferCleanupTXN(rb, subtxn);
    }

    /* cleanup changes in the toplevel txn */
    dlist_foreach_modify(iter, &txn->changes)
    {
        ReorderBufferChange *change;

        change = dlist_container(ReorderBufferChange, node, iter.cur);

        /* Check we're not mixing changes from different transactions. */
        Assert(change->txn == txn);

        ReorderBufferReturnChange(rb, change);
    }

    /*
     * Cleanup the tuplecids we stored for decoding catalog snapshot access.
     * They are always stored in the toplevel transaction.
     */
    dlist_foreach_modify(iter, &txn->tuplecids)
    {
        ReorderBufferChange *change;

        change = dlist_container(ReorderBufferChange, node, iter.cur);

        /* Check we're not mixing changes from different transactions. */
        Assert(change->txn == txn);
        Assert(change->action == REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID);

        ReorderBufferReturnChange(rb, change);
    }

    /*
     * Cleanup the base snapshot, if set.
     */
    if (txn->base_snapshot != NULL)
    {
        SnapBuildSnapDecRefcount(txn->base_snapshot);
        dlist_delete(&txn->base_snapshot_node);
    }

    /*
     * Remove TXN from its containing list.
     *
     * Note: if txn is known as subxact, we are deleting the TXN from its
     * parent's list of known subxacts; this leaves the parent's nsubxacts
     * count too high, but we don't care.  Otherwise, we are deleting the TXN
     * from the LSN-ordered list of toplevel TXNs.
     */
    dlist_delete(&txn->node);

    /* now remove reference from buffer */
    hash_search(rb->by_txn,
                (void *) &txn->xid,
                HASH_REMOVE,
                &found);
    Assert(found);

    /* remove entries spilled to disk */
    if (rbtxn_is_serialized(txn))
        ReorderBufferRestoreCleanup(rb, txn);

    /* deallocate */
    ReorderBufferReturnTXN(rb, txn);
}

/*
 * Build a hash with a (relfilenode, ctid) -> (cmin, cmax) mapping for use by
 * HeapTupleSatisfiesHistoricMVCC.
 */
static void
ReorderBufferBuildTupleCidHash(ReorderBuffer *rb, ReorderBufferTXN *txn)
{
    dlist_iter  iter;
    HASHCTL     hash_ctl;

    if (!rbtxn_has_catalog_changes(txn) || dlist_is_empty(&txn->tuplecids))
        return;

    memset(&hash_ctl, 0, sizeof(hash_ctl));

    hash_ctl.keysize = sizeof(ReorderBufferTupleCidKey);
    hash_ctl.entrysize = sizeof(ReorderBufferTupleCidEnt);
    hash_ctl.hcxt = rb->context;

    /*
     * create the hash with the exact number of to-be-stored tuplecids from
     * the start
     */
    txn->tuplecid_hash =
        hash_create("ReorderBufferTupleCid", txn->ntuplecids, &hash_ctl,
                    HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

    dlist_foreach(iter, &txn->tuplecids)
    {
        ReorderBufferTupleCidKey key;
        ReorderBufferTupleCidEnt *ent;
        bool        found;
        ReorderBufferChange *change;

        change = dlist_container(ReorderBufferChange, node, iter.cur);

        Assert(change->action == REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID);

        /* be careful about padding */
        memset(&key, 0, sizeof(ReorderBufferTupleCidKey));

        key.relnode = change->data.tuplecid.node;

        ItemPointerCopy(&change->data.tuplecid.tid,
                        &key.tid);

        ent = (ReorderBufferTupleCidEnt *)
            hash_search(txn->tuplecid_hash,
                        (void *) &key,
                        HASH_ENTER | HASH_FIND,
                        &found);
        if (!found)
        {
            ent->cmin = change->data.tuplecid.cmin;
            ent->cmax = change->data.tuplecid.cmax;
            ent->combocid = change->data.tuplecid.combocid;
        }
        else
        {
            /*
             * Maybe we already saw this tuple before in this transaction, but
             * if so it must have the same cmin.
             */
            Assert(ent->cmin == change->data.tuplecid.cmin);

            /*
             * cmax may be initially invalid, but once set it can only grow,
             * and never become invalid again.
             */
            Assert((ent->cmax == InvalidCommandId) ||
                   ((change->data.tuplecid.cmax != InvalidCommandId) &&
                    (change->data.tuplecid.cmax > ent->cmax)));
            ent->cmax = change->data.tuplecid.cmax;
        }
    }
}

/*
 * Copy a provided snapshot so we can modify it privately. This is needed so
 * that catalog modifying transactions can look into intermediate catalog
 * states.
 */
static Snapshot
ReorderBufferCopySnap(ReorderBuffer *rb, Snapshot orig_snap,
                      ReorderBufferTXN *txn, CommandId cid)
{
    Snapshot    snap;
    dlist_iter  iter;
    int         i = 0;
    Size        size;

    size = sizeof(SnapshotData) +
        sizeof(TransactionId) * orig_snap->xcnt +
        sizeof(TransactionId) * (txn->nsubtxns + 1);

    snap = MemoryContextAllocZero(rb->context, size);
    memcpy(snap, orig_snap, sizeof(SnapshotData));

    snap->copied = true;
    snap->active_count = 1;     /* mark as active so nobody frees it */
    snap->regd_count = 0;
    snap->xip = (TransactionId *) (snap + 1);

    memcpy(snap->xip, orig_snap->xip, sizeof(TransactionId) * snap->xcnt);

    /*
     * snap->subxip contains all txids that belong to our transaction which we
     * need to check via cmin/cmax. That's why we store the toplevel
     * transaction in there as well.
     */
    snap->subxip = snap->xip + snap->xcnt;
    snap->subxip[i++] = txn->xid;

    /*
     * subxcnt isn't decreased when subtransactions abort, so count manually.
     * Since it's an upper boundary it is safe to use it for the allocation
     * above.
     */
    snap->subxcnt = 1;

    dlist_foreach(iter, &txn->subtxns)
    {
        ReorderBufferTXN *sub_txn;

        sub_txn = dlist_container(ReorderBufferTXN, node, iter.cur);
        snap->subxip[i++] = sub_txn->xid;
        snap->subxcnt++;
    }

    /* sort so we can bsearch() later */
    qsort(snap->subxip, snap->subxcnt, sizeof(TransactionId), xidComparator);

    /* store the specified current CommandId */
    snap->curcid = cid;

    return snap;
}

/*
 * Free a previously ReorderBufferCopySnap'ed snapshot
 */
static void
ReorderBufferFreeSnap(ReorderBuffer *rb, Snapshot snap)
{
    if (snap->copied)
        pfree(snap);
    else
        SnapBuildSnapDecRefcount(snap);
}

/*
 * Perform the replay of a transaction and its non-aborted subtransactions.
 *
 * Subtransactions previously have to be processed by
 * ReorderBufferCommitChild(), even if previously assigned to the toplevel
 * transaction with ReorderBufferAssignChild.
 *
 * We currently can only decode a transaction's contents when its commit
 * record is read because that's the only place where we know about cache
 * invalidations. Thus, once a toplevel commit is read, we iterate over the top
 * and subtransactions (using a k-way merge) and replay the changes in lsn
 * order.
 */
void
ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
                    XLogRecPtr commit_lsn, XLogRecPtr end_lsn,
                    TimestampTz commit_time,
                    RepOriginId origin_id, XLogRecPtr origin_lsn)
{
    ReorderBufferTXN *txn;
    volatile Snapshot snapshot_now;
    volatile CommandId command_id = FirstCommandId;
    bool        using_subtxn;
    ReorderBufferIterTXNState *volatile iterstate = NULL;

    txn = ReorderBufferTXNByXid(rb, xid, false, NULL, InvalidXLogRecPtr,
                                false);

    /* unknown transaction, nothing to replay */
    if (txn == NULL)
        return;

    txn->final_lsn = commit_lsn;
    txn->end_lsn = end_lsn;
    txn->commit_time = commit_time;
    txn->origin_id = origin_id;
    txn->origin_lsn = origin_lsn;

    /*
     * If this transaction has no snapshot, it didn't make any changes to the
     * database, so there's nothing to decode.  Note that
     * ReorderBufferCommitChild will have transferred any snapshots from
     * subtransactions if there were any.
     */
    if (txn->base_snapshot == NULL)
    {
        Assert(txn->ninvalidations == 0);
        ReorderBufferCleanupTXN(rb, txn);
        return;
    }

    snapshot_now = txn->base_snapshot;

    /* build data to be able to lookup the CommandIds of catalog tuples */
    ReorderBufferBuildTupleCidHash(rb, txn);

    /* setup the initial snapshot */
    SetupHistoricSnapshot(snapshot_now, txn->tuplecid_hash);

    /*
     * Decoding needs access to syscaches et al., which in turn use
     * heavyweight locks and such. Thus we need to have enough state around to
     * keep track of those.  The easiest way is to simply use a transaction
     * internally.  That also allows us to easily enforce that nothing writes
     * to the database by checking for xid assignments.
     *
     * When we're called via the SQL SRF there's already a transaction
     * started, so start an explicit subtransaction there.
     */
    using_subtxn = IsTransactionOrTransactionBlock();

    PG_TRY();
    {
        ReorderBufferChange *change;
        ReorderBufferChange *specinsert = NULL;

        if (using_subtxn)
            BeginInternalSubTransaction("replay");
        else
            StartTransactionCommand();

        rb->begin(rb, txn);

        ReorderBufferIterTXNInit(rb, txn, &iterstate);
        while ((change = ReorderBufferIterTXNNext(rb, iterstate)) != NULL)
        {
            Relation    relation = NULL;
            Oid         reloid;

            switch (change->action)
            {
                case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:

                    /*
                     * Confirmation for speculative insertion arrived. Simply
                     * use as a normal record. It'll be cleaned up at the end
                     * of INSERT processing.
                     */
                    if (specinsert == NULL)
                        elog(ERROR, "invalid ordering of speculative insertion changes");
                    Assert(specinsert->data.tp.oldtuple == NULL);
                    change = specinsert;
                    change->action = REORDER_BUFFER_CHANGE_INSERT;

                    /* intentionally fall through */
                case REORDER_BUFFER_CHANGE_INSERT:
                case REORDER_BUFFER_CHANGE_UPDATE:
                case REORDER_BUFFER_CHANGE_DELETE:
                    Assert(snapshot_now);

                    reloid = RelidByRelfilenode(change->data.tp.relnode.spcNode,
                                                change->data.tp.relnode.relNode);

                    /*
                     * Mapped catalog tuple without data, emitted while
                     * catalog table was in the process of being rewritten. We
                     * can fail to look up the relfilenode, because the
                     * relmapper has no "historic" view, in contrast to normal
                     * the normal catalog during decoding. Thus repeated
                     * rewrites can cause a lookup failure. That's OK because
                     * we do not decode catalog changes anyway. Normally such
                     * tuples would be skipped over below, but we can't
                     * identify whether the table should be logically logged
                     * without mapping the relfilenode to the oid.
                     */
                    if (reloid == InvalidOid &&
                        change->data.tp.newtuple == NULL &&
                        change->data.tp.oldtuple == NULL)
                        goto change_done;
                    else if (reloid == InvalidOid)
                        elog(ERROR, "could not map filenode \"%s\" to relation OID",
                             relpathperm(change->data.tp.relnode,
                                         MAIN_FORKNUM));

                    relation = RelationIdGetRelation(reloid);

                    if (!RelationIsValid(relation))
                        elog(ERROR, "could not open relation with OID %u (for filenode \"%s\")",
                             reloid,
                             relpathperm(change->data.tp.relnode,
                                         MAIN_FORKNUM));

                    if (!RelationIsLogicallyLogged(relation))
                        goto change_done;

                    /*
                     * Ignore temporary heaps created during DDL unless the
                     * plugin has asked for them.
                     */
                    if (relation->rd_rel->relrewrite && !rb->output_rewrites)
                        goto change_done;

                    /*
                     * For now ignore sequence changes entirely. Most of the
                     * time they don't log changes using records we
                     * understand, so it doesn't make sense to handle the few
                     * cases we do.
                     */
                    if (relation->rd_rel->relkind == RELKIND_SEQUENCE)
                        goto change_done;

                    /* user-triggered change */
                    if (!IsToastRelation(relation))
                    {
                        ReorderBufferToastReplace(rb, txn, relation, change);
                        rb->apply_change(rb, txn, relation, change);

                        /*
                         * Only clear reassembled toast chunks if we're sure
                         * they're not required anymore. The creator of the
                         * tuple tells us.
                         */
                        if (change->data.tp.clear_toast_afterwards)
                            ReorderBufferToastReset(rb, txn);
                    }
                    /* we're not interested in toast deletions */
                    else if (change->action == REORDER_BUFFER_CHANGE_INSERT)
                    {
                        /*
                         * Need to reassemble the full toasted Datum in
                         * memory, to ensure the chunks don't get reused till
                         * we're done remove it from the list of this
                         * transaction's changes. Otherwise it will get
                         * freed/reused while restoring spooled data from
                         * disk.
                         */
                        Assert(change->data.tp.newtuple != NULL);

                        dlist_delete(&change->node);
                        ReorderBufferToastAppendChunk(rb, txn, relation,
                                                      change);
                    }

            change_done:

                    /*
                     * Either speculative insertion was confirmed, or it was
                     * unsuccessful and the record isn't needed anymore.
                     */
                    if (specinsert != NULL)
                    {
                        ReorderBufferReturnChange(rb, specinsert);
                        specinsert = NULL;
                    }

                    if (relation != NULL)
                    {
                        RelationClose(relation);
                        relation = NULL;
                    }
                    break;

                case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_INSERT:

                    /*
                     * Speculative insertions are dealt with by delaying the
                     * processing of the insert until the confirmation record
                     * arrives. For that we simply unlink the record from the
                     * chain, so it does not get freed/reused while restoring
                     * spooled data from disk.
                     *
                     * This is safe in the face of concurrent catalog changes
                     * because the relevant relation can't be changed between
                     * speculative insertion and confirmation due to
                     * CheckTableNotInUse() and locking.
                     */

                    /* clear out a pending (and thus failed) speculation */
                    if (specinsert != NULL)
                    {
                        ReorderBufferReturnChange(rb, specinsert);
                        specinsert = NULL;
                    }

                    /* and memorize the pending insertion */
                    dlist_delete(&change->node);
                    specinsert = change;
                    break;

                case REORDER_BUFFER_CHANGE_TRUNCATE:
                    {
                        int         i;
                        int         nrelids = change->data.truncate.nrelids;
                        int         nrelations = 0;
                        Relation   *relations;

                        relations = palloc0(nrelids * sizeof(Relation));
                        for (i = 0; i < nrelids; i++)
                        {
                            Oid         relid = change->data.truncate.relids[i];
                            Relation    relation;

                            relation = RelationIdGetRelation(relid);

                            if (!RelationIsValid(relation))
                                elog(ERROR, "could not open relation with OID %u", relid);

                            if (!RelationIsLogicallyLogged(relation))
                                continue;

                            relations[nrelations++] = relation;
                        }

                        rb->apply_truncate(rb, txn, nrelations, relations, change);

                        for (i = 0; i < nrelations; i++)
                            RelationClose(relations[i]);

                        break;
                    }

                case REORDER_BUFFER_CHANGE_MESSAGE:
                    rb->message(rb, txn, change->lsn, true,
                                change->data.msg.prefix,
                                change->data.msg.message_size,
                                change->data.msg.message);
                    break;

                case REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT:
                    /* get rid of the old */
                    TeardownHistoricSnapshot(false);

                    if (snapshot_now->copied)
                    {
                        ReorderBufferFreeSnap(rb, snapshot_now);
                        snapshot_now =
                            ReorderBufferCopySnap(rb, change->data.snapshot,
                                                  txn, command_id);
                    }

                    /*
                     * Restored from disk, need to be careful not to double
                     * free. We could introduce refcounting for that, but for
                     * now this seems infrequent enough not to care.
                     */
                    else if (change->data.snapshot->copied)
                    {
                        snapshot_now =
                            ReorderBufferCopySnap(rb, change->data.snapshot,
                                                  txn, command_id);
                    }
                    else
                    {
                        snapshot_now = change->data.snapshot;
                    }


                    /* and continue with the new one */
                    SetupHistoricSnapshot(snapshot_now, txn->tuplecid_hash);
                    break;

                case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
                    Assert(change->data.command_id != InvalidCommandId);

                    if (command_id < change->data.command_id)
                    {
                        command_id = change->data.command_id;

                        if (!snapshot_now->copied)
                        {
                            /* we don't use the global one anymore */
                            snapshot_now = ReorderBufferCopySnap(rb, snapshot_now,
                                                                 txn, command_id);
                        }

                        snapshot_now->curcid = command_id;

                        TeardownHistoricSnapshot(false);
                        SetupHistoricSnapshot(snapshot_now, txn->tuplecid_hash);

                        /*
                         * Every time the CommandId is incremented, we could
                         * see new catalog contents, so execute all
                         * invalidations.
                         */
                        ReorderBufferExecuteInvalidations(rb, txn);
                    }

                    break;

                case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
                    elog(ERROR, "tuplecid value in changequeue");
                    break;
            }
        }

        /*
         * There's a speculative insertion remaining, just clean in up, it
         * can't have been successful, otherwise we'd gotten a confirmation
         * record.
         */
        if (specinsert)
        {
            ReorderBufferReturnChange(rb, specinsert);
            specinsert = NULL;
        }

        /* clean up the iterator */
        ReorderBufferIterTXNFinish(rb, iterstate);
        iterstate = NULL;

        /* call commit callback */
        rb->commit(rb, txn, commit_lsn);

        /* this is just a sanity check against bad output plugin behaviour */
        if (GetCurrentTransactionIdIfAny() != InvalidTransactionId)
            elog(ERROR, "output plugin used XID %u",
                 GetCurrentTransactionId());

        /* cleanup */
        TeardownHistoricSnapshot(false);

        /*
         * Aborting the current (sub-)transaction as a whole has the right
         * semantics. We want all locks acquired in here to be released, not
         * reassigned to the parent and we do not want any database access
         * have persistent effects.
         */
        AbortCurrentTransaction();

        /* make sure there's no cache pollution */
        ReorderBufferExecuteInvalidations(rb, txn);

        if (using_subtxn)
            RollbackAndReleaseCurrentSubTransaction();

        if (snapshot_now->copied)
            ReorderBufferFreeSnap(rb, snapshot_now);

        /* remove potential on-disk data, and deallocate */
        ReorderBufferCleanupTXN(rb, txn);
    }
    PG_CATCH();
    {
        /* TODO: Encapsulate cleanup from the PG_TRY and PG_CATCH blocks */
        if (iterstate)
            ReorderBufferIterTXNFinish(rb, iterstate);

        TeardownHistoricSnapshot(true);

        /*
         * Force cache invalidation to happen outside of a valid transaction
         * to prevent catalog access as we just caught an error.
         */
        AbortCurrentTransaction();

        /* make sure there's no cache pollution */
        ReorderBufferExecuteInvalidations(rb, txn);

        if (using_subtxn)
            RollbackAndReleaseCurrentSubTransaction();

        if (snapshot_now->copied)
            ReorderBufferFreeSnap(rb, snapshot_now);

        /* remove potential on-disk data, and deallocate */
        ReorderBufferCleanupTXN(rb, txn);

        PG_RE_THROW();
    }
    PG_END_TRY();
}

/*
 * Abort a transaction that possibly has previous changes. Needs to be first
 * called for subtransactions and then for the toplevel xid.
 *
 * NB: Transactions handled here have to have actively aborted (i.e. have
 * produced an abort record). Implicitly aborted transactions are handled via
 * ReorderBufferAbortOld(); transactions we're just not interested in, but
 * which have committed are handled in ReorderBufferForget().
 *
 * This function purges this transaction and its contents from memory and
 * disk.
 */
void
ReorderBufferAbort(ReorderBuffer *rb, TransactionId xid, XLogRecPtr lsn)
{
    ReorderBufferTXN *txn;

    txn = ReorderBufferTXNByXid(rb, xid, false, NULL, InvalidXLogRecPtr,
                                false);

    /* unknown, nothing to remove */
    if (txn == NULL)
        return;

    /* cosmetic... */
    txn->final_lsn = lsn;

    /* remove potential on-disk data, and deallocate */
    ReorderBufferCleanupTXN(rb, txn);
}

/*
 * Abort all transactions that aren't actually running anymore because the
 * server restarted.
 *
 * NB: These really have to be transactions that have aborted due to a server
 * crash/immediate restart, as we don't deal with invalidations here.
 */
void
ReorderBufferAbortOld(ReorderBuffer *rb, TransactionId oldestRunningXid)
{
    dlist_mutable_iter it;

    /*
     * Iterate through all (potential) toplevel TXNs and abort all that are
     * older than what possibly can be running. Once we've found the first
     * that is alive we stop, there might be some that acquired an xid earlier
     * but started writing later, but it's unlikely and they will be cleaned
     * up in a later call to this function.
     */
    dlist_foreach_modify(it, &rb->toplevel_by_lsn)
    {
        ReorderBufferTXN *txn;

        txn = dlist_container(ReorderBufferTXN, node, it.cur);

        if (TransactionIdPrecedes(txn->xid, oldestRunningXid))
        {
            /*
             * We set final_lsn on a transaction when we decode its commit or
             * abort record, but we never see those records for crashed
             * transactions.  To ensure cleanup of these transactions, set
             * final_lsn to that of their last change; this causes
             * ReorderBufferRestoreCleanup to do the right thing.
             */
            if (rbtxn_is_serialized(txn) && txn->final_lsn == 0)
            {
                ReorderBufferChange *last =
                dlist_tail_element(ReorderBufferChange, node, &txn->changes);

                txn->final_lsn = last->lsn;
            }

            elog(DEBUG2, "aborting old transaction %u", txn->xid);

            /* remove potential on-disk data, and deallocate this tx */
            ReorderBufferCleanupTXN(rb, txn);
        }
        else
            return;
    }
}

/*
 * Forget the contents of a transaction if we aren't interested in its
 * contents. Needs to be first called for subtransactions and then for the
 * toplevel xid.
 *
 * This is significantly different to ReorderBufferAbort() because
 * transactions that have committed need to be treated differently from aborted
 * ones since they may have modified the catalog.
 *
 * Note that this is only allowed to be called in the moment a transaction
 * commit has just been read, not earlier; otherwise later records referring
 * to this xid might re-create the transaction incompletely.
 */
void
ReorderBufferForget(ReorderBuffer *rb, TransactionId xid, XLogRecPtr lsn)
{
    ReorderBufferTXN *txn;

    txn = ReorderBufferTXNByXid(rb, xid, false, NULL, InvalidXLogRecPtr,
                                false);

    /* unknown, nothing to forget */
    if (txn == NULL)
        return;

    /* cosmetic... */
    txn->final_lsn = lsn;

    /*
     * Process cache invalidation messages if there are any. Even if we're not
     * interested in the transaction's contents, it could have manipulated the
     * catalog and we need to update the caches according to that.
     */
    if (txn->base_snapshot != NULL && txn->ninvalidations > 0)
        ReorderBufferImmediateInvalidation(rb, txn->ninvalidations,
                                           txn->invalidations);
    else
        Assert(txn->ninvalidations == 0);

    /* remove potential on-disk data, and deallocate */
    ReorderBufferCleanupTXN(rb, txn);
}

/*
 * Execute invalidations happening outside the context of a decoded
 * transaction. That currently happens either for xid-less commits
 * (cf. RecordTransactionCommit()) or for invalidations in uninteresting
 * transactions (via ReorderBufferForget()).
 */
void
ReorderBufferImmediateInvalidation(ReorderBuffer *rb, uint32 ninvalidations,
                                   SharedInvalidationMessage *invalidations)
{
    bool        use_subtxn = IsTransactionOrTransactionBlock();
    int         i;

    if (use_subtxn)
        BeginInternalSubTransaction("replay");

    /*
     * Force invalidations to happen outside of a valid transaction - that way
     * entries will just be marked as invalid without accessing the catalog.
     * That's advantageous because we don't need to setup the full state
     * necessary for catalog access.
     */
    if (use_subtxn)
        AbortCurrentTransaction();

    for (i = 0; i < ninvalidations; i++)
        LocalExecuteInvalidationMessage(&invalidations[i]);

    if (use_subtxn)
        RollbackAndReleaseCurrentSubTransaction();
}

/*
 * Tell reorderbuffer about an xid seen in the WAL stream. Has to be called at
 * least once for every xid in XLogRecord->xl_xid (other places in records
 * may, but do not have to be passed through here).
 *
 * Reorderbuffer keeps some datastructures about transactions in LSN order,
 * for efficiency. To do that it has to know about when transactions are seen
 * first in the WAL. As many types of records are not actually interesting for
 * logical decoding, they do not necessarily pass though here.
 */
void
ReorderBufferProcessXid(ReorderBuffer *rb, TransactionId xid, XLogRecPtr lsn)
{
    /* many records won't have an xid assigned, centralize check here */
    if (xid != InvalidTransactionId)
        ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);
}

/*
 * Add a new snapshot to this transaction that may only used after lsn 'lsn'
 * because the previous snapshot doesn't describe the catalog correctly for
 * following rows.
 */
void
ReorderBufferAddSnapshot(ReorderBuffer *rb, TransactionId xid,
                         XLogRecPtr lsn, Snapshot snap)
{
    ReorderBufferChange *change = ReorderBufferGetChange(rb);

    change->data.snapshot = snap;
    change->action = REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT;

    ReorderBufferQueueChange(rb, xid, lsn, change);
}

/*
 * Set up the transaction's base snapshot.
 *
 * If we know that xid is a subtransaction, set the base snapshot on the
 * top-level transaction instead.
 */
void
ReorderBufferSetBaseSnapshot(ReorderBuffer *rb, TransactionId xid,
                             XLogRecPtr lsn, Snapshot snap)
{
    ReorderBufferTXN *txn;
    bool        is_new;

    AssertArg(snap != NULL);

    /*
     * Fetch the transaction to operate on.  If we know it's a subtransaction,
     * operate on its top-level transaction instead.
     */
    txn = ReorderBufferTXNByXid(rb, xid, true, &is_new, lsn, true);
    if (rbtxn_is_known_subxact(txn))
        txn = ReorderBufferTXNByXid(rb, txn->toplevel_xid, false,
                                    NULL, InvalidXLogRecPtr, false);
    Assert(txn->base_snapshot == NULL);

    txn->base_snapshot = snap;
    txn->base_snapshot_lsn = lsn;
    dlist_push_tail(&rb->txns_by_base_snapshot_lsn, &txn->base_snapshot_node);

    AssertTXNLsnOrder(rb);
}

/*
 * Access the catalog with this CommandId at this point in the changestream.
 *
 * May only be called for command ids > 1
 */
void
ReorderBufferAddNewCommandId(ReorderBuffer *rb, TransactionId xid,
                             XLogRecPtr lsn, CommandId cid)
{
    ReorderBufferChange *change = ReorderBufferGetChange(rb);

    change->data.command_id = cid;
    change->action = REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID;

    ReorderBufferQueueChange(rb, xid, lsn, change);
}

/*
 * Update the memory accounting info. We track memory used by the whole
 * reorder buffer and the transaction containing the change.
 */
static void
ReorderBufferChangeMemoryUpdate(ReorderBuffer *rb,
                                ReorderBufferChange *change,
                                bool addition)
{
    Size        sz;

    Assert(change->txn);

    /*
     * Ignore tuple CID changes, because those are not evicted when reaching
     * memory limit. So we just don't count them, because it might easily
     * trigger a pointless attempt to spill.
     */
    if (change->action == REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID)
        return;

    sz = ReorderBufferChangeSize(change);

    if (addition)
    {
        change->txn->size += sz;
        rb->size += sz;
    }
    else
    {
        Assert((rb->size >= sz) && (change->txn->size >= sz));
        change->txn->size -= sz;
        rb->size -= sz;
    }
}

/*
 * Add new (relfilenode, tid) -> (cmin, cmax) mappings.
 *
 * We do not include this change type in memory accounting, because we
 * keep CIDs in a separate list and do not evict them when reaching
 * the memory limit.
 */
void
ReorderBufferAddNewTupleCids(ReorderBuffer *rb, TransactionId xid,
                             XLogRecPtr lsn, RelFileNode node,
                             ItemPointerData tid, CommandId cmin,
                             CommandId cmax, CommandId combocid)
{
    ReorderBufferChange *change = ReorderBufferGetChange(rb);
    ReorderBufferTXN *txn;

    txn = ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);

    change->data.tuplecid.node = node;
    change->data.tuplecid.tid = tid;
    change->data.tuplecid.cmin = cmin;
    change->data.tuplecid.cmax = cmax;
    change->data.tuplecid.combocid = combocid;
    change->lsn = lsn;
    change->txn = txn;
    change->action = REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID;

    dlist_push_tail(&txn->tuplecids, &change->node);
    txn->ntuplecids++;
}

/*
 * Setup the invalidation of the toplevel transaction.
 *
 * This needs to be done before ReorderBufferCommit is called!
 */
void
ReorderBufferAddInvalidations(ReorderBuffer *rb, TransactionId xid,
                              XLogRecPtr lsn, Size nmsgs,
                              SharedInvalidationMessage *msgs)
{
    ReorderBufferTXN *txn;

    txn = ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);

    if (txn->ninvalidations != 0)
        elog(ERROR, "only ever add one set of invalidations");

    Assert(nmsgs > 0);

    txn->ninvalidations = nmsgs;
    txn->invalidations = (SharedInvalidationMessage *)
        MemoryContextAlloc(rb->context,
                           sizeof(SharedInvalidationMessage) * nmsgs);
    memcpy(txn->invalidations, msgs,
           sizeof(SharedInvalidationMessage) * nmsgs);
}

/*
 * Apply all invalidations we know. Possibly we only need parts at this point
 * in the changestream but we don't know which those are.
 */
static void
ReorderBufferExecuteInvalidations(ReorderBuffer *rb, ReorderBufferTXN *txn)
{
    int         i;

    for (i = 0; i < txn->ninvalidations; i++)
        LocalExecuteInvalidationMessage(&txn->invalidations[i]);
}

/*
 * Mark a transaction as containing catalog changes
 */
void
ReorderBufferXidSetCatalogChanges(ReorderBuffer *rb, TransactionId xid,
                                  XLogRecPtr lsn)
{
    ReorderBufferTXN *txn;

    txn = ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);

    txn->txn_flags |= RBTXN_HAS_CATALOG_CHANGES;
}

/*
 * Query whether a transaction is already *known* to contain catalog
 * changes. This can be wrong until directly before the commit!
 */
bool
ReorderBufferXidHasCatalogChanges(ReorderBuffer *rb, TransactionId xid)
{
    ReorderBufferTXN *txn;

    txn = ReorderBufferTXNByXid(rb, xid, false, NULL, InvalidXLogRecPtr,
                                false);
    if (txn == NULL)
        return false;

    return rbtxn_has_catalog_changes(txn);
}

/*
 * ReorderBufferXidHasBaseSnapshot
 *      Have we already set the base snapshot for the given txn/subtxn?
 */
bool
ReorderBufferXidHasBaseSnapshot(ReorderBuffer *rb, TransactionId xid)
{
    ReorderBufferTXN *txn;

    txn = ReorderBufferTXNByXid(rb, xid, false,
                                NULL, InvalidXLogRecPtr, false);

    /* transaction isn't known yet, ergo no snapshot */
    if (txn == NULL)
        return false;

    /* a known subtxn? operate on top-level txn instead */
    if (rbtxn_is_known_subxact(txn))
        txn = ReorderBufferTXNByXid(rb, txn->toplevel_xid, false,
                                    NULL, InvalidXLogRecPtr, false);

    return txn->base_snapshot != NULL;
}


/*
 * ---------------------------------------
 * Disk serialization support
 * ---------------------------------------
 */

/*
 * Ensure the IO buffer is >= sz.
 */
static void
ReorderBufferSerializeReserve(ReorderBuffer *rb, Size sz)
{
    if (!rb->outbufsize)
    {
        rb->outbuf = MemoryContextAlloc(rb->context, sz);
        rb->outbufsize = sz;
    }
    else if (rb->outbufsize < sz)
    {
        rb->outbuf = repalloc(rb->outbuf, sz);
        rb->outbufsize = sz;
    }
}

/*
 * Find the largest transaction (toplevel or subxact) to evict (spill to disk).
 *
 * XXX With many subtransactions this might be quite slow, because we'll have
 * to walk through all of them. There are some options how we could improve
 * that: (a) maintain some secondary structure with transactions sorted by
 * amount of changes, (b) not looking for the entirely largest transaction,
 * but e.g. for transaction using at least some fraction of the memory limit,
 * and (c) evicting multiple transactions at once, e.g. to free a given portion
 * of the memory limit (e.g. 50%).
 */
static ReorderBufferTXN *
ReorderBufferLargestTXN(ReorderBuffer *rb)
{
    HASH_SEQ_STATUS hash_seq;
    ReorderBufferTXNByIdEnt *ent;
    ReorderBufferTXN *largest = NULL;

    hash_seq_init(&hash_seq, rb->by_txn);
    while ((ent = hash_seq_search(&hash_seq)) != NULL)
    {
        ReorderBufferTXN *txn = ent->txn;

        /* if the current transaction is larger, remember it */
        if ((!largest) || (txn->size > largest->size))
            largest = txn;
    }

    Assert(largest);
    Assert(largest->size > 0);
    Assert(largest->size <= rb->size);

    return largest;
}

/*
 * Check whether the logical_decoding_work_mem limit was reached, and if yes
 * pick the transaction to evict and spill the changes to disk.
 *
 * XXX At this point we select just a single (largest) transaction, but
 * we might also adapt a more elaborate eviction strategy - for example
 * evicting enough transactions to free certain fraction (e.g. 50%) of
 * the memory limit.
 */
static void
ReorderBufferCheckMemoryLimit(ReorderBuffer *rb)
{
    ReorderBufferTXN *txn;

    /* bail out if we haven't exceeded the memory limit */
    if (rb->size < logical_decoding_work_mem * 1024L)
        return;

    /*
     * Pick the largest transaction (or subtransaction) and evict it from
     * memory by serializing it to disk.
     */
    txn = ReorderBufferLargestTXN(rb);

    ReorderBufferSerializeTXN(rb, txn);

    /*
     * After eviction, the transaction should have no entries in memory, and
     * should use 0 bytes for changes.
     */
    Assert(txn->size == 0);
    Assert(txn->nentries_mem == 0);

    /*
     * And furthermore, evicting the transaction should get us below the
     * memory limit again - it is not possible that we're still exceeding the
     * memory limit after evicting the transaction.
     *
     * This follows from the simple fact that the selected transaction is at
     * least as large as the most recent change (which caused us to go over
     * the memory limit). So by evicting it we're definitely back below the
     * memory limit.
     */
    Assert(rb->size < logical_decoding_work_mem * 1024L);
}

/*
 * Spill data of a large transaction (and its subtransactions) to disk.
 */
static void
ReorderBufferSerializeTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
{
    dlist_iter  subtxn_i;
    dlist_mutable_iter change_i;
    int         fd = -1;
    XLogSegNo   curOpenSegNo = 0;
    Size        spilled = 0;
    Size        size = txn->size;

    elog(DEBUG2, "spill %u changes in XID %u to disk",
         (uint32) txn->nentries_mem, txn->xid);

    /* do the same to all child TXs */
    dlist_foreach(subtxn_i, &txn->subtxns)
    {
        ReorderBufferTXN *subtxn;

        subtxn = dlist_container(ReorderBufferTXN, node, subtxn_i.cur);
        ReorderBufferSerializeTXN(rb, subtxn);
    }

    /* serialize changestream */
    dlist_foreach_modify(change_i, &txn->changes)
    {
        ReorderBufferChange *change;

        change = dlist_container(ReorderBufferChange, node, change_i.cur);

        /*
         * store in segment in which it belongs by start lsn, don't split over
         * multiple segments tho
         */
        if (fd == -1 ||
            !XLByteInSeg(change->lsn, curOpenSegNo, wal_segment_size))
        {
            char        path[MAXPGPATH];

            if (fd != -1)
                CloseTransientFile(fd);

            XLByteToSeg(change->lsn, curOpenSegNo, wal_segment_size);

            /*
             * No need to care about TLIs here, only used during a single run,
             * so each LSN only maps to a specific WAL record.
             */
            ReorderBufferSerializedPath(path, MyReplicationSlot, txn->xid,
                                        curOpenSegNo);

            /* open segment, create it if necessary */
            fd = OpenTransientFile(path,
                                   O_CREAT | O_WRONLY | O_APPEND | PG_BINARY);

            if (fd < 0)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not open file \"%s\": %m", path)));
        }

        ReorderBufferSerializeChange(rb, txn, fd, change);
        dlist_delete(&change->node);
        ReorderBufferReturnChange(rb, change);

        spilled++;
    }

    /* update the statistics */
    rb->spillCount += 1;
    rb->spillBytes += size;

    /* Don't consider already serialized transactions. */
    rb->spillTxns += rbtxn_is_serialized(txn) ? 0 : 1;

    Assert(spilled == txn->nentries_mem);
    Assert(dlist_is_empty(&txn->changes));
    txn->nentries_mem = 0;
    txn->txn_flags |= RBTXN_IS_SERIALIZED;

    if (fd != -1)
        CloseTransientFile(fd);
}

/*
 * Serialize individual change to disk.
 */
static void
ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
                             int fd, ReorderBufferChange *change)
{
    ReorderBufferDiskChange *ondisk;
    Size        sz = sizeof(ReorderBufferDiskChange);

    ReorderBufferSerializeReserve(rb, sz);

    ondisk = (ReorderBufferDiskChange *) rb->outbuf;
    memcpy(&ondisk->change, change, sizeof(ReorderBufferChange));

    switch (change->action)
    {
            /* fall through these, they're all similar enough */
        case REORDER_BUFFER_CHANGE_INSERT:
        case REORDER_BUFFER_CHANGE_UPDATE:
        case REORDER_BUFFER_CHANGE_DELETE:
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_INSERT:
            {
                char       *data;
                ReorderBufferTupleBuf *oldtup,
                           *newtup;
                Size        oldlen = 0;
                Size        newlen = 0;

                oldtup = change->data.tp.oldtuple;
                newtup = change->data.tp.newtuple;

                if (oldtup)
                {
                    sz += sizeof(HeapTupleData);
                    oldlen = oldtup->tuple.t_len;
                    sz += oldlen;
                }

                if (newtup)
                {
                    sz += sizeof(HeapTupleData);
                    newlen = newtup->tuple.t_len;
                    sz += newlen;
                }

                /* make sure we have enough space */
                ReorderBufferSerializeReserve(rb, sz);

                data = ((char *) rb->outbuf) + sizeof(ReorderBufferDiskChange);
                /* might have been reallocated above */
                ondisk = (ReorderBufferDiskChange *) rb->outbuf;

                if (oldlen)
                {
                    memcpy(data, &oldtup->tuple, sizeof(HeapTupleData));
                    data += sizeof(HeapTupleData);

                    memcpy(data, oldtup->tuple.t_data, oldlen);
                    data += oldlen;
                }

                if (newlen)
                {
                    memcpy(data, &newtup->tuple, sizeof(HeapTupleData));
                    data += sizeof(HeapTupleData);

                    memcpy(data, newtup->tuple.t_data, newlen);
                    data += newlen;
                }
                break;
            }
        case REORDER_BUFFER_CHANGE_MESSAGE:
            {
                char       *data;
                Size        prefix_size = strlen(change->data.msg.prefix) + 1;

                sz += prefix_size + change->data.msg.message_size +
                    sizeof(Size) + sizeof(Size);
                ReorderBufferSerializeReserve(rb, sz);

                data = ((char *) rb->outbuf) + sizeof(ReorderBufferDiskChange);

                /* might have been reallocated above */
                ondisk = (ReorderBufferDiskChange *) rb->outbuf;

                /* write the prefix including the size */
                memcpy(data, &prefix_size, sizeof(Size));
                data += sizeof(Size);
                memcpy(data, change->data.msg.prefix,
                       prefix_size);
                data += prefix_size;

                /* write the message including the size */
                memcpy(data, &change->data.msg.message_size, sizeof(Size));
                data += sizeof(Size);
                memcpy(data, change->data.msg.message,
                       change->data.msg.message_size);
                data += change->data.msg.message_size;

                break;
            }
        case REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT:
            {
                Snapshot    snap;
                char       *data;

                snap = change->data.snapshot;

                sz += sizeof(SnapshotData) +
                    sizeof(TransactionId) * snap->xcnt +
                    sizeof(TransactionId) * snap->subxcnt
                    ;

                /* make sure we have enough space */
                ReorderBufferSerializeReserve(rb, sz);
                data = ((char *) rb->outbuf) + sizeof(ReorderBufferDiskChange);
                /* might have been reallocated above */
                ondisk = (ReorderBufferDiskChange *) rb->outbuf;

                memcpy(data, snap, sizeof(SnapshotData));
                data += sizeof(SnapshotData);

                if (snap->xcnt)
                {
                    memcpy(data, snap->xip,
                           sizeof(TransactionId) * snap->xcnt);
                    data += sizeof(TransactionId) * snap->xcnt;
                }

                if (snap->subxcnt)
                {
                    memcpy(data, snap->subxip,
                           sizeof(TransactionId) * snap->subxcnt);
                    data += sizeof(TransactionId) * snap->subxcnt;
                }
                break;
            }
        case REORDER_BUFFER_CHANGE_TRUNCATE:
            {
                Size        size;
                char       *data;

                /* account for the OIDs of truncated relations */
                size = sizeof(Oid) * change->data.truncate.nrelids;
                sz += size;

                /* make sure we have enough space */
                ReorderBufferSerializeReserve(rb, sz);

                data = ((char *) rb->outbuf) + sizeof(ReorderBufferDiskChange);
                /* might have been reallocated above */
                ondisk = (ReorderBufferDiskChange *) rb->outbuf;

                memcpy(data, change->data.truncate.relids, size);
                data += size;

                break;
            }
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
        case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
        case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
            /* ReorderBufferChange contains everything important */
            break;
    }

    ondisk->size = sz;

    errno = 0;
    pgstat_report_wait_start(WAIT_EVENT_REORDER_BUFFER_WRITE);
    if (write(fd, rb->outbuf, ondisk->size) != ondisk->size)
    {
        int         save_errno = errno;

        CloseTransientFile(fd);

        /* if write didn't set errno, assume problem is no disk space */
        errno = save_errno ? save_errno : ENOSPC;
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not write to data file for XID %u: %m",
                        txn->xid)));
    }
    pgstat_report_wait_end();

    Assert(ondisk->change.action == change->action);
}

/*
 * Size of a change in memory.
 */
static Size
ReorderBufferChangeSize(ReorderBufferChange *change)
{
    Size        sz = sizeof(ReorderBufferChange);

    switch (change->action)
    {
            /* fall through these, they're all similar enough */
        case REORDER_BUFFER_CHANGE_INSERT:
        case REORDER_BUFFER_CHANGE_UPDATE:
        case REORDER_BUFFER_CHANGE_DELETE:
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_INSERT:
            {
                ReorderBufferTupleBuf *oldtup,
                           *newtup;
                Size        oldlen = 0;
                Size        newlen = 0;

                oldtup = change->data.tp.oldtuple;
                newtup = change->data.tp.newtuple;

                if (oldtup)
                {
                    sz += sizeof(HeapTupleData);
                    oldlen = oldtup->tuple.t_len;
                    sz += oldlen;
                }

                if (newtup)
                {
                    sz += sizeof(HeapTupleData);
                    newlen = newtup->tuple.t_len;
                    sz += newlen;
                }

                break;
            }
        case REORDER_BUFFER_CHANGE_MESSAGE:
            {
                Size        prefix_size = strlen(change->data.msg.prefix) + 1;

                sz += prefix_size + change->data.msg.message_size +
                    sizeof(Size) + sizeof(Size);

                break;
            }
        case REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT:
            {
                Snapshot    snap;

                snap = change->data.snapshot;

                sz += sizeof(SnapshotData) +
                    sizeof(TransactionId) * snap->xcnt +
                    sizeof(TransactionId) * snap->subxcnt;

                break;
            }
        case REORDER_BUFFER_CHANGE_TRUNCATE:
            {
                sz += sizeof(Oid) * change->data.truncate.nrelids;

                break;
            }
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
        case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
        case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
            /* ReorderBufferChange contains everything important */
            break;
    }

    return sz;
}


/*
 * Restore a number of changes spilled to disk back into memory.
 */
static Size
ReorderBufferRestoreChanges(ReorderBuffer *rb, ReorderBufferTXN *txn,
                            TXNEntryFile *file, XLogSegNo *segno)
{
    Size        restored = 0;
    XLogSegNo   last_segno;
    dlist_mutable_iter cleanup_iter;
    File       *fd = &file->vfd;

    Assert(txn->first_lsn != InvalidXLogRecPtr);
    Assert(txn->final_lsn != InvalidXLogRecPtr);

    /* free current entries, so we have memory for more */
    dlist_foreach_modify(cleanup_iter, &txn->changes)
    {
        ReorderBufferChange *cleanup =
        dlist_container(ReorderBufferChange, node, cleanup_iter.cur);

        dlist_delete(&cleanup->node);
        ReorderBufferReturnChange(rb, cleanup);
    }
    txn->nentries_mem = 0;
    Assert(dlist_is_empty(&txn->changes));

    XLByteToSeg(txn->final_lsn, last_segno, wal_segment_size);

    while (restored < max_changes_in_memory && *segno <= last_segno)
    {
        int         readBytes;
        ReorderBufferDiskChange *ondisk;

        if (*fd == -1)
        {
            char        path[MAXPGPATH];

            /* first time in */
            if (*segno == 0)
                XLByteToSeg(txn->first_lsn, *segno, wal_segment_size);

            Assert(*segno != 0 || dlist_is_empty(&txn->changes));

            /*
             * No need to care about TLIs here, only used during a single run,
             * so each LSN only maps to a specific WAL record.
             */
            ReorderBufferSerializedPath(path, MyReplicationSlot, txn->xid,
                                        *segno);

            *fd = PathNameOpenFile(path, O_RDONLY | PG_BINARY);

            /* No harm in resetting the offset even in case of failure */
            file->curOffset = 0;

            if (*fd < 0 && errno == ENOENT)
            {
                *fd = -1;
                (*segno)++;
                continue;
            }
            else if (*fd < 0)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not open file \"%s\": %m",
                                path)));
        }

        /*
         * Read the statically sized part of a change which has information
         * about the total size. If we couldn't read a record, we're at the
         * end of this file.
         */
        ReorderBufferSerializeReserve(rb, sizeof(ReorderBufferDiskChange));
        readBytes = FileRead(file->vfd, rb->outbuf,
                             sizeof(ReorderBufferDiskChange),
                             file->curOffset, WAIT_EVENT_REORDER_BUFFER_READ);

        /* eof */
        if (readBytes == 0)
        {
            FileClose(*fd);
            *fd = -1;
            (*segno)++;
            continue;
        }
        else if (readBytes < 0)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read from reorderbuffer spill file: %m")));
        else if (readBytes != sizeof(ReorderBufferDiskChange))
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read from reorderbuffer spill file: read %d instead of %u bytes",
                            readBytes,
                            (uint32) sizeof(ReorderBufferDiskChange))));

        file->curOffset += readBytes;

        ondisk = (ReorderBufferDiskChange *) rb->outbuf;

        ReorderBufferSerializeReserve(rb,
                                      sizeof(ReorderBufferDiskChange) + ondisk->size);
        ondisk = (ReorderBufferDiskChange *) rb->outbuf;

        readBytes = FileRead(file->vfd,
                             rb->outbuf + sizeof(ReorderBufferDiskChange),
                             ondisk->size - sizeof(ReorderBufferDiskChange),
                             file->curOffset,
                             WAIT_EVENT_REORDER_BUFFER_READ);

        if (readBytes < 0)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read from reorderbuffer spill file: %m")));
        else if (readBytes != ondisk->size - sizeof(ReorderBufferDiskChange))
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read from reorderbuffer spill file: read %d instead of %u bytes",
                            readBytes,
                            (uint32) (ondisk->size - sizeof(ReorderBufferDiskChange)))));

        file->curOffset += readBytes;

        /*
         * ok, read a full change from disk, now restore it into proper
         * in-memory format
         */
        ReorderBufferRestoreChange(rb, txn, rb->outbuf);
        restored++;
    }

    return restored;
}

/*
 * Convert change from its on-disk format to in-memory format and queue it onto
 * the TXN's ->changes list.
 *
 * Note: although "data" is declared char*, at entry it points to a
 * maxalign'd buffer, making it safe in most of this function to assume
 * that the pointed-to data is suitably aligned for direct access.
 */
static void
ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
                           char *data)
{
    ReorderBufferDiskChange *ondisk;
    ReorderBufferChange *change;

    ondisk = (ReorderBufferDiskChange *) data;

    change = ReorderBufferGetChange(rb);

    /* copy static part */
    memcpy(change, &ondisk->change, sizeof(ReorderBufferChange));

    data += sizeof(ReorderBufferDiskChange);

    /* restore individual stuff */
    switch (change->action)
    {
            /* fall through these, they're all similar enough */
        case REORDER_BUFFER_CHANGE_INSERT:
        case REORDER_BUFFER_CHANGE_UPDATE:
        case REORDER_BUFFER_CHANGE_DELETE:
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_INSERT:
            if (change->data.tp.oldtuple)
            {
                uint32      tuplelen = ((HeapTuple) data)->t_len;

                change->data.tp.oldtuple =
                    ReorderBufferGetTupleBuf(rb, tuplelen - SizeofHeapTupleHeader);

                /* restore ->tuple */
                memcpy(&change->data.tp.oldtuple->tuple, data,
                       sizeof(HeapTupleData));
                data += sizeof(HeapTupleData);

                /* reset t_data pointer into the new tuplebuf */
                change->data.tp.oldtuple->tuple.t_data =
                    ReorderBufferTupleBufData(change->data.tp.oldtuple);

                /* restore tuple data itself */
                memcpy(change->data.tp.oldtuple->tuple.t_data, data, tuplelen);
                data += tuplelen;
            }

            if (change->data.tp.newtuple)
            {
                /* here, data might not be suitably aligned! */
                uint32      tuplelen;

                memcpy(&tuplelen, data + offsetof(HeapTupleData, t_len),
                       sizeof(uint32));

                change->data.tp.newtuple =
                    ReorderBufferGetTupleBuf(rb, tuplelen - SizeofHeapTupleHeader);

                /* restore ->tuple */
                memcpy(&change->data.tp.newtuple->tuple, data,
                       sizeof(HeapTupleData));
                data += sizeof(HeapTupleData);

                /* reset t_data pointer into the new tuplebuf */
                change->data.tp.newtuple->tuple.t_data =
                    ReorderBufferTupleBufData(change->data.tp.newtuple);

                /* restore tuple data itself */
                memcpy(change->data.tp.newtuple->tuple.t_data, data, tuplelen);
                data += tuplelen;
            }

            break;
        case REORDER_BUFFER_CHANGE_MESSAGE:
            {
                Size        prefix_size;

                /* read prefix */
                memcpy(&prefix_size, data, sizeof(Size));
                data += sizeof(Size);
                change->data.msg.prefix = MemoryContextAlloc(rb->context,
                                                             prefix_size);
                memcpy(change->data.msg.prefix, data, prefix_size);
                Assert(change->data.msg.prefix[prefix_size - 1] == '\0');
                data += prefix_size;

                /* read the message */
                memcpy(&change->data.msg.message_size, data, sizeof(Size));
                data += sizeof(Size);
                change->data.msg.message = MemoryContextAlloc(rb->context,
                                                              change->data.msg.message_size);
                memcpy(change->data.msg.message, data,
                       change->data.msg.message_size);
                data += change->data.msg.message_size;

                break;
            }
        case REORDER_BUFFER_CHANGE_INTERNAL_SNAPSHOT:
            {
                Snapshot    oldsnap;
                Snapshot    newsnap;
                Size        size;

                oldsnap = (Snapshot) data;

                size = sizeof(SnapshotData) +
                    sizeof(TransactionId) * oldsnap->xcnt +
                    sizeof(TransactionId) * (oldsnap->subxcnt + 0);

                change->data.snapshot = MemoryContextAllocZero(rb->context, size);

                newsnap = change->data.snapshot;

                memcpy(newsnap, data, size);
                newsnap->xip = (TransactionId *)
                    (((char *) newsnap) + sizeof(SnapshotData));
                newsnap->subxip = newsnap->xip + newsnap->xcnt;
                newsnap->copied = true;
                break;
            }
            /* the base struct contains all the data, easy peasy */
        case REORDER_BUFFER_CHANGE_TRUNCATE:
            {
                Oid        *relids;

                relids = ReorderBufferGetRelids(rb,
                                                change->data.truncate.nrelids);
                memcpy(relids, data, change->data.truncate.nrelids * sizeof(Oid));
                change->data.truncate.relids = relids;

                break;
            }
        case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
        case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
        case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
            break;
    }

    dlist_push_tail(&txn->changes, &change->node);
    txn->nentries_mem++;

    /*
     * Update memory accounting for the restored change.  We need to do this
     * although we don't check the memory limit when restoring the changes in
     * this branch (we only do that when initially queueing the changes after
     * decoding), because we will release the changes later, and that will
     * update the accounting too (subtracting the size from the counters). And
     * we don't want to underflow there.
     */
    ReorderBufferChangeMemoryUpdate(rb, change, true);
}

/*
 * Remove all on-disk stored for the passed in transaction.
 */
static void
ReorderBufferRestoreCleanup(ReorderBuffer *rb, ReorderBufferTXN *txn)
{
    XLogSegNo   first;
    XLogSegNo   cur;
    XLogSegNo   last;

    Assert(txn->first_lsn != InvalidXLogRecPtr);
    Assert(txn->final_lsn != InvalidXLogRecPtr);

    XLByteToSeg(txn->first_lsn, first, wal_segment_size);
    XLByteToSeg(txn->final_lsn, last, wal_segment_size);

    /* iterate over all possible filenames, and delete them */
    for (cur = first; cur <= last; cur++)
    {
        char        path[MAXPGPATH];

        ReorderBufferSerializedPath(path, MyReplicationSlot, txn->xid, cur);
        if (unlink(path) != 0 && errno != ENOENT)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not remove file \"%s\": %m", path)));
    }
}

/*
 * Remove any leftover serialized reorder buffers from a slot directory after a
 * prior crash or decoding session exit.
 */
static void
ReorderBufferCleanupSerializedTXNs(const char *slotname)
{
    DIR        *spill_dir;
    struct dirent *spill_de;
    struct stat statbuf;
    char        path[MAXPGPATH * 2 + 12];

    sprintf(path, "pg_replslot/%s", slotname);

    /* we're only handling directories here, skip if it's not ours */
    if (lstat(path, &statbuf) == 0 && !S_ISDIR(statbuf.st_mode))
        return;

    spill_dir = AllocateDir(path);
    while ((spill_de = ReadDirExtended(spill_dir, path, INFO)) != NULL)
    {
        /* only look at names that can be ours */
        if (strncmp(spill_de->d_name, "xid", 3) == 0)
        {
            snprintf(path, sizeof(path),
                     "pg_replslot/%s/%s", slotname,
                     spill_de->d_name);

            if (unlink(path) != 0)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not remove file \"%s\" during removal of pg_replslot/%s/xid*: %m",
                                path, slotname)));
        }
    }
    FreeDir(spill_dir);
}

/*
 * Given a replication slot, transaction ID and segment number, fill in the
 * corresponding spill file into 'path', which is a caller-owned buffer of size
 * at least MAXPGPATH.
 */
static void
ReorderBufferSerializedPath(char *path, ReplicationSlot *slot, TransactionId xid,
                            XLogSegNo segno)
{
    XLogRecPtr  recptr;

    XLogSegNoOffsetToRecPtr(segno, 0, wal_segment_size, recptr);

    snprintf(path, MAXPGPATH, "pg_replslot/%s/xid-%u-lsn-%X-%X.spill",
             NameStr(MyReplicationSlot->data.name),
             xid,
             (uint32) (recptr >> 32), (uint32) recptr);
}

/*
 * Delete all data spilled to disk after we've restarted/crashed. It will be
 * recreated when the respective slots are reused.
 */
void
StartupReorderBuffer(void)
{
    DIR        *logical_dir;
    struct dirent *logical_de;

    logical_dir = AllocateDir("pg_replslot");
    while ((logical_de = ReadDir(logical_dir, "pg_replslot")) != NULL)
    {
        if (strcmp(logical_de->d_name, ".") == 0 ||
            strcmp(logical_de->d_name, "..") == 0)
            continue;

        /* if it cannot be a slot, skip the directory */
        if (!ReplicationSlotValidateName(logical_de->d_name, DEBUG2))
            continue;

        /*
         * ok, has to be a surviving logical slot, iterate and delete
         * everything starting with xid-*
         */
        ReorderBufferCleanupSerializedTXNs(logical_de->d_name);
    }
    FreeDir(logical_dir);
}

/* ---------------------------------------
 * toast reassembly support
 * ---------------------------------------
 */

/*
 * Initialize per tuple toast reconstruction support.
 */
static void
ReorderBufferToastInitHash(ReorderBuffer *rb, ReorderBufferTXN *txn)
{
    HASHCTL     hash_ctl;

    Assert(txn->toast_hash == NULL);

    memset(&hash_ctl, 0, sizeof(hash_ctl));
    hash_ctl.keysize = sizeof(Oid);
    hash_ctl.entrysize = sizeof(ReorderBufferToastEnt);
    hash_ctl.hcxt = rb->context;
    txn->toast_hash = hash_create("ReorderBufferToastHash", 5, &hash_ctl,
                                  HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
}

/*
 * Per toast-chunk handling for toast reconstruction
 *
 * Appends a toast chunk so we can reconstruct it when the tuple "owning" the
 * toasted Datum comes along.
 */
static void
ReorderBufferToastAppendChunk(ReorderBuffer *rb, ReorderBufferTXN *txn,
                              Relation relation, ReorderBufferChange *change)
{
    ReorderBufferToastEnt *ent;
    ReorderBufferTupleBuf *newtup;
    bool        found;
    int32       chunksize;
    bool        isnull;
    Pointer     chunk;
    TupleDesc   desc = RelationGetDescr(relation);
    Oid         chunk_id;
    int32       chunk_seq;

    if (txn->toast_hash == NULL)
        ReorderBufferToastInitHash(rb, txn);

    Assert(IsToastRelation(relation));

    newtup = change->data.tp.newtuple;
    chunk_id = DatumGetObjectId(fastgetattr(&newtup->tuple, 1, desc, &isnull));
    Assert(!isnull);
    chunk_seq = DatumGetInt32(fastgetattr(&newtup->tuple, 2, desc, &isnull));
    Assert(!isnull);

    ent = (ReorderBufferToastEnt *)
        hash_search(txn->toast_hash,
                    (void *) &chunk_id,
                    HASH_ENTER,
                    &found);

    if (!found)
    {
        Assert(ent->chunk_id == chunk_id);
        ent->num_chunks = 0;
        ent->last_chunk_seq = 0;
        ent->size = 0;
        ent->reconstructed = NULL;
        dlist_init(&ent->chunks);

        if (chunk_seq != 0)
            elog(ERROR, "got sequence entry %d for toast chunk %u instead of seq 0",
                 chunk_seq, chunk_id);
    }
    else if (found && chunk_seq != ent->last_chunk_seq + 1)
        elog(ERROR, "got sequence entry %d for toast chunk %u instead of seq %d",
             chunk_seq, chunk_id, ent->last_chunk_seq + 1);

    chunk = DatumGetPointer(fastgetattr(&newtup->tuple, 3, desc, &isnull));
    Assert(!isnull);

    /* calculate size so we can allocate the right size at once later */
    if (!VARATT_IS_EXTENDED(chunk))
        chunksize = VARSIZE(chunk) - VARHDRSZ;
    else if (VARATT_IS_SHORT(chunk))
        /* could happen due to heap_form_tuple doing its thing */
        chunksize = VARSIZE_SHORT(chunk) - VARHDRSZ_SHORT;
    else
        elog(ERROR, "unexpected type of toast chunk");

    ent->size += chunksize;
    ent->last_chunk_seq = chunk_seq;
    ent->num_chunks++;
    dlist_push_tail(&ent->chunks, &change->node);
}

/*
 * Rejigger change->newtuple to point to in-memory toast tuples instead to
 * on-disk toast tuples that may not longer exist (think DROP TABLE or VACUUM).
 *
 * We cannot replace unchanged toast tuples though, so those will still point
 * to on-disk toast data.
 *
 * While updating the existing change with detoasted tuple data, we need to
 * update the memory accounting info, because the change size will differ.
 * Otherwise the accounting may get out of sync, triggering serialization
 * at unexpected times.
 *
 * We simply subtract size of the change before rejiggering the tuple, and
 * then adding the new size. This makes it look like the change was removed
 * and then added back, except it only tweaks the accounting info.
 *
 * In particular it can't trigger serialization, which would be pointless
 * anyway as it happens during commit processing right before handing
 * the change to the output plugin.
 */
static void
ReorderBufferToastReplace(ReorderBuffer *rb, ReorderBufferTXN *txn,
                          Relation relation, ReorderBufferChange *change)
{
    TupleDesc   desc;
    int         natt;
    Datum      *attrs;
    bool       *isnull;
    bool       *free;
    HeapTuple   tmphtup;
    Relation    toast_rel;
    TupleDesc   toast_desc;
    MemoryContext oldcontext;
    ReorderBufferTupleBuf *newtup;

    /* no toast tuples changed */
    if (txn->toast_hash == NULL)
        return;

    /*
     * We're going to modify the size of the change, so to make sure the
     * accounting is correct we'll make it look like we're removing the change
     * now (with the old size), and then re-add it at the end.
     */
    ReorderBufferChangeMemoryUpdate(rb, change, false);

    oldcontext = MemoryContextSwitchTo(rb->context);

    /* we should only have toast tuples in an INSERT or UPDATE */
    Assert(change->data.tp.newtuple);

    desc = RelationGetDescr(relation);

    toast_rel = RelationIdGetRelation(relation->rd_rel->reltoastrelid);
    if (!RelationIsValid(toast_rel))
        elog(ERROR, "could not open relation with OID %u",
             relation->rd_rel->reltoastrelid);

    toast_desc = RelationGetDescr(toast_rel);

    /* should we allocate from stack instead? */
    attrs = palloc0(sizeof(Datum) * desc->natts);
    isnull = palloc0(sizeof(bool) * desc->natts);
    free = palloc0(sizeof(bool) * desc->natts);

    newtup = change->data.tp.newtuple;

    heap_deform_tuple(&newtup->tuple, desc, attrs, isnull);

    for (natt = 0; natt < desc->natts; natt++)
    {
        Form_pg_attribute attr = TupleDescAttr(desc, natt);
        ReorderBufferToastEnt *ent;
        struct varlena *varlena;

        /* va_rawsize is the size of the original datum -- including header */
        struct varatt_external toast_pointer;
        struct varatt_indirect redirect_pointer;
        struct varlena *new_datum = NULL;
        struct varlena *reconstructed;
        dlist_iter  it;
        Size        data_done = 0;

        /* system columns aren't toasted */
        if (attr->attnum < 0)
            continue;

        if (attr->attisdropped)
            continue;

        /* not a varlena datatype */
        if (attr->attlen != -1)
            continue;

        /* no data */
        if (isnull[natt])
            continue;

        /* ok, we know we have a toast datum */
        varlena = (struct varlena *) DatumGetPointer(attrs[natt]);

        /* no need to do anything if the tuple isn't external */
        if (!VARATT_IS_EXTERNAL(varlena))
            continue;

        VARATT_EXTERNAL_GET_POINTER(toast_pointer, varlena);

        /*
         * Check whether the toast tuple changed, replace if so.
         */
        ent = (ReorderBufferToastEnt *)
            hash_search(txn->toast_hash,
                        (void *) &toast_pointer.va_valueid,
                        HASH_FIND,
                        NULL);
        if (ent == NULL)
            continue;

        new_datum =
            (struct varlena *) palloc0(INDIRECT_POINTER_SIZE);

        free[natt] = true;

        reconstructed = palloc0(toast_pointer.va_rawsize);

        ent->reconstructed = reconstructed;

        /* stitch toast tuple back together from its parts */
        dlist_foreach(it, &ent->chunks)
        {
            bool        isnull;
            ReorderBufferChange *cchange;
            ReorderBufferTupleBuf *ctup;
            Pointer     chunk;

            cchange = dlist_container(ReorderBufferChange, node, it.cur);
            ctup = cchange->data.tp.newtuple;
            chunk = DatumGetPointer(
                                    fastgetattr(&ctup->tuple, 3, toast_desc, &isnull));

            Assert(!isnull);
            Assert(!VARATT_IS_EXTERNAL(chunk));
            Assert(!VARATT_IS_SHORT(chunk));

            memcpy(VARDATA(reconstructed) + data_done,
                   VARDATA(chunk),
                   VARSIZE(chunk) - VARHDRSZ);
            data_done += VARSIZE(chunk) - VARHDRSZ;
        }
        Assert(data_done == toast_pointer.va_extsize);

        /* make sure its marked as compressed or not */
        if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer))
            SET_VARSIZE_COMPRESSED(reconstructed, data_done + VARHDRSZ);
        else
            SET_VARSIZE(reconstructed, data_done + VARHDRSZ);

        memset(&redirect_pointer, 0, sizeof(redirect_pointer));
        redirect_pointer.pointer = reconstructed;

        SET_VARTAG_EXTERNAL(new_datum, VARTAG_INDIRECT);
        memcpy(VARDATA_EXTERNAL(new_datum), &redirect_pointer,
               sizeof(redirect_pointer));

        attrs[natt] = PointerGetDatum(new_datum);
    }

    /*
     * Build tuple in separate memory & copy tuple back into the tuplebuf
     * passed to the output plugin. We can't directly heap_fill_tuple() into
     * the tuplebuf because attrs[] will point back into the current content.
     */
    tmphtup = heap_form_tuple(desc, attrs, isnull);
    Assert(newtup->tuple.t_len <= MaxHeapTupleSize);
    Assert(ReorderBufferTupleBufData(newtup) == newtup->tuple.t_data);

    memcpy(newtup->tuple.t_data, tmphtup->t_data, tmphtup->t_len);
    newtup->tuple.t_len = tmphtup->t_len;

    /*
     * free resources we won't further need, more persistent stuff will be
     * free'd in ReorderBufferToastReset().
     */
    RelationClose(toast_rel);
    pfree(tmphtup);
    for (natt = 0; natt < desc->natts; natt++)
    {
        if (free[natt])
            pfree(DatumGetPointer(attrs[natt]));
    }
    pfree(attrs);
    pfree(free);
    pfree(isnull);

    MemoryContextSwitchTo(oldcontext);

    /* now add the change back, with the correct size */
    ReorderBufferChangeMemoryUpdate(rb, change, true);
}

/*
 * Free all resources allocated for toast reconstruction.
 */
static void
ReorderBufferToastReset(ReorderBuffer *rb, ReorderBufferTXN *txn)
{
    HASH_SEQ_STATUS hstat;
    ReorderBufferToastEnt *ent;

    if (txn->toast_hash == NULL)
        return;

    /* sequentially walk over the hash and free everything */
    hash_seq_init(&hstat, txn->toast_hash);
    while ((ent = (ReorderBufferToastEnt *) hash_seq_search(&hstat)) != NULL)
    {
        dlist_mutable_iter it;

        if (ent->reconstructed != NULL)
            pfree(ent->reconstructed);

        dlist_foreach_modify(it, &ent->chunks)
        {
            ReorderBufferChange *change =
            dlist_container(ReorderBufferChange, node, it.cur);

            dlist_delete(&change->node);
            ReorderBufferReturnChange(rb, change);
        }
    }

    hash_destroy(txn->toast_hash);
    txn->toast_hash = NULL;
}


/* ---------------------------------------
 * Visibility support for logical decoding
 *
 *
 * Lookup actual cmin/cmax values when using decoding snapshot. We can't
 * always rely on stored cmin/cmax values because of two scenarios:
 *
 * * A tuple got changed multiple times during a single transaction and thus
 *   has got a combocid. Combocid's are only valid for the duration of a
 *   single transaction.
 * * A tuple with a cmin but no cmax (and thus no combocid) got
 *   deleted/updated in another transaction than the one which created it
 *   which we are looking at right now. As only one of cmin, cmax or combocid
 *   is actually stored in the heap we don't have access to the value we
 *   need anymore.
 *
 * To resolve those problems we have a per-transaction hash of (cmin,
 * cmax) tuples keyed by (relfilenode, ctid) which contains the actual
 * (cmin, cmax) values. That also takes care of combocids by simply
 * not caring about them at all. As we have the real cmin/cmax values
 * combocids aren't interesting.
 *
 * As we only care about catalog tuples here the overhead of this
 * hashtable should be acceptable.
 *
 * Heap rewrites complicate this a bit, check rewriteheap.c for
 * details.
 * -------------------------------------------------------------------------
 */

/* struct for sorting mapping files by LSN efficiently */
typedef struct RewriteMappingFile
{
    XLogRecPtr  lsn;
    char        fname[MAXPGPATH];
} RewriteMappingFile;

#ifdef NOT_USED
static void
DisplayMapping(HTAB *tuplecid_data)
{
    HASH_SEQ_STATUS hstat;
    ReorderBufferTupleCidEnt *ent;

    hash_seq_init(&hstat, tuplecid_data);
    while ((ent = (ReorderBufferTupleCidEnt *) hash_seq_search(&hstat)) != NULL)
    {
        elog(DEBUG3, "mapping: node: %u/%u/%u tid: %u/%u cmin: %u, cmax: %u",
             ent->key.relnode.dbNode,
             ent->key.relnode.spcNode,
             ent->key.relnode.relNode,
             ItemPointerGetBlockNumber(&ent->key.tid),
             ItemPointerGetOffsetNumber(&ent->key.tid),
             ent->cmin,
             ent->cmax
            );
    }
}
#endif

/*
 * Apply a single mapping file to tuplecid_data.
 *
 * The mapping file has to have been verified to be a) committed b) for our
 * transaction c) applied in LSN order.
 */
static void
ApplyLogicalMappingFile(HTAB *tuplecid_data, Oid relid, const char *fname)
{
    char        path[MAXPGPATH];
    int         fd;
    int         readBytes;
    LogicalRewriteMappingData map;

    sprintf(path, "pg_logical/mappings/%s", fname);
    fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
    if (fd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m", path)));

    while (true)
    {
        ReorderBufferTupleCidKey key;
        ReorderBufferTupleCidEnt *ent;
        ReorderBufferTupleCidEnt *new_ent;
        bool        found;

        /* be careful about padding */
        memset(&key, 0, sizeof(ReorderBufferTupleCidKey));

        /* read all mappings till the end of the file */
        pgstat_report_wait_start(WAIT_EVENT_REORDER_LOGICAL_MAPPING_READ);
        readBytes = read(fd, &map, sizeof(LogicalRewriteMappingData));
        pgstat_report_wait_end();

        if (readBytes < 0)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read file \"%s\": %m",
                            path)));
        else if (readBytes == 0)    /* EOF */
            break;
        else if (readBytes != sizeof(LogicalRewriteMappingData))
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read from file \"%s\": read %d instead of %d bytes",
                            path, readBytes,
                            (int32) sizeof(LogicalRewriteMappingData))));

        key.relnode = map.old_node;
        ItemPointerCopy(&map.old_tid,
                        &key.tid);


        ent = (ReorderBufferTupleCidEnt *)
            hash_search(tuplecid_data,
                        (void *) &key,
                        HASH_FIND,
                        NULL);

        /* no existing mapping, no need to update */
        if (!ent)
            continue;

        key.relnode = map.new_node;
        ItemPointerCopy(&map.new_tid,
                        &key.tid);

        new_ent = (ReorderBufferTupleCidEnt *)
            hash_search(tuplecid_data,
                        (void *) &key,
                        HASH_ENTER,
                        &found);

        if (found)
        {
            /*
             * Make sure the existing mapping makes sense. We sometime update
             * old records that did not yet have a cmax (e.g. pg_class' own
             * entry while rewriting it) during rewrites, so allow that.
             */
            Assert(ent->cmin == InvalidCommandId || ent->cmin == new_ent->cmin);
            Assert(ent->cmax == InvalidCommandId || ent->cmax == new_ent->cmax);
        }
        else
        {
            /* update mapping */
            new_ent->cmin = ent->cmin;
            new_ent->cmax = ent->cmax;
            new_ent->combocid = ent->combocid;
        }
    }

    if (CloseTransientFile(fd) != 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not close file \"%s\": %m", path)));
}


/*
 * Check whether the TransactionId 'xid' is in the pre-sorted array 'xip'.
 */
static bool
TransactionIdInArray(TransactionId xid, TransactionId *xip, Size num)
{
    return bsearch(&xid, xip, num,
                   sizeof(TransactionId), xidComparator) != NULL;
}

/*
 * list_sort() comparator for sorting RewriteMappingFiles in LSN order.
 */
static int
file_sort_by_lsn(const ListCell *a_p, const ListCell *b_p)
{
    RewriteMappingFile *a = (RewriteMappingFile *) lfirst(a_p);
    RewriteMappingFile *b = (RewriteMappingFile *) lfirst(b_p);

    if (a->lsn < b->lsn)
        return -1;
    else if (a->lsn > b->lsn)
        return 1;
    return 0;
}

/*
 * Apply any existing logical remapping files if there are any targeted at our
 * transaction for relid.
 */
static void
UpdateLogicalMappings(HTAB *tuplecid_data, Oid relid, Snapshot snapshot)
{
    DIR        *mapping_dir;
    struct dirent *mapping_de;
    List       *files = NIL;
    ListCell   *file;
    Oid         dboid = IsSharedRelation(relid) ? InvalidOid : MyDatabaseId;

    mapping_dir = AllocateDir("pg_logical/mappings");
    while ((mapping_de = ReadDir(mapping_dir, "pg_logical/mappings")) != NULL)
    {
        Oid         f_dboid;
        Oid         f_relid;
        TransactionId f_mapped_xid;
        TransactionId f_create_xid;
        XLogRecPtr  f_lsn;
        uint32      f_hi,
                    f_lo;
        RewriteMappingFile *f;

        if (strcmp(mapping_de->d_name, ".") == 0 ||
            strcmp(mapping_de->d_name, "..") == 0)
            continue;

        /* Ignore files that aren't ours */
        if (strncmp(mapping_de->d_name, "map-", 4) != 0)
            continue;

        if (sscanf(mapping_de->d_name, LOGICAL_REWRITE_FORMAT,
                   &f_dboid, &f_relid, &f_hi, &f_lo,
                   &f_mapped_xid, &f_create_xid) != 6)
            elog(ERROR, "could not parse filename \"%s\"", mapping_de->d_name);

        f_lsn = ((uint64) f_hi) << 32 | f_lo;

        /* mapping for another database */
        if (f_dboid != dboid)
            continue;

        /* mapping for another relation */
        if (f_relid != relid)
            continue;

        /* did the creating transaction abort? */
        if (!TransactionIdDidCommit(f_create_xid))
            continue;

        /* not for our transaction */
        if (!TransactionIdInArray(f_mapped_xid, snapshot->subxip, snapshot->subxcnt))
            continue;

        /* ok, relevant, queue for apply */
        f = palloc(sizeof(RewriteMappingFile));
        f->lsn = f_lsn;
        strcpy(f->fname, mapping_de->d_name);
        files = lappend(files, f);
    }
    FreeDir(mapping_dir);

    /* sort files so we apply them in LSN order */
    list_sort(files, file_sort_by_lsn);

    foreach(file, files)
    {
        RewriteMappingFile *f = (RewriteMappingFile *) lfirst(file);

        elog(DEBUG1, "applying mapping: \"%s\" in %u", f->fname,
             snapshot->subxip[0]);
        ApplyLogicalMappingFile(tuplecid_data, relid, f->fname);
        pfree(f);
    }
}

/*
 * Lookup cmin/cmax of a tuple, during logical decoding where we can't rely on
 * combocids.
 */
bool
ResolveCminCmaxDuringDecoding(HTAB *tuplecid_data,
                              Snapshot snapshot,
                              HeapTuple htup, Buffer buffer,
                              CommandId *cmin, CommandId *cmax)
{
    ReorderBufferTupleCidKey key;
    ReorderBufferTupleCidEnt *ent;
    ForkNumber  forkno;
    BlockNumber blockno;
    bool        updated_mapping = false;

    /* be careful about padding */
    memset(&key, 0, sizeof(key));

    Assert(!BufferIsLocal(buffer));

    /*
     * get relfilenode from the buffer, no convenient way to access it other
     * than that.
     */
    BufferGetTag(buffer, &key.relnode, &forkno, &blockno);

    /* tuples can only be in the main fork */
    Assert(forkno == MAIN_FORKNUM);
    Assert(blockno == ItemPointerGetBlockNumber(&htup->t_self));

    ItemPointerCopy(&htup->t_self,
                    &key.tid);

restart:
    ent = (ReorderBufferTupleCidEnt *)
        hash_search(tuplecid_data,
                    (void *) &key,
                    HASH_FIND,
                    NULL);

    /*
     * failed to find a mapping, check whether the table was rewritten and
     * apply mapping if so, but only do that once - there can be no new
     * mappings while we are in here since we have to hold a lock on the
     * relation.
     */
    if (ent == NULL && !updated_mapping)
    {
        UpdateLogicalMappings(tuplecid_data, htup->t_tableOid, snapshot);
        /* now check but don't update for a mapping again */
        updated_mapping = true;
        goto restart;
    }
    else if (ent == NULL)
        return false;

    if (cmin)
        *cmin = ent->cmin;
    if (cmax)
        *cmax = ent->cmax;
    return true;
}