worker.c

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/*-------------------------------------------------------------------------
 * worker.c
 *     PostgreSQL logical replication worker (apply)
 *
 * Copyright (c) 2016-2020, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/logical/worker.c
 *
 * NOTES
 *    This file contains the worker which applies logical changes as they come
 *    from remote logical replication stream.
 *
 *    The main worker (apply) is started by logical replication worker
 *    launcher for every enabled subscription in a database. It uses
 *    walsender protocol to communicate with publisher.
 *
 *    This module includes server facing code and shares libpqwalreceiver
 *    module with walreceiver for providing the libpq specific functionality.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/table.h"
#include "access/tableam.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "catalog/catalog.h"
#include "catalog/namespace.h"
#include "catalog/partition.h"
#include "catalog/pg_inherits.h"
#include "catalog/pg_subscription.h"
#include "catalog/pg_subscription_rel.h"
#include "commands/tablecmds.h"
#include "commands/trigger.h"
#include "executor/executor.h"
#include "executor/execPartition.h"
#include "executor/nodeModifyTable.h"
#include "funcapi.h"
#include "libpq/pqformat.h"
#include "libpq/pqsignal.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "optimizer/optimizer.h"
#include "parser/analyze.h"
#include "parser/parse_relation.h"
#include "pgstat.h"
#include "postmaster/bgworker.h"
#include "postmaster/interrupt.h"
#include "postmaster/postmaster.h"
#include "postmaster/walwriter.h"
#include "replication/decode.h"
#include "replication/logical.h"
#include "replication/logicalproto.h"
#include "replication/logicalrelation.h"
#include "replication/logicalworker.h"
#include "replication/origin.h"
#include "replication/reorderbuffer.h"
#include "replication/snapbuild.h"
#include "replication/walreceiver.h"
#include "replication/worker_internal.h"
#include "rewrite/rewriteHandler.h"
#include "storage/bufmgr.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/catcache.h"
#include "utils/datum.h"
#include "utils/fmgroids.h"
#include "utils/guc.h"
#include "utils/inval.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/syscache.h"
#include "utils/timeout.h"

#define NAPTIME_PER_CYCLE 1000  /* max sleep time between cycles (1s) */

typedef struct FlushPosition
{
    dlist_node  node;
    XLogRecPtr  local_end;
    XLogRecPtr  remote_end;
} FlushPosition;

static dlist_head lsn_mapping = DLIST_STATIC_INIT(lsn_mapping);

typedef struct SlotErrCallbackArg
{
    LogicalRepRelMapEntry *rel;
    int         local_attnum;
    int         remote_attnum;
} SlotErrCallbackArg;

static MemoryContext ApplyMessageContext = NULL;
MemoryContext ApplyContext = NULL;

WalReceiverConn *wrconn = NULL;

Subscription *MySubscription = NULL;
bool        MySubscriptionValid = false;

bool        in_remote_transaction = false;
static XLogRecPtr remote_final_lsn = InvalidXLogRecPtr;

static void send_feedback(XLogRecPtr recvpos, bool force, bool requestReply);

static void store_flush_position(XLogRecPtr remote_lsn);

static void maybe_reread_subscription(void);

static void apply_handle_insert_internal(ResultRelInfo *relinfo,
                                         EState *estate, TupleTableSlot *remoteslot);
static void apply_handle_update_internal(ResultRelInfo *relinfo,
                                         EState *estate, TupleTableSlot *remoteslot,
                                         LogicalRepTupleData *newtup,
                                         LogicalRepRelMapEntry *relmapentry);
static void apply_handle_delete_internal(ResultRelInfo *relinfo, EState *estate,
                                         TupleTableSlot *remoteslot,
                                         LogicalRepRelation *remoterel);
static bool FindReplTupleInLocalRel(EState *estate, Relation localrel,
                                    LogicalRepRelation *remoterel,
                                    TupleTableSlot *remoteslot,
                                    TupleTableSlot **localslot);
static void apply_handle_tuple_routing(ResultRelInfo *relinfo,
                                       EState *estate,
                                       TupleTableSlot *remoteslot,
                                       LogicalRepTupleData *newtup,
                                       LogicalRepRelMapEntry *relmapentry,
                                       CmdType operation);

/*
 * Should this worker apply changes for given relation.
 *
 * This is mainly needed for initial relation data sync as that runs in
 * separate worker process running in parallel and we need some way to skip
 * changes coming to the main apply worker during the sync of a table.
 *
 * Note we need to do smaller or equals comparison for SYNCDONE state because
 * it might hold position of end of initial slot consistent point WAL
 * record + 1 (ie start of next record) and next record can be COMMIT of
 * transaction we are now processing (which is what we set remote_final_lsn
 * to in apply_handle_begin).
 */
static bool
should_apply_changes_for_rel(LogicalRepRelMapEntry *rel)
{
    if (am_tablesync_worker())
        return MyLogicalRepWorker->relid == rel->localreloid;
    else
        return (rel->state == SUBREL_STATE_READY ||
                (rel->state == SUBREL_STATE_SYNCDONE &&
                 rel->statelsn <= remote_final_lsn));
}

/*
 * Make sure that we started local transaction.
 *
 * Also switches to ApplyMessageContext as necessary.
 */
static bool
ensure_transaction(void)
{
    if (IsTransactionState())
    {
        SetCurrentStatementStartTimestamp();

        if (CurrentMemoryContext != ApplyMessageContext)
            MemoryContextSwitchTo(ApplyMessageContext);

        return false;
    }

    SetCurrentStatementStartTimestamp();
    StartTransactionCommand();

    maybe_reread_subscription();

    MemoryContextSwitchTo(ApplyMessageContext);
    return true;
}


/*
 * Executor state preparation for evaluation of constraint expressions,
 * indexes and triggers.
 *
 * This is based on similar code in copy.c
 */
static EState *
create_estate_for_relation(LogicalRepRelMapEntry *rel)
{
    EState     *estate;
    ResultRelInfo *resultRelInfo;
    RangeTblEntry *rte;

    estate = CreateExecutorState();

    rte = makeNode(RangeTblEntry);
    rte->rtekind = RTE_RELATION;
    rte->relid = RelationGetRelid(rel->localrel);
    rte->relkind = rel->localrel->rd_rel->relkind;
    rte->rellockmode = AccessShareLock;
    ExecInitRangeTable(estate, list_make1(rte));

    resultRelInfo = makeNode(ResultRelInfo);
    InitResultRelInfo(resultRelInfo, rel->localrel, 1, NULL, 0);

    estate->es_result_relations = resultRelInfo;
    estate->es_num_result_relations = 1;
    estate->es_result_relation_info = resultRelInfo;

    estate->es_output_cid = GetCurrentCommandId(true);

    /* Prepare to catch AFTER triggers. */
    AfterTriggerBeginQuery();

    return estate;
}

/*
 * Executes default values for columns for which we can't map to remote
 * relation columns.
 *
 * This allows us to support tables which have more columns on the downstream
 * than on the upstream.
 */
static void
slot_fill_defaults(LogicalRepRelMapEntry *rel, EState *estate,
                   TupleTableSlot *slot)
{
    TupleDesc   desc = RelationGetDescr(rel->localrel);
    int         num_phys_attrs = desc->natts;
    int         i;
    int         attnum,
                num_defaults = 0;
    int        *defmap;
    ExprState **defexprs;
    ExprContext *econtext;

    econtext = GetPerTupleExprContext(estate);

    /* We got all the data via replication, no need to evaluate anything. */
    if (num_phys_attrs == rel->remoterel.natts)
        return;

    defmap = (int *) palloc(num_phys_attrs * sizeof(int));
    defexprs = (ExprState **) palloc(num_phys_attrs * sizeof(ExprState *));

    Assert(rel->attrmap->maplen == num_phys_attrs);
    for (attnum = 0; attnum < num_phys_attrs; attnum++)
    {
        Expr       *defexpr;

        if (TupleDescAttr(desc, attnum)->attisdropped || TupleDescAttr(desc, attnum)->attgenerated)
            continue;

        if (rel->attrmap->attnums[attnum] >= 0)
            continue;

        defexpr = (Expr *) build_column_default(rel->localrel, attnum + 1);

        if (defexpr != NULL)
        {
            /* Run the expression through planner */
            defexpr = expression_planner(defexpr);

            /* Initialize executable expression in copycontext */
            defexprs[num_defaults] = ExecInitExpr(defexpr, NULL);
            defmap[num_defaults] = attnum;
            num_defaults++;
        }

    }

    for (i = 0; i < num_defaults; i++)
        slot->tts_values[defmap[i]] =
            ExecEvalExpr(defexprs[i], econtext, &slot->tts_isnull[defmap[i]]);
}

/*
 * Error callback to give more context info about type conversion failure.
 */
static void
slot_store_error_callback(void *arg)
{
    SlotErrCallbackArg *errarg = (SlotErrCallbackArg *) arg;
    LogicalRepRelMapEntry *rel;
    char       *remotetypname;
    Oid         remotetypoid,
                localtypoid;

    /* Nothing to do if remote attribute number is not set */
    if (errarg->remote_attnum < 0)
        return;

    rel = errarg->rel;
    remotetypoid = rel->remoterel.atttyps[errarg->remote_attnum];

    /* Fetch remote type name from the LogicalRepTypMap cache */
    remotetypname = logicalrep_typmap_gettypname(remotetypoid);

    /* Fetch local type OID from the local sys cache */
    localtypoid = get_atttype(rel->localreloid, errarg->local_attnum + 1);

    errcontext("processing remote data for replication target relation \"%s.%s\" column \"%s\", "
               "remote type %s, local type %s",
               rel->remoterel.nspname, rel->remoterel.relname,
               rel->remoterel.attnames[errarg->remote_attnum],
               remotetypname,
               format_type_be(localtypoid));
}

/*
 * Store data in C string form into slot.
 * This is similar to BuildTupleFromCStrings but TupleTableSlot fits our
 * use better.
 */
static void
slot_store_cstrings(TupleTableSlot *slot, LogicalRepRelMapEntry *rel,
                    char **values)
{
    int         natts = slot->tts_tupleDescriptor->natts;
    int         i;
    SlotErrCallbackArg errarg;
    ErrorContextCallback errcallback;

    ExecClearTuple(slot);

    /* Push callback + info on the error context stack */
    errarg.rel = rel;
    errarg.local_attnum = -1;
    errarg.remote_attnum = -1;
    errcallback.callback = slot_store_error_callback;
    errcallback.arg = (void *) &errarg;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;

    /* Call the "in" function for each non-dropped attribute */
    Assert(natts == rel->attrmap->maplen);
    for (i = 0; i < natts; i++)
    {
        Form_pg_attribute att = TupleDescAttr(slot->tts_tupleDescriptor, i);
        int         remoteattnum = rel->attrmap->attnums[i];

        if (!att->attisdropped && remoteattnum >= 0 &&
            values[remoteattnum] != NULL)
        {
            Oid         typinput;
            Oid         typioparam;

            errarg.local_attnum = i;
            errarg.remote_attnum = remoteattnum;

            getTypeInputInfo(att->atttypid, &typinput, &typioparam);
            slot->tts_values[i] =
                OidInputFunctionCall(typinput, values[remoteattnum],
                                     typioparam, att->atttypmod);
            slot->tts_isnull[i] = false;

            errarg.local_attnum = -1;
            errarg.remote_attnum = -1;
        }
        else
        {
            /*
             * We assign NULL to dropped attributes, NULL values, and missing
             * values (missing values should be later filled using
             * slot_fill_defaults).
             */
            slot->tts_values[i] = (Datum) 0;
            slot->tts_isnull[i] = true;
        }
    }

    /* Pop the error context stack */
    error_context_stack = errcallback.previous;

    ExecStoreVirtualTuple(slot);
}

/*
 * Replace selected columns with user data provided as C strings.
 * This is somewhat similar to heap_modify_tuple but also calls the type
 * input functions on the user data.
 * "slot" is filled with a copy of the tuple in "srcslot", with
 * columns selected by the "replaces" array replaced with data values
 * from "values".
 * Caution: unreplaced pass-by-ref columns in "slot" will point into the
 * storage for "srcslot".  This is OK for current usage, but someday we may
 * need to materialize "slot" at the end to make it independent of "srcslot".
 */
static void
slot_modify_cstrings(TupleTableSlot *slot, TupleTableSlot *srcslot,
                     LogicalRepRelMapEntry *rel,
                     char **values, bool *replaces)
{
    int         natts = slot->tts_tupleDescriptor->natts;
    int         i;
    SlotErrCallbackArg errarg;
    ErrorContextCallback errcallback;

    /* We'll fill "slot" with a virtual tuple, so we must start with ... */
    ExecClearTuple(slot);

    /*
     * Copy all the column data from srcslot, so that we'll have valid values
     * for unreplaced columns.
     */
    Assert(natts == srcslot->tts_tupleDescriptor->natts);
    slot_getallattrs(srcslot);
    memcpy(slot->tts_values, srcslot->tts_values, natts * sizeof(Datum));
    memcpy(slot->tts_isnull, srcslot->tts_isnull, natts * sizeof(bool));

    /* For error reporting, push callback + info on the error context stack */
    errarg.rel = rel;
    errarg.local_attnum = -1;
    errarg.remote_attnum = -1;
    errcallback.callback = slot_store_error_callback;
    errcallback.arg = (void *) &errarg;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;

    /* Call the "in" function for each replaced attribute */
    Assert(natts == rel->attrmap->maplen);
    for (i = 0; i < natts; i++)
    {
        Form_pg_attribute att = TupleDescAttr(slot->tts_tupleDescriptor, i);
        int         remoteattnum = rel->attrmap->attnums[i];

        if (remoteattnum < 0)
            continue;

        if (!replaces[remoteattnum])
            continue;

        if (values[remoteattnum] != NULL)
        {
            Oid         typinput;
            Oid         typioparam;

            errarg.local_attnum = i;
            errarg.remote_attnum = remoteattnum;

            getTypeInputInfo(att->atttypid, &typinput, &typioparam);
            slot->tts_values[i] =
                OidInputFunctionCall(typinput, values[remoteattnum],
                                     typioparam, att->atttypmod);
            slot->tts_isnull[i] = false;

            errarg.local_attnum = -1;
            errarg.remote_attnum = -1;
        }
        else
        {
            slot->tts_values[i] = (Datum) 0;
            slot->tts_isnull[i] = true;
        }
    }

    /* Pop the error context stack */
    error_context_stack = errcallback.previous;

    /* And finally, declare that "slot" contains a valid virtual tuple */
    ExecStoreVirtualTuple(slot);
}

/*
 * Handle BEGIN message.
 */
static void
apply_handle_begin(StringInfo s)
{
    LogicalRepBeginData begin_data;

    logicalrep_read_begin(s, &begin_data);

    remote_final_lsn = begin_data.final_lsn;

    in_remote_transaction = true;

    pgstat_report_activity(STATE_RUNNING, NULL);
}

/*
 * Handle COMMIT message.
 *
 * TODO, support tracking of multiple origins
 */
static void
apply_handle_commit(StringInfo s)
{
    LogicalRepCommitData commit_data;

    logicalrep_read_commit(s, &commit_data);

    Assert(commit_data.commit_lsn == remote_final_lsn);

    /* The synchronization worker runs in single transaction. */
    if (IsTransactionState() && !am_tablesync_worker())
    {
        /*
         * Update origin state so we can restart streaming from correct
         * position in case of crash.
         */
        replorigin_session_origin_lsn = commit_data.end_lsn;
        replorigin_session_origin_timestamp = commit_data.committime;

        CommitTransactionCommand();
        pgstat_report_stat(false);

        store_flush_position(commit_data.end_lsn);
    }
    else
    {
        /* Process any invalidation messages that might have accumulated. */
        AcceptInvalidationMessages();
        maybe_reread_subscription();
    }

    in_remote_transaction = false;

    /* Process any tables that are being synchronized in parallel. */
    process_syncing_tables(commit_data.end_lsn);

    pgstat_report_activity(STATE_IDLE, NULL);
}

/*
 * Handle ORIGIN message.
 *
 * TODO, support tracking of multiple origins
 */
static void
apply_handle_origin(StringInfo s)
{
    /*
     * ORIGIN message can only come inside remote transaction and before any
     * actual writes.
     */
    if (!in_remote_transaction ||
        (IsTransactionState() && !am_tablesync_worker()))
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("ORIGIN message sent out of order")));
}

/*
 * Handle RELATION message.
 *
 * Note we don't do validation against local schema here. The validation
 * against local schema is postponed until first change for given relation
 * comes as we only care about it when applying changes for it anyway and we
 * do less locking this way.
 */
static void
apply_handle_relation(StringInfo s)
{
    LogicalRepRelation *rel;

    rel = logicalrep_read_rel(s);
    logicalrep_relmap_update(rel);
}

/*
 * Handle TYPE message.
 *
 * Note we don't do local mapping here, that's done when the type is
 * actually used.
 */
static void
apply_handle_type(StringInfo s)
{
    LogicalRepTyp typ;

    logicalrep_read_typ(s, &typ);
    logicalrep_typmap_update(&typ);
}

/*
 * Get replica identity index or if it is not defined a primary key.
 *
 * If neither is defined, returns InvalidOid
 */
static Oid
GetRelationIdentityOrPK(Relation rel)
{
    Oid         idxoid;

    idxoid = RelationGetReplicaIndex(rel);

    if (!OidIsValid(idxoid))
        idxoid = RelationGetPrimaryKeyIndex(rel);

    return idxoid;
}

/*
 * Handle INSERT message.
 */

static void
apply_handle_insert(StringInfo s)
{
    LogicalRepRelMapEntry *rel;
    LogicalRepTupleData newtup;
    LogicalRepRelId relid;
    EState     *estate;
    TupleTableSlot *remoteslot;
    MemoryContext oldctx;

    ensure_transaction();

    relid = logicalrep_read_insert(s, &newtup);
    rel = logicalrep_rel_open(relid, RowExclusiveLock);
    if (!should_apply_changes_for_rel(rel))
    {
        /*
         * The relation can't become interesting in the middle of the
         * transaction so it's safe to unlock it.
         */
        logicalrep_rel_close(rel, RowExclusiveLock);
        return;
    }

    /* Initialize the executor state. */
    estate = create_estate_for_relation(rel);
    remoteslot = ExecInitExtraTupleSlot(estate,
                                        RelationGetDescr(rel->localrel),
                                        &TTSOpsVirtual);

    /* Input functions may need an active snapshot, so get one */
    PushActiveSnapshot(GetTransactionSnapshot());

    /* Process and store remote tuple in the slot */
    oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
    slot_store_cstrings(remoteslot, rel, newtup.values);
    slot_fill_defaults(rel, estate, remoteslot);
    MemoryContextSwitchTo(oldctx);

    /* For a partitioned table, insert the tuple into a partition. */
    if (rel->localrel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
        apply_handle_tuple_routing(estate->es_result_relation_info, estate,
                                   remoteslot, NULL, rel, CMD_INSERT);
    else
        apply_handle_insert_internal(estate->es_result_relation_info, estate,
                                     remoteslot);

    PopActiveSnapshot();

    /* Handle queued AFTER triggers. */
    AfterTriggerEndQuery(estate);

    ExecResetTupleTable(estate->es_tupleTable, false);
    FreeExecutorState(estate);

    logicalrep_rel_close(rel, NoLock);

    CommandCounterIncrement();
}

/* Workhorse for apply_handle_insert() */
static void
apply_handle_insert_internal(ResultRelInfo *relinfo,
                             EState *estate, TupleTableSlot *remoteslot)
{
    ExecOpenIndices(relinfo, false);

    /* Do the insert. */
    ExecSimpleRelationInsert(estate, remoteslot);

    /* Cleanup. */
    ExecCloseIndices(relinfo);
}

/*
 * Check if the logical replication relation is updatable and throw
 * appropriate error if it isn't.
 */
static void
check_relation_updatable(LogicalRepRelMapEntry *rel)
{
    /* Updatable, no error. */
    if (rel->updatable)
        return;

    /*
     * We are in error mode so it's fine this is somewhat slow. It's better to
     * give user correct error.
     */
    if (OidIsValid(GetRelationIdentityOrPK(rel->localrel)))
    {
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("publisher did not send replica identity column "
                        "expected by the logical replication target relation \"%s.%s\"",
                        rel->remoterel.nspname, rel->remoterel.relname)));
    }

    ereport(ERROR,
            (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
             errmsg("logical replication target relation \"%s.%s\" has "
                    "neither REPLICA IDENTITY index nor PRIMARY "
                    "KEY and published relation does not have "
                    "REPLICA IDENTITY FULL",
                    rel->remoterel.nspname, rel->remoterel.relname)));
}

/*
 * Handle UPDATE message.
 *
 * TODO: FDW support
 */
static void
apply_handle_update(StringInfo s)
{
    LogicalRepRelMapEntry *rel;
    LogicalRepRelId relid;
    EState     *estate;
    LogicalRepTupleData oldtup;
    LogicalRepTupleData newtup;
    bool        has_oldtup;
    TupleTableSlot *remoteslot;
    RangeTblEntry *target_rte;
    MemoryContext oldctx;

    ensure_transaction();

    relid = logicalrep_read_update(s, &has_oldtup, &oldtup,
                                   &newtup);
    rel = logicalrep_rel_open(relid, RowExclusiveLock);
    if (!should_apply_changes_for_rel(rel))
    {
        /*
         * The relation can't become interesting in the middle of the
         * transaction so it's safe to unlock it.
         */
        logicalrep_rel_close(rel, RowExclusiveLock);
        return;
    }

    /* Check if we can do the update. */
    check_relation_updatable(rel);

    /* Initialize the executor state. */
    estate = create_estate_for_relation(rel);
    remoteslot = ExecInitExtraTupleSlot(estate,
                                        RelationGetDescr(rel->localrel),
                                        &TTSOpsVirtual);

    /*
     * Populate updatedCols so that per-column triggers can fire.  This could
     * include more columns than were actually changed on the publisher
     * because the logical replication protocol doesn't contain that
     * information.  But it would for example exclude columns that only exist
     * on the subscriber, since we are not touching those.
     */
    target_rte = list_nth(estate->es_range_table, 0);
    for (int i = 0; i < remoteslot->tts_tupleDescriptor->natts; i++)
    {
        if (newtup.changed[i])
            target_rte->updatedCols = bms_add_member(target_rte->updatedCols,
                                                     i + 1 - FirstLowInvalidHeapAttributeNumber);
    }

    fill_extraUpdatedCols(target_rte, RelationGetDescr(rel->localrel));

    PushActiveSnapshot(GetTransactionSnapshot());

    /* Build the search tuple. */
    oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
    slot_store_cstrings(remoteslot, rel,
                        has_oldtup ? oldtup.values : newtup.values);
    MemoryContextSwitchTo(oldctx);

    /* For a partitioned table, apply update to correct partition. */
    if (rel->localrel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
        apply_handle_tuple_routing(estate->es_result_relation_info, estate,
                                   remoteslot, &newtup, rel, CMD_UPDATE);
    else
        apply_handle_update_internal(estate->es_result_relation_info, estate,
                                     remoteslot, &newtup, rel);

    PopActiveSnapshot();

    /* Handle queued AFTER triggers. */
    AfterTriggerEndQuery(estate);

    ExecResetTupleTable(estate->es_tupleTable, false);
    FreeExecutorState(estate);

    logicalrep_rel_close(rel, NoLock);

    CommandCounterIncrement();
}

/* Workhorse for apply_handle_update() */
static void
apply_handle_update_internal(ResultRelInfo *relinfo,
                             EState *estate, TupleTableSlot *remoteslot,
                             LogicalRepTupleData *newtup,
                             LogicalRepRelMapEntry *relmapentry)
{
    Relation    localrel = relinfo->ri_RelationDesc;
    EPQState    epqstate;
    TupleTableSlot *localslot;
    bool        found;
    MemoryContext oldctx;

    EvalPlanQualInit(&epqstate, estate, NULL, NIL, -1);
    ExecOpenIndices(relinfo, false);

    found = FindReplTupleInLocalRel(estate, localrel,
                                    &relmapentry->remoterel,
                                    remoteslot, &localslot);
    ExecClearTuple(remoteslot);

    /*
     * Tuple found.
     *
     * Note this will fail if there are other conflicting unique indexes.
     */
    if (found)
    {
        /* Process and store remote tuple in the slot */
        oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
        slot_modify_cstrings(remoteslot, localslot, relmapentry,
                             newtup->values, newtup->changed);
        MemoryContextSwitchTo(oldctx);

        EvalPlanQualSetSlot(&epqstate, remoteslot);

        /* Do the actual update. */
        ExecSimpleRelationUpdate(estate, &epqstate, localslot, remoteslot);
    }
    else
    {
        /*
         * The tuple to be updated could not be found.
         *
         * TODO what to do here, change the log level to LOG perhaps?
         */
        elog(DEBUG1,
             "logical replication did not find row for update "
             "in replication target relation \"%s\"",
             RelationGetRelationName(localrel));
    }

    /* Cleanup. */
    ExecCloseIndices(relinfo);
    EvalPlanQualEnd(&epqstate);
}

/*
 * Handle DELETE message.
 *
 * TODO: FDW support
 */
static void
apply_handle_delete(StringInfo s)
{
    LogicalRepRelMapEntry *rel;
    LogicalRepTupleData oldtup;
    LogicalRepRelId relid;
    EState     *estate;
    TupleTableSlot *remoteslot;
    MemoryContext oldctx;

    ensure_transaction();

    relid = logicalrep_read_delete(s, &oldtup);
    rel = logicalrep_rel_open(relid, RowExclusiveLock);
    if (!should_apply_changes_for_rel(rel))
    {
        /*
         * The relation can't become interesting in the middle of the
         * transaction so it's safe to unlock it.
         */
        logicalrep_rel_close(rel, RowExclusiveLock);
        return;
    }

    /* Check if we can do the delete. */
    check_relation_updatable(rel);

    /* Initialize the executor state. */
    estate = create_estate_for_relation(rel);
    remoteslot = ExecInitExtraTupleSlot(estate,
                                        RelationGetDescr(rel->localrel),
                                        &TTSOpsVirtual);

    PushActiveSnapshot(GetTransactionSnapshot());

    /* Build the search tuple. */
    oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
    slot_store_cstrings(remoteslot, rel, oldtup.values);
    MemoryContextSwitchTo(oldctx);

    /* For a partitioned table, apply delete to correct partition. */
    if (rel->localrel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
        apply_handle_tuple_routing(estate->es_result_relation_info, estate,
                                   remoteslot, NULL, rel, CMD_DELETE);
    else
        apply_handle_delete_internal(estate->es_result_relation_info, estate,
                                     remoteslot, &rel->remoterel);

    PopActiveSnapshot();

    /* Handle queued AFTER triggers. */
    AfterTriggerEndQuery(estate);

    ExecResetTupleTable(estate->es_tupleTable, false);
    FreeExecutorState(estate);

    logicalrep_rel_close(rel, NoLock);

    CommandCounterIncrement();
}

/* Workhorse for apply_handle_delete() */
static void
apply_handle_delete_internal(ResultRelInfo *relinfo, EState *estate,
                             TupleTableSlot *remoteslot,
                             LogicalRepRelation *remoterel)
{
    Relation    localrel = relinfo->ri_RelationDesc;
    EPQState    epqstate;
    TupleTableSlot *localslot;
    bool        found;

    EvalPlanQualInit(&epqstate, estate, NULL, NIL, -1);
    ExecOpenIndices(relinfo, false);

    found = FindReplTupleInLocalRel(estate, localrel, remoterel,
                                    remoteslot, &localslot);

    /* If found delete it. */
    if (found)
    {
        EvalPlanQualSetSlot(&epqstate, localslot);

        /* Do the actual delete. */
        ExecSimpleRelationDelete(estate, &epqstate, localslot);
    }
    else
    {
        /* The tuple to be deleted could not be found. */
        elog(DEBUG1,
             "logical replication could not find row for delete "
             "in replication target relation \"%s\"",
             RelationGetRelationName(localrel));
    }

    /* Cleanup. */
    ExecCloseIndices(relinfo);
    EvalPlanQualEnd(&epqstate);
}

/*
 * Try to find a tuple received from the publication side (in 'remoteslot') in
 * the corresponding local relation using either replica identity index,
 * primary key or if needed, sequential scan.
 *
 * Local tuple, if found, is returned in '*localslot'.
 */
static bool
FindReplTupleInLocalRel(EState *estate, Relation localrel,
                        LogicalRepRelation *remoterel,
                        TupleTableSlot *remoteslot,
                        TupleTableSlot **localslot)
{
    Oid         idxoid;
    bool        found;

    *localslot = table_slot_create(localrel, &estate->es_tupleTable);

    idxoid = GetRelationIdentityOrPK(localrel);
    Assert(OidIsValid(idxoid) ||
           (remoterel->replident == REPLICA_IDENTITY_FULL));

    if (OidIsValid(idxoid))
        found = RelationFindReplTupleByIndex(localrel, idxoid,
                                             LockTupleExclusive,
                                             remoteslot, *localslot);
    else
        found = RelationFindReplTupleSeq(localrel, LockTupleExclusive,
                                         remoteslot, *localslot);

    return found;
}

/*
 * This handles insert, update, delete on a partitioned table.
 */
static void
apply_handle_tuple_routing(ResultRelInfo *relinfo,
                           EState *estate,
                           TupleTableSlot *remoteslot,
                           LogicalRepTupleData *newtup,
                           LogicalRepRelMapEntry *relmapentry,
                           CmdType operation)
{
    Relation    parentrel = relinfo->ri_RelationDesc;
    ModifyTableState *mtstate = NULL;
    PartitionTupleRouting *proute = NULL;
    ResultRelInfo *partrelinfo;
    Relation    partrel;
    TupleTableSlot *remoteslot_part;
    PartitionRoutingInfo *partinfo;
    TupleConversionMap *map;
    MemoryContext oldctx;

    /* ModifyTableState is needed for ExecFindPartition(). */
    mtstate = makeNode(ModifyTableState);
    mtstate->ps.plan = NULL;
    mtstate->ps.state = estate;
    mtstate->operation = operation;
    mtstate->resultRelInfo = relinfo;
    proute = ExecSetupPartitionTupleRouting(estate, mtstate, parentrel);

    /*
     * Find the partition to which the "search tuple" belongs.
     */
    Assert(remoteslot != NULL);
    oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
    partrelinfo = ExecFindPartition(mtstate, relinfo, proute,
                                    remoteslot, estate);
    Assert(partrelinfo != NULL);
    partrel = partrelinfo->ri_RelationDesc;

    /*
     * To perform any of the operations below, the tuple must match the
     * partition's rowtype. Convert if needed or just copy, using a dedicated
     * slot to store the tuple in any case.
     */
    partinfo = partrelinfo->ri_PartitionInfo;
    remoteslot_part = partinfo->pi_PartitionTupleSlot;
    if (remoteslot_part == NULL)
        remoteslot_part = table_slot_create(partrel, &estate->es_tupleTable);
    map = partinfo->pi_RootToPartitionMap;
    if (map != NULL)
        remoteslot_part = execute_attr_map_slot(map->attrMap, remoteslot,
                                                remoteslot_part);
    else
    {
        remoteslot_part = ExecCopySlot(remoteslot_part, remoteslot);
        slot_getallattrs(remoteslot_part);
    }
    MemoryContextSwitchTo(oldctx);

    estate->es_result_relation_info = partrelinfo;
    switch (operation)
    {
        case CMD_INSERT:
            apply_handle_insert_internal(partrelinfo, estate,
                                         remoteslot_part);
            break;

        case CMD_DELETE:
            apply_handle_delete_internal(partrelinfo, estate,
                                         remoteslot_part,
                                         &relmapentry->remoterel);
            break;

        case CMD_UPDATE:

            /*
             * For UPDATE, depending on whether or not the updated tuple
             * satisfies the partition's constraint, perform a simple UPDATE
             * of the partition or move the updated tuple into a different
             * suitable partition.
             */
            {
                AttrMap    *attrmap = map ? map->attrMap : NULL;
                LogicalRepRelMapEntry *part_entry;
                TupleTableSlot *localslot;
                ResultRelInfo *partrelinfo_new;
                bool        found;

                part_entry = logicalrep_partition_open(relmapentry, partrel,
                                                       attrmap);

                /* Get the matching local tuple from the partition. */
                found = FindReplTupleInLocalRel(estate, partrel,
                                                &part_entry->remoterel,
                                                remoteslot_part, &localslot);

                oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
                if (found)
                {
                    /* Apply the update.  */
                    slot_modify_cstrings(remoteslot_part, localslot,
                                         part_entry,
                                         newtup->values, newtup->changed);
                    MemoryContextSwitchTo(oldctx);
                }
                else
                {
                    /*
                     * The tuple to be updated could not be found.
                     *
                     * TODO what to do here, change the log level to LOG
                     * perhaps?
                     */
                    elog(DEBUG1,
                         "logical replication did not find row for update "
                         "in replication target relation \"%s\"",
                         RelationGetRelationName(partrel));
                }

                /*
                 * Does the updated tuple still satisfy the current
                 * partition's constraint?
                 */
                if (partrelinfo->ri_PartitionCheck == NULL ||
                    ExecPartitionCheck(partrelinfo, remoteslot_part, estate,
                                       false))
                {
                    /*
                     * Yes, so simply UPDATE the partition.  We don't call
                     * apply_handle_update_internal() here, which would
                     * normally do the following work, to avoid repeating some
                     * work already done above to find the local tuple in the
                     * partition.
                     */
                    EPQState    epqstate;

                    EvalPlanQualInit(&epqstate, estate, NULL, NIL, -1);
                    ExecOpenIndices(partrelinfo, false);

                    EvalPlanQualSetSlot(&epqstate, remoteslot_part);
                    ExecSimpleRelationUpdate(estate, &epqstate, localslot,
                                             remoteslot_part);
                    ExecCloseIndices(partrelinfo);
                    EvalPlanQualEnd(&epqstate);
                }
                else
                {
                    /* Move the tuple into the new partition. */

                    /*
                     * New partition will be found using tuple routing, which
                     * can only occur via the parent table.  We might need to
                     * convert the tuple to the parent's rowtype.  Note that
                     * this is the tuple found in the partition, not the
                     * original search tuple received by this function.
                     */
                    if (map)
                    {
                        TupleConversionMap *PartitionToRootMap =
                        convert_tuples_by_name(RelationGetDescr(partrel),
                                               RelationGetDescr(parentrel));

                        remoteslot =
                            execute_attr_map_slot(PartitionToRootMap->attrMap,
                                                  remoteslot_part, remoteslot);
                    }
                    else
                    {
                        remoteslot = ExecCopySlot(remoteslot, remoteslot_part);
                        slot_getallattrs(remoteslot);
                    }


                    /* Find the new partition. */
                    oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
                    partrelinfo_new = ExecFindPartition(mtstate, relinfo,
                                                        proute, remoteslot,
                                                        estate);
                    MemoryContextSwitchTo(oldctx);
                    Assert(partrelinfo_new != partrelinfo);

                    /* DELETE old tuple found in the old partition. */
                    estate->es_result_relation_info = partrelinfo;
                    apply_handle_delete_internal(partrelinfo, estate,
                                                 localslot,
                                                 &relmapentry->remoterel);

                    /* INSERT new tuple into the new partition. */

                    /*
                     * Convert the replacement tuple to match the destination
                     * partition rowtype.
                     */
                    oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
                    partrel = partrelinfo_new->ri_RelationDesc;
                    partinfo = partrelinfo_new->ri_PartitionInfo;
                    remoteslot_part = partinfo->pi_PartitionTupleSlot;
                    if (remoteslot_part == NULL)
                        remoteslot_part = table_slot_create(partrel,
                                                            &estate->es_tupleTable);
                    map = partinfo->pi_RootToPartitionMap;
                    if (map != NULL)
                    {
                        remoteslot_part = execute_attr_map_slot(map->attrMap,
                                                                remoteslot,
                                                                remoteslot_part);
                    }
                    else
                    {
                        remoteslot_part = ExecCopySlot(remoteslot_part,
                                                       remoteslot);
                        slot_getallattrs(remoteslot);
                    }
                    MemoryContextSwitchTo(oldctx);
                    estate->es_result_relation_info = partrelinfo_new;
                    apply_handle_insert_internal(partrelinfo_new, estate,
                                                 remoteslot_part);
                }
            }
            break;

        default:
            elog(ERROR, "unrecognized CmdType: %d", (int) operation);
            break;
    }

    ExecCleanupTupleRouting(mtstate, proute);
}

/*
 * Handle TRUNCATE message.
 *
 * TODO: FDW support
 */
static void
apply_handle_truncate(StringInfo s)
{
    bool        cascade = false;
    bool        restart_seqs = false;
    List       *remote_relids = NIL;
    List       *remote_rels = NIL;
    List       *rels = NIL;
    List       *part_rels = NIL;
    List       *relids = NIL;
    List       *relids_logged = NIL;
    ListCell   *lc;

    ensure_transaction();

    remote_relids = logicalrep_read_truncate(s, &cascade, &restart_seqs);

    foreach(lc, remote_relids)
    {
        LogicalRepRelId relid = lfirst_oid(lc);
        LogicalRepRelMapEntry *rel;

        rel = logicalrep_rel_open(relid, RowExclusiveLock);
        if (!should_apply_changes_for_rel(rel))
        {
            /*
             * The relation can't become interesting in the middle of the
             * transaction so it's safe to unlock it.
             */
            logicalrep_rel_close(rel, RowExclusiveLock);
            continue;
        }

        remote_rels = lappend(remote_rels, rel);
        rels = lappend(rels, rel->localrel);
        relids = lappend_oid(relids, rel->localreloid);
        if (RelationIsLogicallyLogged(rel->localrel))
            relids_logged = lappend_oid(relids_logged, rel->localreloid);

        /*
         * Truncate partitions if we got a message to truncate a partitioned
         * table.
         */
        if (rel->localrel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
        {
            ListCell   *child;
            List       *children = find_all_inheritors(rel->localreloid,
                                                       RowExclusiveLock,
                                                       NULL);

            foreach(child, children)
            {
                Oid         childrelid = lfirst_oid(child);
                Relation    childrel;

                if (list_member_oid(relids, childrelid))
                    continue;

                /* find_all_inheritors already got lock */
                childrel = table_open(childrelid, NoLock);

                /*
                 * Ignore temp tables of other backends.  See similar code in
                 * ExecuteTruncate().
                 */
                if (RELATION_IS_OTHER_TEMP(childrel))
                {
                    table_close(childrel, RowExclusiveLock);
                    continue;
                }

                rels = lappend(rels, childrel);
                part_rels = lappend(part_rels, childrel);
                relids = lappend_oid(relids, childrelid);
                /* Log this relation only if needed for logical decoding */
                if (RelationIsLogicallyLogged(childrel))
                    relids_logged = lappend_oid(relids_logged, childrelid);
            }
        }
    }

    /*
     * Even if we used CASCADE on the upstream primary we explicitly default to
     * replaying changes without further cascading. This might be later
     * changeable with a user specified option.
     */
    ExecuteTruncateGuts(rels, relids, relids_logged, DROP_RESTRICT, restart_seqs);

    foreach(lc, remote_rels)
    {
        LogicalRepRelMapEntry *rel = lfirst(lc);

        logicalrep_rel_close(rel, NoLock);
    }
    foreach(lc, part_rels)
    {
        Relation    rel = lfirst(lc);

        table_close(rel, NoLock);
    }

    CommandCounterIncrement();
}


/*
 * Logical replication protocol message dispatcher.
 */
static void
apply_dispatch(StringInfo s)
{
    char        action = pq_getmsgbyte(s);

    switch (action)
    {
            /* BEGIN */
        case 'B':
            apply_handle_begin(s);
            break;
            /* COMMIT */
        case 'C':
            apply_handle_commit(s);
            break;
            /* INSERT */
        case 'I':
            apply_handle_insert(s);
            break;
            /* UPDATE */
        case 'U':
            apply_handle_update(s);
            break;
            /* DELETE */
        case 'D':
            apply_handle_delete(s);
            break;
            /* TRUNCATE */
        case 'T':
            apply_handle_truncate(s);
            break;
            /* RELATION */
        case 'R':
            apply_handle_relation(s);
            break;
            /* TYPE */
        case 'Y':
            apply_handle_type(s);
            break;
            /* ORIGIN */
        case 'O':
            apply_handle_origin(s);
            break;
        default:
            ereport(ERROR,
                    (errcode(ERRCODE_PROTOCOL_VIOLATION),
                     errmsg("invalid logical replication message type \"%c\"", action)));
    }
}

/*
 * Figure out which write/flush positions to report to the walsender process.
 *
 * We can't simply report back the last LSN the walsender sent us because the
 * local transaction might not yet be flushed to disk locally. Instead we
 * build a list that associates local with remote LSNs for every commit. When
 * reporting back the flush position to the sender we iterate that list and
 * check which entries on it are already locally flushed. Those we can report
 * as having been flushed.
 *
 * The have_pending_txes is true if there are outstanding transactions that
 * need to be flushed.
 */
static void
get_flush_position(XLogRecPtr *write, XLogRecPtr *flush,
                   bool *have_pending_txes)
{
    dlist_mutable_iter iter;
    XLogRecPtr  local_flush = GetFlushRecPtr();

    *write = InvalidXLogRecPtr;
    *flush = InvalidXLogRecPtr;

    dlist_foreach_modify(iter, &lsn_mapping)
    {
        FlushPosition *pos =
        dlist_container(FlushPosition, node, iter.cur);

        *write = pos->remote_end;

        if (pos->local_end <= local_flush)
        {
            *flush = pos->remote_end;
            dlist_delete(iter.cur);
            pfree(pos);
        }
        else
        {
            /*
             * Don't want to uselessly iterate over the rest of the list which
             * could potentially be long. Instead get the last element and
             * grab the write position from there.
             */
            pos = dlist_tail_element(FlushPosition, node,
                                     &lsn_mapping);
            *write = pos->remote_end;
            *have_pending_txes = true;
            return;
        }
    }

    *have_pending_txes = !dlist_is_empty(&lsn_mapping);
}

/*
 * Store current remote/local lsn pair in the tracking list.
 */
static void
store_flush_position(XLogRecPtr remote_lsn)
{
    FlushPosition *flushpos;

    /* Need to do this in permanent context */
    MemoryContextSwitchTo(ApplyContext);

    /* Track commit lsn  */
    flushpos = (FlushPosition *) palloc(sizeof(FlushPosition));
    flushpos->local_end = XactLastCommitEnd;
    flushpos->remote_end = remote_lsn;

    dlist_push_tail(&lsn_mapping, &flushpos->node);
    MemoryContextSwitchTo(ApplyMessageContext);
}


/* Update statistics of the worker. */
static void
UpdateWorkerStats(XLogRecPtr last_lsn, TimestampTz send_time, bool reply)
{
    MyLogicalRepWorker->last_lsn = last_lsn;
    MyLogicalRepWorker->last_send_time = send_time;
    MyLogicalRepWorker->last_recv_time = GetCurrentTimestamp();
    if (reply)
    {
        MyLogicalRepWorker->reply_lsn = last_lsn;
        MyLogicalRepWorker->reply_time = send_time;
    }
}

/*
 * Apply main loop.
 */
static void
LogicalRepApplyLoop(XLogRecPtr last_received)
{
    TimestampTz last_recv_timestamp = GetCurrentTimestamp();

    /*
     * Init the ApplyMessageContext which we clean up after each replication
     * protocol message.
     */
    ApplyMessageContext = AllocSetContextCreate(ApplyContext,
                                                "ApplyMessageContext",
                                                ALLOCSET_DEFAULT_SIZES);

    /* mark as idle, before starting to loop */
    pgstat_report_activity(STATE_IDLE, NULL);

    for (;;)
    {
        pgsocket    fd = PGINVALID_SOCKET;
        int         rc;
        int         len;
        char       *buf = NULL;
        bool        endofstream = false;
        bool        ping_sent = false;
        long        wait_time;

        CHECK_FOR_INTERRUPTS();

        MemoryContextSwitchTo(ApplyMessageContext);

        len = walrcv_receive(wrconn, &buf, &fd);

        if (len != 0)
        {
            /* Process the data */
            for (;;)
            {
                CHECK_FOR_INTERRUPTS();

                if (len == 0)
                {
                    break;
                }
                else if (len < 0)
                {
                    ereport(LOG,
                            (errmsg("data stream from publisher has ended")));
                    endofstream = true;
                    break;
                }
                else
                {
                    int         c;
                    StringInfoData s;

                    /* Reset timeout. */
                    last_recv_timestamp = GetCurrentTimestamp();
                    ping_sent = false;

                    /* Ensure we are reading the data into our memory context. */
                    MemoryContextSwitchTo(ApplyMessageContext);

                    s.data = buf;
                    s.len = len;
                    s.cursor = 0;
                    s.maxlen = -1;

                    c = pq_getmsgbyte(&s);

                    if (c == 'w')
                    {
                        XLogRecPtr  start_lsn;
                        XLogRecPtr  end_lsn;
                        TimestampTz send_time;

                        start_lsn = pq_getmsgint64(&s);
                        end_lsn = pq_getmsgint64(&s);
                        send_time = pq_getmsgint64(&s);

                        if (last_received < start_lsn)
                            last_received = start_lsn;

                        if (last_received < end_lsn)
                            last_received = end_lsn;

                        UpdateWorkerStats(last_received, send_time, false);

                        apply_dispatch(&s);
                    }
                    else if (c == 'k')
                    {
                        XLogRecPtr  end_lsn;
                        TimestampTz timestamp;
                        bool        reply_requested;

                        end_lsn = pq_getmsgint64(&s);
                        timestamp = pq_getmsgint64(&s);
                        reply_requested = pq_getmsgbyte(&s);

                        if (last_received < end_lsn)
                            last_received = end_lsn;

                        send_feedback(last_received, reply_requested, false);
                        UpdateWorkerStats(last_received, timestamp, true);
                    }
                    /* other message types are purposefully ignored */

                    MemoryContextReset(ApplyMessageContext);
                }

                len = walrcv_receive(wrconn, &buf, &fd);
            }
        }

        /* confirm all writes so far */
        send_feedback(last_received, false, false);

        if (!in_remote_transaction)
        {
            /*
             * If we didn't get any transactions for a while there might be
             * unconsumed invalidation messages in the queue, consume them
             * now.
             */
            AcceptInvalidationMessages();
            maybe_reread_subscription();

            /* Process any table synchronization changes. */
            process_syncing_tables(last_received);
        }

        /* Cleanup the memory. */
        MemoryContextResetAndDeleteChildren(ApplyMessageContext);
        MemoryContextSwitchTo(TopMemoryContext);

        /* Check if we need to exit the streaming loop. */
        if (endofstream)
        {
            TimeLineID  tli;

            walrcv_endstreaming(wrconn, &tli);
            break;
        }

        /*
         * Wait for more data or latch.  If we have unflushed transactions,
         * wake up after WalWriterDelay to see if they've been flushed yet (in
         * which case we should send a feedback message).  Otherwise, there's
         * no particular urgency about waking up unless we get data or a
         * signal.
         */
        if (!dlist_is_empty(&lsn_mapping))
            wait_time = WalWriterDelay;
        else
            wait_time = NAPTIME_PER_CYCLE;

        rc = WaitLatchOrSocket(MyLatch,
                               WL_SOCKET_READABLE | WL_LATCH_SET |
                               WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                               fd, wait_time,
                               WAIT_EVENT_LOGICAL_APPLY_MAIN);

        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            CHECK_FOR_INTERRUPTS();
        }

        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
        }

        if (rc & WL_TIMEOUT)
        {
            /*
             * We didn't receive anything new. If we haven't heard anything
             * from the server for more than wal_receiver_timeout / 2, ping
             * the server. Also, if it's been longer than
             * wal_receiver_status_interval since the last update we sent,
             * send a status update to the primary anyway, to report any
             * progress in applying WAL.
             */
            bool        requestReply = false;

            /*
             * Check if time since last receive from standby has reached the
             * configured limit.
             */
            if (wal_receiver_timeout > 0)
            {
                TimestampTz now = GetCurrentTimestamp();
                TimestampTz timeout;

                timeout =
                    TimestampTzPlusMilliseconds(last_recv_timestamp,
                                                wal_receiver_timeout);

                if (now >= timeout)
                    ereport(ERROR,
                            (errmsg("terminating logical replication worker due to timeout")));

                /*
                 * We didn't receive anything new, for half of receiver
                 * replication timeout. Ping the server.
                 */
                if (!ping_sent)
                {
                    timeout = TimestampTzPlusMilliseconds(last_recv_timestamp,
                                                          (wal_receiver_timeout / 2));
                    if (now >= timeout)
                    {
                        requestReply = true;
                        ping_sent = true;
                    }
                }
            }

            send_feedback(last_received, requestReply, requestReply);
        }
    }
}

/*
 * Send a Standby Status Update message to server.
 *
 * 'recvpos' is the latest LSN we've received data to, force is set if we need
 * to send a response to avoid timeouts.
 */
static void
send_feedback(XLogRecPtr recvpos, bool force, bool requestReply)
{
    static StringInfo reply_message = NULL;
    static TimestampTz send_time = 0;

    static XLogRecPtr last_recvpos = InvalidXLogRecPtr;
    static XLogRecPtr last_writepos = InvalidXLogRecPtr;
    static XLogRecPtr last_flushpos = InvalidXLogRecPtr;

    XLogRecPtr  writepos;
    XLogRecPtr  flushpos;
    TimestampTz now;
    bool        have_pending_txes;

    /*
     * If the user doesn't want status to be reported to the publisher, be
     * sure to exit before doing anything at all.
     */
    if (!force && wal_receiver_status_interval <= 0)
        return;

    /* It's legal to not pass a recvpos */
    if (recvpos < last_recvpos)
        recvpos = last_recvpos;

    get_flush_position(&writepos, &flushpos, &have_pending_txes);

    /*
     * No outstanding transactions to flush, we can report the latest received
     * position. This is important for synchronous replication.
     */
    if (!have_pending_txes)
        flushpos = writepos = recvpos;

    if (writepos < last_writepos)
        writepos = last_writepos;

    if (flushpos < last_flushpos)
        flushpos = last_flushpos;

    now = GetCurrentTimestamp();

    /* if we've already reported everything we're good */
    if (!force &&
        writepos == last_writepos &&
        flushpos == last_flushpos &&
        !TimestampDifferenceExceeds(send_time, now,
                                    wal_receiver_status_interval * 1000))
        return;
    send_time = now;

    if (!reply_message)
    {
        MemoryContext oldctx = MemoryContextSwitchTo(ApplyContext);

        reply_message = makeStringInfo();
        MemoryContextSwitchTo(oldctx);
    }
    else
        resetStringInfo(reply_message);

    pq_sendbyte(reply_message, 'r');
    pq_sendint64(reply_message, recvpos);   /* write */
    pq_sendint64(reply_message, flushpos);  /* flush */
    pq_sendint64(reply_message, writepos);  /* apply */
    pq_sendint64(reply_message, now);   /* sendTime */
    pq_sendbyte(reply_message, requestReply);   /* replyRequested */

    elog(DEBUG2, "sending feedback (force %d) to recv %X/%X, write %X/%X, flush %X/%X",
         force,
         (uint32) (recvpos >> 32), (uint32) recvpos,
         (uint32) (writepos >> 32), (uint32) writepos,
         (uint32) (flushpos >> 32), (uint32) flushpos
        );

    walrcv_send(wrconn, reply_message->data, reply_message->len);

    if (recvpos > last_recvpos)
        last_recvpos = recvpos;
    if (writepos > last_writepos)
        last_writepos = writepos;
    if (flushpos > last_flushpos)
        last_flushpos = flushpos;
}

/*
 * Reread subscription info if needed. Most changes will be exit.
 */
static void
maybe_reread_subscription(void)
{
    MemoryContext oldctx;
    Subscription *newsub;
    bool        started_tx = false;

    /* When cache state is valid there is nothing to do here. */
    if (MySubscriptionValid)
        return;

    /* This function might be called inside or outside of transaction. */
    if (!IsTransactionState())
    {
        StartTransactionCommand();
        started_tx = true;
    }

    /* Ensure allocations in permanent context. */
    oldctx = MemoryContextSwitchTo(ApplyContext);

    newsub = GetSubscription(MyLogicalRepWorker->subid, true);

    /*
     * Exit if the subscription was removed. This normally should not happen
     * as the worker gets killed during DROP SUBSCRIPTION.
     */
    if (!newsub)
    {
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription \"%s\" will "
                        "stop because the subscription was removed",
                        MySubscription->name)));

        proc_exit(0);
    }

    /*
     * Exit if the subscription was disabled. This normally should not happen
     * as the worker gets killed during ALTER SUBSCRIPTION ... DISABLE.
     */
    if (!newsub->enabled)
    {
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription \"%s\" will "
                        "stop because the subscription was disabled",
                        MySubscription->name)));

        proc_exit(0);
    }

    /*
     * Exit if connection string was changed. The launcher will start new
     * worker.
     */
    if (strcmp(newsub->conninfo, MySubscription->conninfo) != 0)
    {
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription \"%s\" will "
                        "restart because the connection information was changed",
                        MySubscription->name)));

        proc_exit(0);
    }

    /*
     * Exit if subscription name was changed (it's used for
     * fallback_application_name). The launcher will start new worker.
     */
    if (strcmp(newsub->name, MySubscription->name) != 0)
    {
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription \"%s\" will "
                        "restart because subscription was renamed",
                        MySubscription->name)));

        proc_exit(0);
    }

    /* !slotname should never happen when enabled is true. */
    Assert(newsub->slotname);

    /*
     * We need to make new connection to new slot if slot name has changed so
     * exit here as well if that's the case.
     */
    if (strcmp(newsub->slotname, MySubscription->slotname) != 0)
    {
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription \"%s\" will "
                        "restart because the replication slot name was changed",
                        MySubscription->name)));

        proc_exit(0);
    }

    /*
     * Exit if publication list was changed. The launcher will start new
     * worker.
     */
    if (!equal(newsub->publications, MySubscription->publications))
    {
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription \"%s\" will "
                        "restart because subscription's publications were changed",
                        MySubscription->name)));

        proc_exit(0);
    }

    /* Check for other changes that should never happen too. */
    if (newsub->dbid != MySubscription->dbid)
    {
        elog(ERROR, "subscription %u changed unexpectedly",
             MyLogicalRepWorker->subid);
    }

    /* Clean old subscription info and switch to new one. */
    FreeSubscription(MySubscription);
    MySubscription = newsub;

    MemoryContextSwitchTo(oldctx);

    /* Change synchronous commit according to the user's wishes */
    SetConfigOption("synchronous_commit", MySubscription->synccommit,
                    PGC_BACKEND, PGC_S_OVERRIDE);

    if (started_tx)
        CommitTransactionCommand();

    MySubscriptionValid = true;
}

/*
 * Callback from subscription syscache invalidation.
 */
static void
subscription_change_cb(Datum arg, int cacheid, uint32 hashvalue)
{
    MySubscriptionValid = false;
}

/* Logical Replication Apply worker entry point */
void
ApplyWorkerMain(Datum main_arg)
{
    int         worker_slot = DatumGetInt32(main_arg);
    MemoryContext oldctx;
    char        originname[NAMEDATALEN];
    XLogRecPtr  origin_startpos;
    char       *myslotname;
    WalRcvStreamOptions options;

    /* Attach to slot */
    logicalrep_worker_attach(worker_slot);

    /* Setup signal handling */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGTERM, die);
    BackgroundWorkerUnblockSignals();

    /*
     * We don't currently need any ResourceOwner in a walreceiver process, but
     * if we did, we could call CreateAuxProcessResourceOwner here.
     */

    /* Initialise stats to a sanish value */
    MyLogicalRepWorker->last_send_time = MyLogicalRepWorker->last_recv_time =
        MyLogicalRepWorker->reply_time = GetCurrentTimestamp();

    /* Load the libpq-specific functions */
    load_file("libpqwalreceiver", false);

    /* Run as replica session replication role. */
    SetConfigOption("session_replication_role", "replica",
                    PGC_SUSET, PGC_S_OVERRIDE);

    /* Connect to our database. */
    BackgroundWorkerInitializeConnectionByOid(MyLogicalRepWorker->dbid,
                                              MyLogicalRepWorker->userid,
                                              0);

    /* Load the subscription into persistent memory context. */
    ApplyContext = AllocSetContextCreate(TopMemoryContext,
                                         "ApplyContext",
                                         ALLOCSET_DEFAULT_SIZES);
    StartTransactionCommand();
    oldctx = MemoryContextSwitchTo(ApplyContext);

    MySubscription = GetSubscription(MyLogicalRepWorker->subid, true);
    if (!MySubscription)
    {
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription %u will not "
                        "start because the subscription was removed during startup",
                        MyLogicalRepWorker->subid)));
        proc_exit(0);
    }

    MySubscriptionValid = true;
    MemoryContextSwitchTo(oldctx);

    if (!MySubscription->enabled)
    {
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription \"%s\" will not "
                        "start because the subscription was disabled during startup",
                        MySubscription->name)));

        proc_exit(0);
    }

    /* Setup synchronous commit according to the user's wishes */
    SetConfigOption("synchronous_commit", MySubscription->synccommit,
                    PGC_BACKEND, PGC_S_OVERRIDE);

    /* Keep us informed about subscription changes. */
    CacheRegisterSyscacheCallback(SUBSCRIPTIONOID,
                                  subscription_change_cb,
                                  (Datum) 0);

    if (am_tablesync_worker())
        ereport(LOG,
                (errmsg("logical replication table synchronization worker for subscription \"%s\", table \"%s\" has started",
                        MySubscription->name, get_rel_name(MyLogicalRepWorker->relid))));
    else
        ereport(LOG,
                (errmsg("logical replication apply worker for subscription \"%s\" has started",
                        MySubscription->name)));

    CommitTransactionCommand();

    /* Connect to the origin and start the replication. */
    elog(DEBUG1, "connecting to publisher using connection string \"%s\"",
         MySubscription->conninfo);

    if (am_tablesync_worker())
    {
        char       *syncslotname;

        /* This is table synchronization worker, call initial sync. */
        syncslotname = LogicalRepSyncTableStart(&origin_startpos);

        /* The slot name needs to be allocated in permanent memory context. */
        oldctx = MemoryContextSwitchTo(ApplyContext);
        myslotname = pstrdup(syncslotname);
        MemoryContextSwitchTo(oldctx);

        pfree(syncslotname);
    }
    else
    {
        /* This is main apply worker */
        RepOriginId originid;
        TimeLineID  startpointTLI;
        char       *err;

        myslotname = MySubscription->slotname;

        /*
         * This shouldn't happen if the subscription is enabled, but guard
         * against DDL bugs or manual catalog changes.  (libpqwalreceiver will
         * crash if slot is NULL.)
         */
        if (!myslotname)
            ereport(ERROR,
                    (errmsg("subscription has no replication slot set")));

        /* Setup replication origin tracking. */
        StartTransactionCommand();
        snprintf(originname, sizeof(originname), "pg_%u", MySubscription->oid);
        originid = replorigin_by_name(originname, true);
        if (!OidIsValid(originid))
            originid = replorigin_create(originname);
        replorigin_session_setup(originid);
        replorigin_session_origin = originid;
        origin_startpos = replorigin_session_get_progress(false);
        CommitTransactionCommand();

        wrconn = walrcv_connect(MySubscription->conninfo, true, MySubscription->name,
                                &err);
        if (wrconn == NULL)
            ereport(ERROR,
                    (errmsg("could not connect to the publisher: %s", err)));

        /*
         * We don't really use the output identify_system for anything but it
         * does some initializations on the upstream so let's still call it.
         */
        (void) walrcv_identify_system(wrconn, &startpointTLI);

    }

    /*
     * Setup callback for syscache so that we know when something changes in
     * the subscription relation state.
     */
    CacheRegisterSyscacheCallback(SUBSCRIPTIONRELMAP,
                                  invalidate_syncing_table_states,
                                  (Datum) 0);

    /* Build logical replication streaming options. */
    options.logical = true;
    options.startpoint = origin_startpos;
    options.slotname = myslotname;
    options.proto.logical.proto_version = LOGICALREP_PROTO_VERSION_NUM;
    options.proto.logical.publication_names = MySubscription->publications;

    /* Start normal logical streaming replication. */
    walrcv_startstreaming(wrconn, &options);

    /* Run the main loop. */
    LogicalRepApplyLoop(origin_startpos);

    proc_exit(0);
}

/*
 * Is current process a logical replication worker?
 */
bool
IsLogicalWorker(void)
{
    return MyLogicalRepWorker != NULL;
}