slotfuncs.c

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/*-------------------------------------------------------------------------
 *
 * slotfuncs.c
 *     Support functions for replication slots
 *
 * Copyright (c) 2012-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/slotfuncs.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/htup_details.h"
#include "access/xlog_internal.h"
#include "access/xlogutils.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "replication/decode.h"
#include "replication/logical.h"
#include "replication/slot.h"
#include "utils/builtins.h"
#include "utils/inval.h"
#include "utils/pg_lsn.h"
#include "utils/resowner.h"

static void
check_permissions(void)
{
    if (!superuser() && !has_rolreplication(GetUserId()))
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser or replication role to use replication slots")));
}

/*
 * Helper function for creating a new physical replication slot with
 * given arguments. Note that this function doesn't release the created
 * slot.
 *
 * If restart_lsn is a valid value, we use it without WAL reservation
 * routine. So the caller must guarantee that WAL is available.
 */
static void
create_physical_replication_slot(char *name, bool immediately_reserve,
                                 bool temporary, XLogRecPtr restart_lsn)
{
    Assert(!MyReplicationSlot);

    /* acquire replication slot, this will check for conflicting names */
    ReplicationSlotCreate(name, false,
                          temporary ? RS_TEMPORARY : RS_PERSISTENT, false);

    if (immediately_reserve)
    {
        /* Reserve WAL as the user asked for it */
        if (XLogRecPtrIsInvalid(restart_lsn))
            ReplicationSlotReserveWal();
        else
            MyReplicationSlot->data.restart_lsn = restart_lsn;

        /* Write this slot to disk */
        ReplicationSlotMarkDirty();
        ReplicationSlotSave();
    }
}

/*
 * SQL function for creating a new physical (streaming replication)
 * replication slot.
 */
Datum
pg_create_physical_replication_slot(PG_FUNCTION_ARGS)
{
    Name        name = PG_GETARG_NAME(0);
    bool        immediately_reserve = PG_GETARG_BOOL(1);
    bool        temporary = PG_GETARG_BOOL(2);
    Datum       values[2];
    bool        nulls[2];
    TupleDesc   tupdesc;
    HeapTuple   tuple;
    Datum       result;

    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");

    check_permissions();

    CheckSlotRequirements();

    create_physical_replication_slot(NameStr(*name),
                                     immediately_reserve,
                                     temporary,
                                     InvalidXLogRecPtr);

    values[0] = NameGetDatum(&MyReplicationSlot->data.name);
    nulls[0] = false;

    if (immediately_reserve)
    {
        values[1] = LSNGetDatum(MyReplicationSlot->data.restart_lsn);
        nulls[1] = false;
    }
    else
        nulls[1] = true;

    tuple = heap_form_tuple(tupdesc, values, nulls);
    result = HeapTupleGetDatum(tuple);

    ReplicationSlotRelease();

    PG_RETURN_DATUM(result);
}


/*
 * Helper function for creating a new logical replication slot with
 * given arguments. Note that this function doesn't release the created
 * slot.
 *
 * When find_startpoint is false, the slot's confirmed_flush is not set; it's
 * caller's responsibility to ensure it's set to something sensible.
 */
static void
create_logical_replication_slot(char *name, char *plugin,
                                bool temporary, bool two_phase,
                                XLogRecPtr restart_lsn,
                                bool find_startpoint)
{
    LogicalDecodingContext *ctx = NULL;

    Assert(!MyReplicationSlot);

    /*
     * Acquire a logical decoding slot, this will check for conflicting names.
     * Initially create persistent slot as ephemeral - that allows us to
     * nicely handle errors during initialization because it'll get dropped if
     * this transaction fails. We'll make it persistent at the end. Temporary
     * slots can be created as temporary from beginning as they get dropped on
     * error as well.
     */
    ReplicationSlotCreate(name, true,
                          temporary ? RS_TEMPORARY : RS_EPHEMERAL, two_phase);

    /*
     * Create logical decoding context to find start point or, if we don't
     * need it, to 1) bump slot's restart_lsn and xmin 2) check plugin sanity.
     *
     * Note: when !find_startpoint this is still important, because it's at
     * this point that the output plugin is validated.
     */
    ctx = CreateInitDecodingContext(plugin, NIL,
                                    false,  /* just catalogs is OK */
                                    restart_lsn,
                                    XL_ROUTINE(.page_read = read_local_xlog_page,
                                               .segment_open = wal_segment_open,
                                               .segment_close = wal_segment_close),
                                    NULL, NULL, NULL);

    /*
     * If caller needs us to determine the decoding start point, do so now.
     * This might take a while.
     */
    if (find_startpoint)
        DecodingContextFindStartpoint(ctx);

    /* don't need the decoding context anymore */
    FreeDecodingContext(ctx);
}

/*
 * SQL function for creating a new logical replication slot.
 */
Datum
pg_create_logical_replication_slot(PG_FUNCTION_ARGS)
{
    Name        name = PG_GETARG_NAME(0);
    Name        plugin = PG_GETARG_NAME(1);
    bool        temporary = PG_GETARG_BOOL(2);
    bool        two_phase = PG_GETARG_BOOL(3);
    Datum       result;
    TupleDesc   tupdesc;
    HeapTuple   tuple;
    Datum       values[2];
    bool        nulls[2];

    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");

    check_permissions();

    CheckLogicalDecodingRequirements();

    create_logical_replication_slot(NameStr(*name),
                                    NameStr(*plugin),
                                    temporary,
                                    two_phase,
                                    InvalidXLogRecPtr,
                                    true);

    values[0] = NameGetDatum(&MyReplicationSlot->data.name);
    values[1] = LSNGetDatum(MyReplicationSlot->data.confirmed_flush);

    memset(nulls, 0, sizeof(nulls));

    tuple = heap_form_tuple(tupdesc, values, nulls);
    result = HeapTupleGetDatum(tuple);

    /* ok, slot is now fully created, mark it as persistent if needed */
    if (!temporary)
        ReplicationSlotPersist();
    ReplicationSlotRelease();

    PG_RETURN_DATUM(result);
}


/*
 * SQL function for dropping a replication slot.
 */
Datum
pg_drop_replication_slot(PG_FUNCTION_ARGS)
{
    Name        name = PG_GETARG_NAME(0);

    check_permissions();

    CheckSlotRequirements();

    ReplicationSlotDrop(NameStr(*name), true);

    PG_RETURN_VOID();
}

/*
 * pg_get_replication_slots - SQL SRF showing active replication slots.
 */
Datum
pg_get_replication_slots(PG_FUNCTION_ARGS)
{
#define PG_GET_REPLICATION_SLOTS_COLS 14
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    TupleDesc   tupdesc;
    Tuplestorestate *tupstore;
    MemoryContext per_query_ctx;
    MemoryContext oldcontext;
    XLogRecPtr  currlsn;
    int         slotno;

    /* check to see if caller supports us returning a tuplestore */
    if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("set-valued function called in context that cannot accept a set")));
    if (!(rsinfo->allowedModes & SFRM_Materialize))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("materialize mode required, but it is not allowed in this context")));

    /* Build a tuple descriptor for our result type */
    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");

    /*
     * We don't require any special permission to see this function's data
     * because nothing should be sensitive. The most critical being the slot
     * name, which shouldn't contain anything particularly sensitive.
     */

    per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
    oldcontext = MemoryContextSwitchTo(per_query_ctx);

    tupstore = tuplestore_begin_heap(true, false, work_mem);
    rsinfo->returnMode = SFRM_Materialize;
    rsinfo->setResult = tupstore;
    rsinfo->setDesc = tupdesc;

    MemoryContextSwitchTo(oldcontext);

    currlsn = GetXLogWriteRecPtr();

    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    for (slotno = 0; slotno < max_replication_slots; slotno++)
    {
        ReplicationSlot *slot = &ReplicationSlotCtl->replication_slots[slotno];
        ReplicationSlot slot_contents;
        Datum       values[PG_GET_REPLICATION_SLOTS_COLS];
        bool        nulls[PG_GET_REPLICATION_SLOTS_COLS];
        WALAvailability walstate;
        int         i;

        if (!slot->in_use)
            continue;

        /* Copy slot contents while holding spinlock, then examine at leisure */
        SpinLockAcquire(&slot->mutex);
        slot_contents = *slot;
        SpinLockRelease(&slot->mutex);

        memset(values, 0, sizeof(values));
        memset(nulls, 0, sizeof(nulls));

        i = 0;
        values[i++] = NameGetDatum(&slot_contents.data.name);

        if (slot_contents.data.database == InvalidOid)
            nulls[i++] = true;
        else
            values[i++] = NameGetDatum(&slot_contents.data.plugin);

        if (slot_contents.data.database == InvalidOid)
            values[i++] = CStringGetTextDatum("physical");
        else
            values[i++] = CStringGetTextDatum("logical");

        if (slot_contents.data.database == InvalidOid)
            nulls[i++] = true;
        else
            values[i++] = ObjectIdGetDatum(slot_contents.data.database);

        values[i++] = BoolGetDatum(slot_contents.data.persistency == RS_TEMPORARY);
        values[i++] = BoolGetDatum(slot_contents.active_pid != 0);

        if (slot_contents.active_pid != 0)
            values[i++] = Int32GetDatum(slot_contents.active_pid);
        else
            nulls[i++] = true;

        if (slot_contents.data.xmin != InvalidTransactionId)
            values[i++] = TransactionIdGetDatum(slot_contents.data.xmin);
        else
            nulls[i++] = true;

        if (slot_contents.data.catalog_xmin != InvalidTransactionId)
            values[i++] = TransactionIdGetDatum(slot_contents.data.catalog_xmin);
        else
            nulls[i++] = true;

        if (slot_contents.data.restart_lsn != InvalidXLogRecPtr)
            values[i++] = LSNGetDatum(slot_contents.data.restart_lsn);
        else
            nulls[i++] = true;

        if (slot_contents.data.confirmed_flush != InvalidXLogRecPtr)
            values[i++] = LSNGetDatum(slot_contents.data.confirmed_flush);
        else
            nulls[i++] = true;

        /*
         * If invalidated_at is valid and restart_lsn is invalid, we know for
         * certain that the slot has been invalidated.  Otherwise, test
         * availability from restart_lsn.
         */
        if (XLogRecPtrIsInvalid(slot_contents.data.restart_lsn) &&
            !XLogRecPtrIsInvalid(slot_contents.data.invalidated_at))
            walstate = WALAVAIL_REMOVED;
        else
            walstate = GetWALAvailability(slot_contents.data.restart_lsn);

        switch (walstate)
        {
            case WALAVAIL_INVALID_LSN:
                nulls[i++] = true;
                break;

            case WALAVAIL_RESERVED:
                values[i++] = CStringGetTextDatum("reserved");
                break;

            case WALAVAIL_EXTENDED:
                values[i++] = CStringGetTextDatum("extended");
                break;

            case WALAVAIL_UNRESERVED:
                values[i++] = CStringGetTextDatum("unreserved");
                break;

            case WALAVAIL_REMOVED:

                /*
                 * If we read the restart_lsn long enough ago, maybe that file
                 * has been removed by now.  However, the walsender could have
                 * moved forward enough that it jumped to another file after
                 * we looked.  If checkpointer signalled the process to
                 * termination, then it's definitely lost; but if a process is
                 * still alive, then "unreserved" seems more appropriate.
                 *
                 * If we do change it, save the state for safe_wal_size below.
                 */
                if (!XLogRecPtrIsInvalid(slot_contents.data.restart_lsn))
                {
                    int         pid;

                    SpinLockAcquire(&slot->mutex);
                    pid = slot->active_pid;
                    slot_contents.data.restart_lsn = slot->data.restart_lsn;
                    SpinLockRelease(&slot->mutex);
                    if (pid != 0)
                    {
                        values[i++] = CStringGetTextDatum("unreserved");
                        walstate = WALAVAIL_UNRESERVED;
                        break;
                    }
                }
                values[i++] = CStringGetTextDatum("lost");
                break;
        }

        /*
         * safe_wal_size is only computed for slots that have not been lost,
         * and only if there's a configured maximum size.
         */
        if (walstate == WALAVAIL_REMOVED || max_slot_wal_keep_size_mb < 0)
            nulls[i++] = true;
        else
        {
            XLogSegNo   targetSeg;
            uint64      slotKeepSegs;
            uint64      keepSegs;
            XLogSegNo   failSeg;
            XLogRecPtr  failLSN;

            XLByteToSeg(slot_contents.data.restart_lsn, targetSeg, wal_segment_size);

            /* determine how many segments slots can be kept by slots */
            slotKeepSegs = XLogMBVarToSegs(max_slot_wal_keep_size_mb, wal_segment_size);
            /* ditto for wal_keep_size */
            keepSegs = XLogMBVarToSegs(wal_keep_size_mb, wal_segment_size);

            /* if currpos reaches failLSN, we lose our segment */
            failSeg = targetSeg + Max(slotKeepSegs, keepSegs) + 1;
            XLogSegNoOffsetToRecPtr(failSeg, 0, wal_segment_size, failLSN);

            values[i++] = Int64GetDatum(failLSN - currlsn);
        }

        values[i++] = BoolGetDatum(slot_contents.data.two_phase);

        Assert(i == PG_GET_REPLICATION_SLOTS_COLS);

        tuplestore_putvalues(tupstore, tupdesc, values, nulls);
    }

    LWLockRelease(ReplicationSlotControlLock);

    tuplestore_donestoring(tupstore);

    return (Datum) 0;
}

/*
 * Helper function for advancing our physical replication slot forward.
 *
 * The LSN position to move to is compared simply to the slot's restart_lsn,
 * knowing that any position older than that would be removed by successive
 * checkpoints.
 */
static XLogRecPtr
pg_physical_replication_slot_advance(XLogRecPtr moveto)
{
    XLogRecPtr  startlsn = MyReplicationSlot->data.restart_lsn;
    XLogRecPtr  retlsn = startlsn;

    Assert(moveto != InvalidXLogRecPtr);

    if (startlsn < moveto)
    {
        SpinLockAcquire(&MyReplicationSlot->mutex);
        MyReplicationSlot->data.restart_lsn = moveto;
        SpinLockRelease(&MyReplicationSlot->mutex);
        retlsn = moveto;

        /*
         * Dirty the slot so as it is written out at the next checkpoint. Note
         * that the LSN position advanced may still be lost in the event of a
         * crash, but this makes the data consistent after a clean shutdown.
         */
        ReplicationSlotMarkDirty();
    }

    return retlsn;
}

/*
 * Helper function for advancing our logical replication slot forward.
 *
 * The slot's restart_lsn is used as start point for reading records, while
 * confirmed_flush is used as base point for the decoding context.
 *
 * We cannot just do LogicalConfirmReceivedLocation to update confirmed_flush,
 * because we need to digest WAL to advance restart_lsn allowing to recycle
 * WAL and removal of old catalog tuples.  As decoding is done in fast_forward
 * mode, no changes are generated anyway.
 */
static XLogRecPtr
pg_logical_replication_slot_advance(XLogRecPtr moveto)
{
    LogicalDecodingContext *ctx;
    ResourceOwner old_resowner = CurrentResourceOwner;
    XLogRecPtr  retlsn;

    Assert(moveto != InvalidXLogRecPtr);

    PG_TRY();
    {
        /*
         * Create our decoding context in fast_forward mode, passing start_lsn
         * as InvalidXLogRecPtr, so that we start processing from my slot's
         * confirmed_flush.
         */
        ctx = CreateDecodingContext(InvalidXLogRecPtr,
                                    NIL,
                                    true,   /* fast_forward */
                                    XL_ROUTINE(.page_read = read_local_xlog_page,
                                               .segment_open = wal_segment_open,
                                               .segment_close = wal_segment_close),
                                    NULL, NULL, NULL);

        /*
         * Start reading at the slot's restart_lsn, which we know to point to
         * a valid record.
         */
        XLogBeginRead(ctx->reader, MyReplicationSlot->data.restart_lsn);

        /* invalidate non-timetravel entries */
        InvalidateSystemCaches();

        /* Decode at least one record, until we run out of records */
        while (ctx->reader->EndRecPtr < moveto)
        {
            char       *errm = NULL;
            XLogRecord *record;

            /*
             * Read records.  No changes are generated in fast_forward mode,
             * but snapbuilder/slot statuses are updated properly.
             */
            record = XLogReadRecord(ctx->reader, &errm);
            if (errm)
                elog(ERROR, "%s", errm);

            /*
             * Process the record.  Storage-level changes are ignored in
             * fast_forward mode, but other modules (such as snapbuilder)
             * might still have critical updates to do.
             */
            if (record)
                LogicalDecodingProcessRecord(ctx, ctx->reader);

            /* Stop once the requested target has been reached */
            if (moveto <= ctx->reader->EndRecPtr)
                break;

            CHECK_FOR_INTERRUPTS();
        }

        /*
         * Logical decoding could have clobbered CurrentResourceOwner during
         * transaction management, so restore the executor's value.  (This is
         * a kluge, but it's not worth cleaning up right now.)
         */
        CurrentResourceOwner = old_resowner;

        if (ctx->reader->EndRecPtr != InvalidXLogRecPtr)
        {
            LogicalConfirmReceivedLocation(moveto);

            /*
             * If only the confirmed_flush LSN has changed the slot won't get
             * marked as dirty by the above. Callers on the walsender
             * interface are expected to keep track of their own progress and
             * don't need it written out. But SQL-interface users cannot
             * specify their own start positions and it's harder for them to
             * keep track of their progress, so we should make more of an
             * effort to save it for them.
             *
             * Dirty the slot so it is written out at the next checkpoint. The
             * LSN position advanced to may still be lost on a crash but this
             * makes the data consistent after a clean shutdown.
             */
            ReplicationSlotMarkDirty();
        }

        retlsn = MyReplicationSlot->data.confirmed_flush;

        /* free context, call shutdown callback */
        FreeDecodingContext(ctx);

        InvalidateSystemCaches();
    }
    PG_CATCH();
    {
        /* clear all timetravel entries */
        InvalidateSystemCaches();

        PG_RE_THROW();
    }
    PG_END_TRY();

    return retlsn;
}

/*
 * SQL function for moving the position in a replication slot.
 */
Datum
pg_replication_slot_advance(PG_FUNCTION_ARGS)
{
    Name        slotname = PG_GETARG_NAME(0);
    XLogRecPtr  moveto = PG_GETARG_LSN(1);
    XLogRecPtr  endlsn;
    XLogRecPtr  minlsn;
    TupleDesc   tupdesc;
    Datum       values[2];
    bool        nulls[2];
    HeapTuple   tuple;
    Datum       result;

    Assert(!MyReplicationSlot);

    check_permissions();

    if (XLogRecPtrIsInvalid(moveto))
        ereport(ERROR,
                (errmsg("invalid target WAL LSN")));

    /* Build a tuple descriptor for our result type */
    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");

    /*
     * We can't move slot past what's been flushed/replayed so clamp the
     * target position accordingly.
     */
    if (!RecoveryInProgress())
        moveto = Min(moveto, GetFlushRecPtr());
    else
        moveto = Min(moveto, GetXLogReplayRecPtr(&ThisTimeLineID));

    /* Acquire the slot so we "own" it */
    ReplicationSlotAcquire(NameStr(*slotname), true);

    /* A slot whose restart_lsn has never been reserved cannot be advanced */
    if (XLogRecPtrIsInvalid(MyReplicationSlot->data.restart_lsn))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("replication slot \"%s\" cannot be advanced",
                        NameStr(*slotname)),
                 errdetail("This slot has never previously reserved WAL, or it has been invalidated.")));

    /*
     * Check if the slot is not moving backwards.  Physical slots rely simply
     * on restart_lsn as a minimum point, while logical slots have confirmed
     * consumption up to confirmed_flush, meaning that in both cases data
     * older than that is not available anymore.
     */
    if (OidIsValid(MyReplicationSlot->data.database))
        minlsn = MyReplicationSlot->data.confirmed_flush;
    else
        minlsn = MyReplicationSlot->data.restart_lsn;

    if (moveto < minlsn)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("cannot advance replication slot to %X/%X, minimum is %X/%X",
                        LSN_FORMAT_ARGS(moveto), LSN_FORMAT_ARGS(minlsn))));

    /* Do the actual slot update, depending on the slot type */
    if (OidIsValid(MyReplicationSlot->data.database))
        endlsn = pg_logical_replication_slot_advance(moveto);
    else
        endlsn = pg_physical_replication_slot_advance(moveto);

    values[0] = NameGetDatum(&MyReplicationSlot->data.name);
    nulls[0] = false;

    /*
     * Recompute the minimum LSN and xmin across all slots to adjust with the
     * advancing potentially done.
     */
    ReplicationSlotsComputeRequiredXmin(false);
    ReplicationSlotsComputeRequiredLSN();

    ReplicationSlotRelease();

    /* Return the reached position. */
    values[1] = LSNGetDatum(endlsn);
    nulls[1] = false;

    tuple = heap_form_tuple(tupdesc, values, nulls);
    result = HeapTupleGetDatum(tuple);

    PG_RETURN_DATUM(result);
}

/*
 * Helper function of copying a replication slot.
 */
static Datum
copy_replication_slot(FunctionCallInfo fcinfo, bool logical_slot)
{
    Name        src_name = PG_GETARG_NAME(0);
    Name        dst_name = PG_GETARG_NAME(1);
    ReplicationSlot *src = NULL;
    ReplicationSlot first_slot_contents;
    ReplicationSlot second_slot_contents;
    XLogRecPtr  src_restart_lsn;
    bool        src_islogical;
    bool        temporary;
    char       *plugin;
    Datum       values[2];
    bool        nulls[2];
    Datum       result;
    TupleDesc   tupdesc;
    HeapTuple   tuple;

    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");

    check_permissions();

    if (logical_slot)
        CheckLogicalDecodingRequirements();
    else
        CheckSlotRequirements();

    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);

    /*
     * We need to prevent the source slot's reserved WAL from being removed,
     * but we don't want to lock that slot for very long, and it can advance
     * in the meantime.  So obtain the source slot's data, and create a new
     * slot using its restart_lsn.  Afterwards we lock the source slot again
     * and verify that the data we copied (name, type) has not changed
     * incompatibly.  No inconvenient WAL removal can occur once the new slot
     * is created -- but since WAL removal could have occurred before we
     * managed to create the new slot, we advance the new slot's restart_lsn
     * to the source slot's updated restart_lsn the second time we lock it.
     */
    for (int i = 0; i < max_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];

        if (s->in_use && strcmp(NameStr(s->data.name), NameStr(*src_name)) == 0)
        {
            /* Copy the slot contents while holding spinlock */
            SpinLockAcquire(&s->mutex);
            first_slot_contents = *s;
            SpinLockRelease(&s->mutex);
            src = s;
            break;
        }
    }

    LWLockRelease(ReplicationSlotControlLock);

    if (src == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("replication slot \"%s\" does not exist", NameStr(*src_name))));

    src_islogical = SlotIsLogical(&first_slot_contents);
    src_restart_lsn = first_slot_contents.data.restart_lsn;
    temporary = (first_slot_contents.data.persistency == RS_TEMPORARY);
    plugin = logical_slot ? NameStr(first_slot_contents.data.plugin) : NULL;

    /* Check type of replication slot */
    if (src_islogical != logical_slot)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 src_islogical ?
                 errmsg("cannot copy physical replication slot \"%s\" as a logical replication slot",
                        NameStr(*src_name)) :
                 errmsg("cannot copy logical replication slot \"%s\" as a physical replication slot",
                        NameStr(*src_name))));

    /* Copying non-reserved slot doesn't make sense */
    if (XLogRecPtrIsInvalid(src_restart_lsn))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("cannot copy a replication slot that doesn't reserve WAL")));

    /* Overwrite params from optional arguments */
    if (PG_NARGS() >= 3)
        temporary = PG_GETARG_BOOL(2);
    if (PG_NARGS() >= 4)
    {
        Assert(logical_slot);
        plugin = NameStr(*(PG_GETARG_NAME(3)));
    }

    /* Create new slot and acquire it */
    if (logical_slot)
    {
        /*
         * We must not try to read WAL, since we haven't reserved it yet --
         * hence pass find_startpoint false.  confirmed_flush will be set
         * below, by copying from the source slot.
         */
        create_logical_replication_slot(NameStr(*dst_name),
                                        plugin,
                                        temporary,
                                        false,
                                        src_restart_lsn,
                                        false);
    }
    else
        create_physical_replication_slot(NameStr(*dst_name),
                                         true,
                                         temporary,
                                         src_restart_lsn);

    /*
     * Update the destination slot to current values of the source slot;
     * recheck that the source slot is still the one we saw previously.
     */
    {
        TransactionId copy_effective_xmin;
        TransactionId copy_effective_catalog_xmin;
        TransactionId copy_xmin;
        TransactionId copy_catalog_xmin;
        XLogRecPtr  copy_restart_lsn;
        XLogRecPtr  copy_confirmed_flush;
        bool        copy_islogical;
        char       *copy_name;

        /* Copy data of source slot again */
        SpinLockAcquire(&src->mutex);
        second_slot_contents = *src;
        SpinLockRelease(&src->mutex);

        copy_effective_xmin = second_slot_contents.effective_xmin;
        copy_effective_catalog_xmin = second_slot_contents.effective_catalog_xmin;

        copy_xmin = second_slot_contents.data.xmin;
        copy_catalog_xmin = second_slot_contents.data.catalog_xmin;
        copy_restart_lsn = second_slot_contents.data.restart_lsn;
        copy_confirmed_flush = second_slot_contents.data.confirmed_flush;

        /* for existence check */
        copy_name = NameStr(second_slot_contents.data.name);
        copy_islogical = SlotIsLogical(&second_slot_contents);

        /*
         * Check if the source slot still exists and is valid. We regard it as
         * invalid if the type of replication slot or name has been changed,
         * or the restart_lsn either is invalid or has gone backward. (The
         * restart_lsn could go backwards if the source slot is dropped and
         * copied from an older slot during installation.)
         *
         * Since erroring out will release and drop the destination slot we
         * don't need to release it here.
         */
        if (copy_restart_lsn < src_restart_lsn ||
            src_islogical != copy_islogical ||
            strcmp(copy_name, NameStr(*src_name)) != 0)
            ereport(ERROR,
                    (errmsg("could not copy replication slot \"%s\"",
                            NameStr(*src_name)),
                     errdetail("The source replication slot was modified incompatibly during the copy operation.")));

        /* The source slot must have a consistent snapshot */
        if (src_islogical && XLogRecPtrIsInvalid(copy_confirmed_flush))
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("cannot copy unfinished logical replication slot \"%s\"",
                            NameStr(*src_name)),
                     errhint("Retry when the source replication slot's confirmed_flush_lsn is valid.")));

        /* Install copied values again */
        SpinLockAcquire(&MyReplicationSlot->mutex);
        MyReplicationSlot->effective_xmin = copy_effective_xmin;
        MyReplicationSlot->effective_catalog_xmin = copy_effective_catalog_xmin;

        MyReplicationSlot->data.xmin = copy_xmin;
        MyReplicationSlot->data.catalog_xmin = copy_catalog_xmin;
        MyReplicationSlot->data.restart_lsn = copy_restart_lsn;
        MyReplicationSlot->data.confirmed_flush = copy_confirmed_flush;
        SpinLockRelease(&MyReplicationSlot->mutex);

        ReplicationSlotMarkDirty();
        ReplicationSlotsComputeRequiredXmin(false);
        ReplicationSlotsComputeRequiredLSN();
        ReplicationSlotSave();

#ifdef USE_ASSERT_CHECKING
        /* Check that the restart_lsn is available */
        {
            XLogSegNo   segno;

            XLByteToSeg(copy_restart_lsn, segno, wal_segment_size);
            Assert(XLogGetLastRemovedSegno() < segno);
        }
#endif
    }

    /* target slot fully created, mark as persistent if needed */
    if (logical_slot && !temporary)
        ReplicationSlotPersist();

    /* All done.  Set up the return values */
    values[0] = NameGetDatum(dst_name);
    nulls[0] = false;
    if (!XLogRecPtrIsInvalid(MyReplicationSlot->data.confirmed_flush))
    {
        values[1] = LSNGetDatum(MyReplicationSlot->data.confirmed_flush);
        nulls[1] = false;
    }
    else
        nulls[1] = true;

    tuple = heap_form_tuple(tupdesc, values, nulls);
    result = HeapTupleGetDatum(tuple);

    ReplicationSlotRelease();

    PG_RETURN_DATUM(result);
}

/* The wrappers below are all to appease opr_sanity */
Datum
pg_copy_logical_replication_slot_a(PG_FUNCTION_ARGS)
{
    return copy_replication_slot(fcinfo, true);
}

Datum
pg_copy_logical_replication_slot_b(PG_FUNCTION_ARGS)
{
    return copy_replication_slot(fcinfo, true);
}

Datum
pg_copy_logical_replication_slot_c(PG_FUNCTION_ARGS)
{
    return copy_replication_slot(fcinfo, true);
}

Datum
pg_copy_physical_replication_slot_a(PG_FUNCTION_ARGS)
{
    return copy_replication_slot(fcinfo, false);
}

Datum
pg_copy_physical_replication_slot_b(PG_FUNCTION_ARGS)
{
    return copy_replication_slot(fcinfo, false);
}