logical.c

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/*-------------------------------------------------------------------------
 * logical.c
 *     PostgreSQL logical decoding coordination
 *
 * Copyright (c) 2012-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/logical/logical.c
 *
 * NOTES
 *    This file coordinates interaction between the various modules that
 *    together provide logical decoding, primarily by providing so
 *    called LogicalDecodingContexts. The goal is to encapsulate most of the
 *    internal complexity for consumers of logical decoding, so they can
 *    create and consume a changestream with a low amount of code. Builtin
 *    consumers are the walsender and SQL SRF interface, but it's possible to
 *    add further ones without changing core code, e.g. to consume changes in
 *    a bgworker.
 *
 *    The idea is that a consumer provides three callbacks, one to read WAL,
 *    one to prepare a data write, and a final one for actually writing since
 *    their implementation depends on the type of consumer.  Check
 *    logicalfuncs.c for an example implementation of a fairly simple consumer
 *    and an implementation of a WAL reading callback that's suitable for
 *    simple consumers.
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogutils.h"
#include "fmgr.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "replication/decode.h"
#include "replication/logical.h"
#include "replication/reorderbuffer.h"
#include "replication/slotsync.h"
#include "replication/snapbuild.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/builtins.h"
#include "utils/injection_point.h"
#include "utils/inval.h"
#include "utils/memutils.h"
 
/* data for errcontext callback */
typedef struct LogicalErrorCallbackState
{
    LogicalDecodingContext *ctx;
    const char *callback_name;
    XLogRecPtr  report_location;
} LogicalErrorCallbackState;
 
/* wrappers around output plugin callbacks */
static void output_plugin_error_callback(void *arg);
static void startup_cb_wrapper(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
                               bool is_init);
static void shutdown_cb_wrapper(LogicalDecodingContext *ctx);
static void begin_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn);
static void commit_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                              XLogRecPtr commit_lsn);
static void begin_prepare_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn);
static void prepare_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                               XLogRecPtr prepare_lsn);
static void commit_prepared_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                       XLogRecPtr commit_lsn);
static void rollback_prepared_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                         XLogRecPtr prepare_end_lsn, TimestampTz prepare_time);
static void change_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                              Relation relation, ReorderBufferChange *change);
static void truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                int nrelations, Relation relations[], ReorderBufferChange *change);
static void message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                               XLogRecPtr message_lsn, bool transactional,
                               const char *prefix, Size message_size, const char *message);
 
/* streaming callbacks */
static void stream_start_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                    XLogRecPtr first_lsn);
static void stream_stop_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                   XLogRecPtr last_lsn);
static void stream_abort_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                    XLogRecPtr abort_lsn);
static void stream_prepare_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                      XLogRecPtr prepare_lsn);
static void stream_commit_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                     XLogRecPtr commit_lsn);
static void stream_change_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                     Relation relation, ReorderBufferChange *change);
static void stream_message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                      XLogRecPtr message_lsn, bool transactional,
                                      const char *prefix, Size message_size, const char *message);
static void stream_truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                                       int nrelations, Relation relations[], ReorderBufferChange *change);
 
/* callback to update txn's progress */
static void update_progress_txn_cb_wrapper(ReorderBuffer *cache,
                                           ReorderBufferTXN *txn,
                                           XLogRecPtr lsn);
 
static void LoadOutputPlugin(OutputPluginCallbacks *callbacks, const char *plugin);
 
/*
 * Make sure the current settings & environment are capable of doing logical
 * decoding.
 */
void
CheckLogicalDecodingRequirements(void)
{
    CheckSlotRequirements();
 
    /*
     * NB: Adding a new requirement likely means that RestoreSlotFromDisk()
     * needs the same check.
     */
 
    if (wal_level < WAL_LEVEL_LOGICAL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical decoding requires \"wal_level\" >= \"logical\"")));
 
    if (MyDatabaseId == InvalidOid)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical decoding requires a database connection")));
 
    if (RecoveryInProgress())
    {
        /*
         * This check may have race conditions, but whenever
         * XLOG_PARAMETER_CHANGE indicates that wal_level has changed, we
         * verify that there are no existing logical replication slots. And to
         * avoid races around creating a new slot,
         * CheckLogicalDecodingRequirements() is called once before creating
         * the slot, and once when logical decoding is initially starting up.
         */
        if (GetActiveWalLevelOnStandby() < WAL_LEVEL_LOGICAL)
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("logical decoding on standby requires \"wal_level\" >= \"logical\" on the primary")));
    }
}
 
/*
 * Helper function for CreateInitDecodingContext() and
 * CreateDecodingContext() performing common tasks.
 */
static LogicalDecodingContext *
StartupDecodingContext(List *output_plugin_options,
                       XLogRecPtr start_lsn,
                       TransactionId xmin_horizon,
                       bool need_full_snapshot,
                       bool fast_forward,
                       bool in_create,
                       XLogReaderRoutine *xl_routine,
                       LogicalOutputPluginWriterPrepareWrite prepare_write,
                       LogicalOutputPluginWriterWrite do_write,
                       LogicalOutputPluginWriterUpdateProgress update_progress)
{
    ReplicationSlot *slot;
    MemoryContext context,
                old_context;
    LogicalDecodingContext *ctx;
 
    /* shorter lines... */
    slot = MyReplicationSlot;
 
    context = AllocSetContextCreate(CurrentMemoryContext,
                                    "Logical decoding context",
                                    ALLOCSET_DEFAULT_SIZES);
    old_context = MemoryContextSwitchTo(context);
    ctx = palloc0(sizeof(LogicalDecodingContext));
 
    ctx->context = context;
 
    /*
     * (re-)load output plugins, so we detect a bad (removed) output plugin
     * now.
     */
    if (!fast_forward)
        LoadOutputPlugin(&ctx->callbacks, NameStr(slot->data.plugin));
 
    /*
     * Now that the slot's xmin has been set, we can announce ourselves as a
     * logical decoding backend which doesn't need to be checked individually
     * when computing the xmin horizon because the xmin is enforced via
     * replication slots.
     *
     * We can only do so if we're outside of a transaction (i.e. the case when
     * streaming changes via walsender), otherwise an already setup
     * snapshot/xid would end up being ignored. That's not a particularly
     * bothersome restriction since the SQL interface can't be used for
     * streaming anyway.
     */
    if (!IsTransactionOrTransactionBlock())
    {
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
        MyProc->statusFlags |= PROC_IN_LOGICAL_DECODING;
        ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
        LWLockRelease(ProcArrayLock);
    }
 
    ctx->slot = slot;
 
    ctx->reader = XLogReaderAllocate(wal_segment_size, NULL, xl_routine, ctx);
    if (!ctx->reader)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory"),
                 errdetail("Failed while allocating a WAL reading processor.")));
 
    ctx->reorder = ReorderBufferAllocate();
    ctx->snapshot_builder =
        AllocateSnapshotBuilder(ctx->reorder, xmin_horizon, start_lsn,
                                need_full_snapshot, in_create, slot->data.two_phase_at);
 
    ctx->reorder->private_data = ctx;
 
    /* wrap output plugin callbacks, so we can add error context information */
    ctx->reorder->begin = begin_cb_wrapper;
    ctx->reorder->apply_change = change_cb_wrapper;
    ctx->reorder->apply_truncate = truncate_cb_wrapper;
    ctx->reorder->commit = commit_cb_wrapper;
    ctx->reorder->message = message_cb_wrapper;
 
    /*
     * To support streaming, we require start/stop/abort/commit/change
     * callbacks. The message and truncate callbacks are optional, similar to
     * regular output plugins. We however enable streaming when at least one
     * of the methods is enabled so that we can easily identify missing
     * methods.
     *
     * We decide it here, but only check it later in the wrappers.
     */
    ctx->streaming = (ctx->callbacks.stream_start_cb != NULL) ||
        (ctx->callbacks.stream_stop_cb != NULL) ||
        (ctx->callbacks.stream_abort_cb != NULL) ||
        (ctx->callbacks.stream_commit_cb != NULL) ||
        (ctx->callbacks.stream_change_cb != NULL) ||
        (ctx->callbacks.stream_message_cb != NULL) ||
        (ctx->callbacks.stream_truncate_cb != NULL);
 
    /*
     * streaming callbacks
     *
     * stream_message and stream_truncate callbacks are optional, so we do not
     * fail with ERROR when missing, but the wrappers simply do nothing. We
     * must set the ReorderBuffer callbacks to something, otherwise the calls
     * from there will crash (we don't want to move the checks there).
     */
    ctx->reorder->stream_start = stream_start_cb_wrapper;
    ctx->reorder->stream_stop = stream_stop_cb_wrapper;
    ctx->reorder->stream_abort = stream_abort_cb_wrapper;
    ctx->reorder->stream_prepare = stream_prepare_cb_wrapper;
    ctx->reorder->stream_commit = stream_commit_cb_wrapper;
    ctx->reorder->stream_change = stream_change_cb_wrapper;
    ctx->reorder->stream_message = stream_message_cb_wrapper;
    ctx->reorder->stream_truncate = stream_truncate_cb_wrapper;
 
 
    /*
     * To support two-phase logical decoding, we require
     * begin_prepare/prepare/commit-prepare/abort-prepare callbacks. The
     * filter_prepare callback is optional. We however enable two-phase
     * logical decoding when at least one of the methods is enabled so that we
     * can easily identify missing methods.
     *
     * We decide it here, but only check it later in the wrappers.
     */
    ctx->twophase = (ctx->callbacks.begin_prepare_cb != NULL) ||
        (ctx->callbacks.prepare_cb != NULL) ||
        (ctx->callbacks.commit_prepared_cb != NULL) ||
        (ctx->callbacks.rollback_prepared_cb != NULL) ||
        (ctx->callbacks.stream_prepare_cb != NULL) ||
        (ctx->callbacks.filter_prepare_cb != NULL);
 
    /*
     * Callback to support decoding at prepare time.
     */
    ctx->reorder->begin_prepare = begin_prepare_cb_wrapper;
    ctx->reorder->prepare = prepare_cb_wrapper;
    ctx->reorder->commit_prepared = commit_prepared_cb_wrapper;
    ctx->reorder->rollback_prepared = rollback_prepared_cb_wrapper;
 
    /*
     * Callback to support updating progress during sending data of a
     * transaction (and its subtransactions) to the output plugin.
     */
    ctx->reorder->update_progress_txn = update_progress_txn_cb_wrapper;
 
    ctx->out = makeStringInfo();
    ctx->prepare_write = prepare_write;
    ctx->write = do_write;
    ctx->update_progress = update_progress;
 
    ctx->output_plugin_options = output_plugin_options;
 
    ctx->fast_forward = fast_forward;
 
    MemoryContextSwitchTo(old_context);
 
    return ctx;
}
 
/*
 * Create a new decoding context, for a new logical slot.
 *
 * plugin -- contains the name of the output plugin
 * output_plugin_options -- contains options passed to the output plugin
 * need_full_snapshot -- if true, must obtain a snapshot able to read all
 *      tables; if false, one that can read only catalogs is acceptable.
 * restart_lsn -- if given as invalid, it's this routine's responsibility to
 *      mark WAL as reserved by setting a convenient restart_lsn for the slot.
 *      Otherwise, we set for decoding to start from the given LSN without
 *      marking WAL reserved beforehand.  In that scenario, it's up to the
 *      caller to guarantee that WAL remains available.
 * xl_routine -- XLogReaderRoutine for underlying XLogReader
 * prepare_write, do_write, update_progress --
 *      callbacks that perform the use-case dependent, actual, work.
 *
 * Needs to be called while in a memory context that's at least as long lived
 * as the decoding context because further memory contexts will be created
 * inside it.
 *
 * Returns an initialized decoding context after calling the output plugin's
 * startup function.
 */
LogicalDecodingContext *
CreateInitDecodingContext(const char *plugin,
                          List *output_plugin_options,
                          bool need_full_snapshot,
                          XLogRecPtr restart_lsn,
                          XLogReaderRoutine *xl_routine,
                          LogicalOutputPluginWriterPrepareWrite prepare_write,
                          LogicalOutputPluginWriterWrite do_write,
                          LogicalOutputPluginWriterUpdateProgress update_progress)
{
    TransactionId xmin_horizon = InvalidTransactionId;
    ReplicationSlot *slot;
    NameData    plugin_name;
    LogicalDecodingContext *ctx;
    MemoryContext old_context;
 
    /*
     * On a standby, this check is also required while creating the slot.
     * Check the comments in the function.
     */
    CheckLogicalDecodingRequirements();
 
    /* shorter lines... */
    slot = MyReplicationSlot;
 
    /* first some sanity checks that are unlikely to be violated */
    if (slot == NULL)
        elog(ERROR, "cannot perform logical decoding without an acquired slot");
 
    if (plugin == NULL)
        elog(ERROR, "cannot initialize logical decoding without a specified plugin");
 
    /* Make sure the passed slot is suitable. These are user facing errors. */
    if (SlotIsPhysical(slot))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("cannot use physical replication slot for logical decoding")));
 
    if (slot->data.database != MyDatabaseId)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("replication slot \"%s\" was not created in this database",
                        NameStr(slot->data.name))));
 
    if (IsTransactionState() &&
        GetTopTransactionIdIfAny() != InvalidTransactionId)
        ereport(ERROR,
                (errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
                 errmsg("cannot create logical replication slot in transaction that has performed writes")));
 
    /*
     * Register output plugin name with slot.  We need the mutex to avoid
     * concurrent reading of a partially copied string.  But we don't want any
     * complicated code while holding a spinlock, so do namestrcpy() outside.
     */
    namestrcpy(&plugin_name, plugin);
    SpinLockAcquire(&slot->mutex);
    slot->data.plugin = plugin_name;
    SpinLockRelease(&slot->mutex);
 
    if (XLogRecPtrIsInvalid(restart_lsn))
        ReplicationSlotReserveWal();
    else
    {
        SpinLockAcquire(&slot->mutex);
        slot->data.restart_lsn = restart_lsn;
        SpinLockRelease(&slot->mutex);
    }
 
    /* ----
     * This is a bit tricky: We need to determine a safe xmin horizon to start
     * decoding from, to avoid starting from a running xacts record referring
     * to xids whose rows have been vacuumed or pruned
     * already. GetOldestSafeDecodingTransactionId() returns such a value, but
     * without further interlock its return value might immediately be out of
     * date.
     *
     * So we have to acquire the ProcArrayLock to prevent computation of new
     * xmin horizons by other backends, get the safe decoding xid, and inform
     * the slot machinery about the new limit. Once that's done the
     * ProcArrayLock can be released as the slot machinery now is
     * protecting against vacuum.
     *
     * Note that, temporarily, the data, not just the catalog, xmin has to be
     * reserved if a data snapshot is to be exported.  Otherwise the initial
     * data snapshot created here is not guaranteed to be valid. After that
     * the data xmin doesn't need to be managed anymore and the global xmin
     * should be recomputed. As we are fine with losing the pegged data xmin
     * after crash - no chance a snapshot would get exported anymore - we can
     * get away with just setting the slot's
     * effective_xmin. ReplicationSlotRelease will reset it again.
     *
     * ----
     */
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
    xmin_horizon = GetOldestSafeDecodingTransactionId(!need_full_snapshot);
 
    SpinLockAcquire(&slot->mutex);
    slot->effective_catalog_xmin = xmin_horizon;
    slot->data.catalog_xmin = xmin_horizon;
    if (need_full_snapshot)
        slot->effective_xmin = xmin_horizon;
    SpinLockRelease(&slot->mutex);
 
    ReplicationSlotsComputeRequiredXmin(true);
 
    LWLockRelease(ProcArrayLock);
 
    ReplicationSlotMarkDirty();
    ReplicationSlotSave();
 
    ctx = StartupDecodingContext(NIL, restart_lsn, xmin_horizon,
                                 need_full_snapshot, false, true,
                                 xl_routine, prepare_write, do_write,
                                 update_progress);
 
    /* call output plugin initialization callback */
    old_context = MemoryContextSwitchTo(ctx->context);
    if (ctx->callbacks.startup_cb != NULL)
        startup_cb_wrapper(ctx, &ctx->options, true);
    MemoryContextSwitchTo(old_context);
 
    /*
     * We allow decoding of prepared transactions when the two_phase is
     * enabled at the time of slot creation, or when the two_phase option is
     * given at the streaming start, provided the plugin supports all the
     * callbacks for two-phase.
     */
    ctx->twophase &= slot->data.two_phase;
 
    ctx->reorder->output_rewrites = ctx->options.receive_rewrites;
 
    return ctx;
}
 
/*
 * Create a new decoding context, for a logical slot that has previously been
 * used already.
 *
 * start_lsn
 *      The LSN at which to start decoding.  If InvalidXLogRecPtr, restart
 *      from the slot's confirmed_flush; otherwise, start from the specified
 *      location (but move it forwards to confirmed_flush if it's older than
 *      that, see below).
 *
 * output_plugin_options
 *      options passed to the output plugin.
 *
 * fast_forward
 *      bypass the generation of logical changes.
 *
 * xl_routine
 *      XLogReaderRoutine used by underlying xlogreader
 *
 * prepare_write, do_write, update_progress
 *      callbacks that have to be filled to perform the use-case dependent,
 *      actual work.
 *
 * Needs to be called while in a memory context that's at least as long lived
 * as the decoding context because further memory contexts will be created
 * inside it.
 *
 * Returns an initialized decoding context after calling the output plugin's
 * startup function.
 */
LogicalDecodingContext *
CreateDecodingContext(XLogRecPtr start_lsn,
                      List *output_plugin_options,
                      bool fast_forward,
                      XLogReaderRoutine *xl_routine,
                      LogicalOutputPluginWriterPrepareWrite prepare_write,
                      LogicalOutputPluginWriterWrite do_write,
                      LogicalOutputPluginWriterUpdateProgress update_progress)
{
    LogicalDecodingContext *ctx;
    ReplicationSlot *slot;
    MemoryContext old_context;
 
    /* shorter lines... */
    slot = MyReplicationSlot;
 
    /* first some sanity checks that are unlikely to be violated */
    if (slot == NULL)
        elog(ERROR, "cannot perform logical decoding without an acquired slot");
 
    /* make sure the passed slot is suitable, these are user facing errors */
    if (SlotIsPhysical(slot))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("cannot use physical replication slot for logical decoding")));
 
    /*
     * We need to access the system tables during decoding to build the
     * logical changes unless we are in fast_forward mode where no changes are
     * generated.
     */
    if (slot->data.database != MyDatabaseId && !fast_forward)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("replication slot \"%s\" was not created in this database",
                        NameStr(slot->data.name))));
 
    /*
     * The slots being synced from the primary can't be used for decoding as
     * they are used after failover. However, we do allow advancing the LSNs
     * during the synchronization of slots. See update_local_synced_slot.
     */
    if (RecoveryInProgress() && slot->data.synced && !IsSyncingReplicationSlots())
        ereport(ERROR,
                errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                errmsg("cannot use replication slot \"%s\" for logical decoding",
                       NameStr(slot->data.name)),
                errdetail("This replication slot is being synchronized from the primary server."),
                errhint("Specify another replication slot."));
 
    /* slot must be valid to allow decoding */
    Assert(slot->data.invalidated == RS_INVAL_NONE);
    Assert(slot->data.restart_lsn != InvalidXLogRecPtr);
 
    if (start_lsn == InvalidXLogRecPtr)
    {
        /* continue from last position */
        start_lsn = slot->data.confirmed_flush;
    }
    else if (start_lsn < slot->data.confirmed_flush)
    {
        /*
         * It might seem like we should error out in this case, but it's
         * pretty common for a client to acknowledge a LSN it doesn't have to
         * do anything for, and thus didn't store persistently, because the
         * xlog records didn't result in anything relevant for logical
         * decoding. Clients have to be able to do that to support synchronous
         * replication.
         *
         * Starting at a different LSN than requested might not catch certain
         * kinds of client errors; so the client may wish to check that
         * confirmed_flush_lsn matches its expectations.
         */
        elog(LOG, "%X/%08X has been already streamed, forwarding to %X/%08X",
             LSN_FORMAT_ARGS(start_lsn),
             LSN_FORMAT_ARGS(slot->data.confirmed_flush));
 
        start_lsn = slot->data.confirmed_flush;
    }
 
    ctx = StartupDecodingContext(output_plugin_options,
                                 start_lsn, InvalidTransactionId, false,
                                 fast_forward, false, xl_routine, prepare_write,
                                 do_write, update_progress);
 
    /* call output plugin initialization callback */
    old_context = MemoryContextSwitchTo(ctx->context);
    if (ctx->callbacks.startup_cb != NULL)
        startup_cb_wrapper(ctx, &ctx->options, false);
    MemoryContextSwitchTo(old_context);
 
    /*
     * We allow decoding of prepared transactions when the two_phase is
     * enabled at the time of slot creation, or when the two_phase option is
     * given at the streaming start, provided the plugin supports all the
     * callbacks for two-phase.
     */
    ctx->twophase &= (slot->data.two_phase || ctx->twophase_opt_given);
 
    /* Mark slot to allow two_phase decoding if not already marked */
    if (ctx->twophase && !slot->data.two_phase)
    {
        SpinLockAcquire(&slot->mutex);
        slot->data.two_phase = true;
        slot->data.two_phase_at = start_lsn;
        SpinLockRelease(&slot->mutex);
        ReplicationSlotMarkDirty();
        ReplicationSlotSave();
        SnapBuildSetTwoPhaseAt(ctx->snapshot_builder, start_lsn);
    }
 
    ctx->reorder->output_rewrites = ctx->options.receive_rewrites;
 
    ereport(LOG,
            (errmsg("starting logical decoding for slot \"%s\"",
                    NameStr(slot->data.name)),
             errdetail("Streaming transactions committing after %X/%08X, reading WAL from %X/%08X.",
                       LSN_FORMAT_ARGS(slot->data.confirmed_flush),
                       LSN_FORMAT_ARGS(slot->data.restart_lsn))));
 
    return ctx;
}
 
/*
 * Returns true if a consistent initial decoding snapshot has been built.
 */
bool
DecodingContextReady(LogicalDecodingContext *ctx)
{
    return SnapBuildCurrentState(ctx->snapshot_builder) == SNAPBUILD_CONSISTENT;
}
 
/*
 * Read from the decoding slot, until it is ready to start extracting changes.
 */
void
DecodingContextFindStartpoint(LogicalDecodingContext *ctx)
{
    ReplicationSlot *slot = ctx->slot;
 
    /* Initialize from where to start reading WAL. */
    XLogBeginRead(ctx->reader, slot->data.restart_lsn);
 
    elog(DEBUG1, "searching for logical decoding starting point, starting at %X/%08X",
         LSN_FORMAT_ARGS(slot->data.restart_lsn));
 
    /* Wait for a consistent starting point */
    for (;;)
    {
        XLogRecord *record;
        char       *err = NULL;
 
        /* the read_page callback waits for new WAL */
        record = XLogReadRecord(ctx->reader, &err);
        if (err)
            elog(ERROR, "could not find logical decoding starting point: %s", err);
        if (!record)
            elog(ERROR, "could not find logical decoding starting point");
 
        LogicalDecodingProcessRecord(ctx, ctx->reader);
 
        /* only continue till we found a consistent spot */
        if (DecodingContextReady(ctx))
            break;
 
        CHECK_FOR_INTERRUPTS();
    }
 
    SpinLockAcquire(&slot->mutex);
    slot->data.confirmed_flush = ctx->reader->EndRecPtr;
    if (slot->data.two_phase)
        slot->data.two_phase_at = ctx->reader->EndRecPtr;
    SpinLockRelease(&slot->mutex);
}
 
/*
 * Free a previously allocated decoding context, invoking the shutdown
 * callback if necessary.
 */
void
FreeDecodingContext(LogicalDecodingContext *ctx)
{
    if (ctx->callbacks.shutdown_cb != NULL)
        shutdown_cb_wrapper(ctx);
 
    ReorderBufferFree(ctx->reorder);
    FreeSnapshotBuilder(ctx->snapshot_builder);
    XLogReaderFree(ctx->reader);
    MemoryContextDelete(ctx->context);
}
 
/*
 * Prepare a write using the context's output routine.
 */
void
OutputPluginPrepareWrite(struct LogicalDecodingContext *ctx, bool last_write)
{
    if (!ctx->accept_writes)
        elog(ERROR, "writes are only accepted in commit, begin and change callbacks");
 
    ctx->prepare_write(ctx, ctx->write_location, ctx->write_xid, last_write);
    ctx->prepared_write = true;
}
 
/*
 * Perform a write using the context's output routine.
 */
void
OutputPluginWrite(struct LogicalDecodingContext *ctx, bool last_write)
{
    if (!ctx->prepared_write)
        elog(ERROR, "OutputPluginPrepareWrite needs to be called before OutputPluginWrite");
 
    ctx->write(ctx, ctx->write_location, ctx->write_xid, last_write);
    ctx->prepared_write = false;
}
 
/*
 * Update progress tracking (if supported).
 */
void
OutputPluginUpdateProgress(struct LogicalDecodingContext *ctx,
                           bool skipped_xact)
{
    if (!ctx->update_progress)
        return;
 
    ctx->update_progress(ctx, ctx->write_location, ctx->write_xid,
                         skipped_xact);
}
 
/*
 * Load the output plugin, lookup its output plugin init function, and check
 * that it provides the required callbacks.
 */
static void
LoadOutputPlugin(OutputPluginCallbacks *callbacks, const char *plugin)
{
    LogicalOutputPluginInit plugin_init;
 
    plugin_init = (LogicalOutputPluginInit)
        load_external_function(plugin, "_PG_output_plugin_init", false, NULL);
 
    if (plugin_init == NULL)
        elog(ERROR, "output plugins have to declare the _PG_output_plugin_init symbol");
 
    /* ask the output plugin to fill the callback struct */
    plugin_init(callbacks);
 
    if (callbacks->begin_cb == NULL)
        elog(ERROR, "output plugins have to register a begin callback");
    if (callbacks->change_cb == NULL)
        elog(ERROR, "output plugins have to register a change callback");
    if (callbacks->commit_cb == NULL)
        elog(ERROR, "output plugins have to register a commit callback");
}
 
static void
output_plugin_error_callback(void *arg)
{
    LogicalErrorCallbackState *state = (LogicalErrorCallbackState *) arg;
 
    /* not all callbacks have an associated LSN  */
    if (state->report_location != InvalidXLogRecPtr)
        errcontext("slot \"%s\", output plugin \"%s\", in the %s callback, associated LSN %X/%08X",
                   NameStr(state->ctx->slot->data.name),
                   NameStr(state->ctx->slot->data.plugin),
                   state->callback_name,
                   LSN_FORMAT_ARGS(state->report_location));
    else
        errcontext("slot \"%s\", output plugin \"%s\", in the %s callback",
                   NameStr(state->ctx->slot->data.name),
                   NameStr(state->ctx->slot->data.plugin),
                   state->callback_name);
}
 
static void
startup_cb_wrapper(LogicalDecodingContext *ctx, OutputPluginOptions *opt, bool is_init)
{
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "startup";
    state.report_location = InvalidXLogRecPtr;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = false;
    ctx->end_xact = false;
 
    /* do the actual work: call callback */
    ctx->callbacks.startup_cb(ctx, opt, is_init);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
shutdown_cb_wrapper(LogicalDecodingContext *ctx)
{
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "shutdown";
    state.report_location = InvalidXLogRecPtr;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = false;
    ctx->end_xact = false;
 
    /* do the actual work: call callback */
    ctx->callbacks.shutdown_cb(ctx);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
 
/*
 * Callbacks for ReorderBuffer which add in some more information and then call
 * output_plugin.h plugins.
 */
static void
begin_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "begin";
    state.report_location = txn->first_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = txn->first_lsn;
    ctx->end_xact = false;
 
    /* do the actual work: call callback */
    ctx->callbacks.begin_cb(ctx, txn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
commit_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                  XLogRecPtr commit_lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "commit";
    state.report_location = txn->final_lsn; /* beginning of commit record */
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = txn->end_lsn; /* points to the end of the record */
    ctx->end_xact = true;
 
    /* do the actual work: call callback */
    ctx->callbacks.commit_cb(ctx, txn, commit_lsn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
/*
 * The functionality of begin_prepare is quite similar to begin with the
 * exception that this will have gid (global transaction id) information which
 * can be used by plugin. Now, we thought about extending the existing begin
 * but that would break the replication protocol and additionally this looks
 * cleaner.
 */
static void
begin_prepare_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when two-phase commits are supported */
    Assert(ctx->twophase);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "begin_prepare";
    state.report_location = txn->first_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = txn->first_lsn;
    ctx->end_xact = false;
 
    /*
     * If the plugin supports two-phase commits then begin prepare callback is
     * mandatory
     */
    if (ctx->callbacks.begin_prepare_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical replication at prepare time requires a %s callback",
                        "begin_prepare_cb")));
 
    /* do the actual work: call callback */
    ctx->callbacks.begin_prepare_cb(ctx, txn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
prepare_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                   XLogRecPtr prepare_lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when two-phase commits are supported */
    Assert(ctx->twophase);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "prepare";
    state.report_location = txn->final_lsn; /* beginning of prepare record */
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = txn->end_lsn; /* points to the end of the record */
    ctx->end_xact = true;
 
    /*
     * If the plugin supports two-phase commits then prepare callback is
     * mandatory
     */
    if (ctx->callbacks.prepare_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical replication at prepare time requires a %s callback",
                        "prepare_cb")));
 
    /* do the actual work: call callback */
    ctx->callbacks.prepare_cb(ctx, txn, prepare_lsn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
commit_prepared_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                           XLogRecPtr commit_lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when two-phase commits are supported */
    Assert(ctx->twophase);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "commit_prepared";
    state.report_location = txn->final_lsn; /* beginning of commit record */
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = txn->end_lsn; /* points to the end of the record */
    ctx->end_xact = true;
 
    /*
     * If the plugin support two-phase commits then commit prepared callback
     * is mandatory
     */
    if (ctx->callbacks.commit_prepared_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical replication at prepare time requires a %s callback",
                        "commit_prepared_cb")));
 
    /* do the actual work: call callback */
    ctx->callbacks.commit_prepared_cb(ctx, txn, commit_lsn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
rollback_prepared_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                             XLogRecPtr prepare_end_lsn,
                             TimestampTz prepare_time)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when two-phase commits are supported */
    Assert(ctx->twophase);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "rollback_prepared";
    state.report_location = txn->final_lsn; /* beginning of commit record */
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = txn->end_lsn; /* points to the end of the record */
    ctx->end_xact = true;
 
    /*
     * If the plugin support two-phase commits then rollback prepared callback
     * is mandatory
     */
    if (ctx->callbacks.rollback_prepared_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical replication at prepare time requires a %s callback",
                        "rollback_prepared_cb")));
 
    /* do the actual work: call callback */
    ctx->callbacks.rollback_prepared_cb(ctx, txn, prepare_end_lsn,
                                        prepare_time);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
change_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                  Relation relation, ReorderBufferChange *change)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "change";
    state.report_location = change->lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
 
    /*
     * Report this change's lsn so replies from clients can give an up-to-date
     * answer. This won't ever be enough (and shouldn't be!) to confirm
     * receipt of this transaction, but it might allow another transaction's
     * commit to be confirmed with one message.
     */
    ctx->write_location = change->lsn;
 
    ctx->end_xact = false;
 
    ctx->callbacks.change_cb(ctx, txn, relation, change);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                    int nrelations, Relation relations[], ReorderBufferChange *change)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    if (!ctx->callbacks.truncate_cb)
        return;
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "truncate";
    state.report_location = change->lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
 
    /*
     * Report this change's lsn so replies from clients can give an up-to-date
     * answer. This won't ever be enough (and shouldn't be!) to confirm
     * receipt of this transaction, but it might allow another transaction's
     * commit to be confirmed with one message.
     */
    ctx->write_location = change->lsn;
 
    ctx->end_xact = false;
 
    ctx->callbacks.truncate_cb(ctx, txn, nrelations, relations, change);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
bool
filter_prepare_cb_wrapper(LogicalDecodingContext *ctx, TransactionId xid,
                          const char *gid)
{
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
    bool        ret;
 
    Assert(!ctx->fast_forward);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "filter_prepare";
    state.report_location = InvalidXLogRecPtr;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = false;
    ctx->end_xact = false;
 
    /* do the actual work: call callback */
    ret = ctx->callbacks.filter_prepare_cb(ctx, xid, gid);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
 
    return ret;
}
 
bool
filter_by_origin_cb_wrapper(LogicalDecodingContext *ctx, RepOriginId origin_id)
{
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
    bool        ret;
 
    Assert(!ctx->fast_forward);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "filter_by_origin";
    state.report_location = InvalidXLogRecPtr;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = false;
    ctx->end_xact = false;
 
    /* do the actual work: call callback */
    ret = ctx->callbacks.filter_by_origin_cb(ctx, origin_id);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
 
    return ret;
}
 
static void
message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                   XLogRecPtr message_lsn, bool transactional,
                   const char *prefix, Size message_size, const char *message)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    if (ctx->callbacks.message_cb == NULL)
        return;
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "message";
    state.report_location = message_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn != NULL ? txn->xid : InvalidTransactionId;
    ctx->write_location = message_lsn;
    ctx->end_xact = false;
 
    /* do the actual work: call callback */
    ctx->callbacks.message_cb(ctx, txn, message_lsn, transactional, prefix,
                              message_size, message);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
stream_start_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                        XLogRecPtr first_lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when streaming is supported. */
    Assert(ctx->streaming);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "stream_start";
    state.report_location = first_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
 
    /*
     * Report this message's lsn so replies from clients can give an
     * up-to-date answer. This won't ever be enough (and shouldn't be!) to
     * confirm receipt of this transaction, but it might allow another
     * transaction's commit to be confirmed with one message.
     */
    ctx->write_location = first_lsn;
 
    ctx->end_xact = false;
 
    /* in streaming mode, stream_start_cb is required */
    if (ctx->callbacks.stream_start_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical streaming requires a %s callback",
                        "stream_start_cb")));
 
    ctx->callbacks.stream_start_cb(ctx, txn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
stream_stop_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                       XLogRecPtr last_lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when streaming is supported. */
    Assert(ctx->streaming);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "stream_stop";
    state.report_location = last_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
 
    /*
     * Report this message's lsn so replies from clients can give an
     * up-to-date answer. This won't ever be enough (and shouldn't be!) to
     * confirm receipt of this transaction, but it might allow another
     * transaction's commit to be confirmed with one message.
     */
    ctx->write_location = last_lsn;
 
    ctx->end_xact = false;
 
    /* in streaming mode, stream_stop_cb is required */
    if (ctx->callbacks.stream_stop_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical streaming requires a %s callback",
                        "stream_stop_cb")));
 
    ctx->callbacks.stream_stop_cb(ctx, txn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
stream_abort_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                        XLogRecPtr abort_lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when streaming is supported. */
    Assert(ctx->streaming);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "stream_abort";
    state.report_location = abort_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = abort_lsn;
    ctx->end_xact = true;
 
    /* in streaming mode, stream_abort_cb is required */
    if (ctx->callbacks.stream_abort_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical streaming requires a %s callback",
                        "stream_abort_cb")));
 
    ctx->callbacks.stream_abort_cb(ctx, txn, abort_lsn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
stream_prepare_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                          XLogRecPtr prepare_lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /*
     * We're only supposed to call this when streaming and two-phase commits
     * are supported.
     */
    Assert(ctx->streaming);
    Assert(ctx->twophase);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "stream_prepare";
    state.report_location = txn->final_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = txn->end_lsn;
    ctx->end_xact = true;
 
    /* in streaming mode with two-phase commits, stream_prepare_cb is required */
    if (ctx->callbacks.stream_prepare_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical streaming at prepare time requires a %s callback",
                        "stream_prepare_cb")));
 
    ctx->callbacks.stream_prepare_cb(ctx, txn, prepare_lsn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
stream_commit_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                         XLogRecPtr commit_lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when streaming is supported. */
    Assert(ctx->streaming);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "stream_commit";
    state.report_location = txn->final_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
    ctx->write_location = txn->end_lsn;
    ctx->end_xact = true;
 
    /* in streaming mode, stream_commit_cb is required */
    if (ctx->callbacks.stream_commit_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical streaming requires a %s callback",
                        "stream_commit_cb")));
 
    ctx->callbacks.stream_commit_cb(ctx, txn, commit_lsn);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
stream_change_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                         Relation relation, ReorderBufferChange *change)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when streaming is supported. */
    Assert(ctx->streaming);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "stream_change";
    state.report_location = change->lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
 
    /*
     * Report this change's lsn so replies from clients can give an up-to-date
     * answer. This won't ever be enough (and shouldn't be!) to confirm
     * receipt of this transaction, but it might allow another transaction's
     * commit to be confirmed with one message.
     */
    ctx->write_location = change->lsn;
 
    ctx->end_xact = false;
 
    /* in streaming mode, stream_change_cb is required */
    if (ctx->callbacks.stream_change_cb == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical streaming requires a %s callback",
                        "stream_change_cb")));
 
    ctx->callbacks.stream_change_cb(ctx, txn, relation, change);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
stream_message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                          XLogRecPtr message_lsn, bool transactional,
                          const char *prefix, Size message_size, const char *message)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when streaming is supported. */
    Assert(ctx->streaming);
 
    /* this callback is optional */
    if (ctx->callbacks.stream_message_cb == NULL)
        return;
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "stream_message";
    state.report_location = message_lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn != NULL ? txn->xid : InvalidTransactionId;
    ctx->write_location = message_lsn;
    ctx->end_xact = false;
 
    /* do the actual work: call callback */
    ctx->callbacks.stream_message_cb(ctx, txn, message_lsn, transactional, prefix,
                                     message_size, message);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
stream_truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                           int nrelations, Relation relations[],
                           ReorderBufferChange *change)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* We're only supposed to call this when streaming is supported. */
    Assert(ctx->streaming);
 
    /* this callback is optional */
    if (!ctx->callbacks.stream_truncate_cb)
        return;
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "stream_truncate";
    state.report_location = change->lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = true;
    ctx->write_xid = txn->xid;
 
    /*
     * Report this change's lsn so replies from clients can give an up-to-date
     * answer. This won't ever be enough (and shouldn't be!) to confirm
     * receipt of this transaction, but it might allow another transaction's
     * commit to be confirmed with one message.
     */
    ctx->write_location = change->lsn;
 
    ctx->end_xact = false;
 
    ctx->callbacks.stream_truncate_cb(ctx, txn, nrelations, relations, change);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
static void
update_progress_txn_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                               XLogRecPtr lsn)
{
    LogicalDecodingContext *ctx = cache->private_data;
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
 
    Assert(!ctx->fast_forward);
 
    /* Push callback + info on the error context stack */
    state.ctx = ctx;
    state.callback_name = "update_progress_txn";
    state.report_location = lsn;
    errcallback.callback = output_plugin_error_callback;
    errcallback.arg = &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;
 
    /* set output state */
    ctx->accept_writes = false;
    ctx->write_xid = txn->xid;
 
    /*
     * Report this change's lsn so replies from clients can give an up-to-date
     * answer. This won't ever be enough (and shouldn't be!) to confirm
     * receipt of this transaction, but it might allow another transaction's
     * commit to be confirmed with one message.
     */
    ctx->write_location = lsn;
 
    ctx->end_xact = false;
 
    OutputPluginUpdateProgress(ctx, false);
 
    /* Pop the error context stack */
    error_context_stack = errcallback.previous;
}
 
/*
 * Set the required catalog xmin horizon for historic snapshots in the current
 * replication slot.
 *
 * Note that in the most cases, we won't be able to immediately use the xmin
 * to increase the xmin horizon: we need to wait till the client has confirmed
 * receiving current_lsn with LogicalConfirmReceivedLocation().
 */
void
LogicalIncreaseXminForSlot(XLogRecPtr current_lsn, TransactionId xmin)
{
    bool        updated_xmin = false;
    ReplicationSlot *slot;
    bool        got_new_xmin = false;
 
    slot = MyReplicationSlot;
 
    Assert(slot != NULL);
 
    SpinLockAcquire(&slot->mutex);
 
    /*
     * don't overwrite if we already have a newer xmin. This can happen if we
     * restart decoding in a slot.
     */
    if (TransactionIdPrecedesOrEquals(xmin, slot->data.catalog_xmin))
    {
    }
 
    /*
     * If the client has already confirmed up to this lsn, we directly can
     * mark this as accepted. This can happen if we restart decoding in a
     * slot.
     */
    else if (current_lsn <= slot->data.confirmed_flush)
    {
        slot->candidate_catalog_xmin = xmin;
        slot->candidate_xmin_lsn = current_lsn;
 
        /* our candidate can directly be used */
        updated_xmin = true;
    }
 
    /*
     * Only increase if the previous values have been applied, otherwise we
     * might never end up updating if the receiver acks too slowly.
     */
    else if (slot->candidate_xmin_lsn == InvalidXLogRecPtr)
    {
        slot->candidate_catalog_xmin = xmin;
        slot->candidate_xmin_lsn = current_lsn;
 
        /*
         * Log new xmin at an appropriate log level after releasing the
         * spinlock.
         */
        got_new_xmin = true;
    }
    SpinLockRelease(&slot->mutex);
 
    if (got_new_xmin)
        elog(DEBUG1, "got new catalog xmin %u at %X/%08X", xmin,
             LSN_FORMAT_ARGS(current_lsn));
 
    /* candidate already valid with the current flush position, apply */
    if (updated_xmin)
        LogicalConfirmReceivedLocation(slot->data.confirmed_flush);
}
 
/*
 * Mark the minimal LSN (restart_lsn) we need to read to replay all
 * transactions that have not yet committed at current_lsn.
 *
 * Just like LogicalIncreaseXminForSlot this only takes effect when the
 * client has confirmed to have received current_lsn.
 */
void
LogicalIncreaseRestartDecodingForSlot(XLogRecPtr current_lsn, XLogRecPtr restart_lsn)
{
    bool        updated_lsn = false;
    ReplicationSlot *slot;
 
    slot = MyReplicationSlot;
 
    Assert(slot != NULL);
    Assert(restart_lsn != InvalidXLogRecPtr);
    Assert(current_lsn != InvalidXLogRecPtr);
 
    SpinLockAcquire(&slot->mutex);
 
    /* don't overwrite if have a newer restart lsn */
    if (restart_lsn <= slot->data.restart_lsn)
    {
        SpinLockRelease(&slot->mutex);
    }
 
    /*
     * We might have already flushed far enough to directly accept this lsn,
     * in this case there is no need to check for existing candidate LSNs
     */
    else if (current_lsn <= slot->data.confirmed_flush)
    {
        slot->candidate_restart_valid = current_lsn;
        slot->candidate_restart_lsn = restart_lsn;
        SpinLockRelease(&slot->mutex);
 
        /* our candidate can directly be used */
        updated_lsn = true;
    }
 
    /*
     * Only increase if the previous values have been applied, otherwise we
     * might never end up updating if the receiver acks too slowly. A missed
     * value here will just cause some extra effort after reconnecting.
     */
    else if (slot->candidate_restart_valid == InvalidXLogRecPtr)
    {
        slot->candidate_restart_valid = current_lsn;
        slot->candidate_restart_lsn = restart_lsn;
        SpinLockRelease(&slot->mutex);
 
        elog(DEBUG1, "got new restart lsn %X/%08X at %X/%08X",
             LSN_FORMAT_ARGS(restart_lsn),
             LSN_FORMAT_ARGS(current_lsn));
    }
    else
    {
        XLogRecPtr  candidate_restart_lsn;
        XLogRecPtr  candidate_restart_valid;
        XLogRecPtr  confirmed_flush;
 
        candidate_restart_lsn = slot->candidate_restart_lsn;
        candidate_restart_valid = slot->candidate_restart_valid;
        confirmed_flush = slot->data.confirmed_flush;
        SpinLockRelease(&slot->mutex);
 
        elog(DEBUG1, "failed to increase restart lsn: proposed %X/%08X, after %X/%08X, current candidate %X/%08X, current after %X/%08X, flushed up to %X/%08X",
             LSN_FORMAT_ARGS(restart_lsn),
             LSN_FORMAT_ARGS(current_lsn),
             LSN_FORMAT_ARGS(candidate_restart_lsn),
             LSN_FORMAT_ARGS(candidate_restart_valid),
             LSN_FORMAT_ARGS(confirmed_flush));
    }
 
    /* candidates are already valid with the current flush position, apply */
    if (updated_lsn)
        LogicalConfirmReceivedLocation(slot->data.confirmed_flush);
}
 
/*
 * Handle a consumer's confirmation having received all changes up to lsn.
 */
void
LogicalConfirmReceivedLocation(XLogRecPtr lsn)
{
    Assert(lsn != InvalidXLogRecPtr);
 
    /* Do an unlocked check for candidate_lsn first. */
    if (MyReplicationSlot->candidate_xmin_lsn != InvalidXLogRecPtr ||
        MyReplicationSlot->candidate_restart_valid != InvalidXLogRecPtr)
    {
        bool        updated_xmin = false;
        bool        updated_restart = false;
        XLogRecPtr  restart_lsn pg_attribute_unused();
 
        SpinLockAcquire(&MyReplicationSlot->mutex);
 
        /* remember the old restart lsn */
        restart_lsn = MyReplicationSlot->data.restart_lsn;
 
        /*
         * Prevent moving the confirmed_flush backwards, as this could lead to
         * data duplication issues caused by replicating already replicated
         * changes.
         *
         * This can happen when a client acknowledges an LSN it doesn't have
         * to do anything for, and thus didn't store persistently. After a
         * restart, the client can send the prior LSN that it stored
         * persistently as an acknowledgement, but we need to ignore such an
         * LSN. See similar case handling in CreateDecodingContext.
         */
        if (lsn > MyReplicationSlot->data.confirmed_flush)
            MyReplicationSlot->data.confirmed_flush = lsn;
 
        /* if we're past the location required for bumping xmin, do so */
        if (MyReplicationSlot->candidate_xmin_lsn != InvalidXLogRecPtr &&
            MyReplicationSlot->candidate_xmin_lsn <= lsn)
        {
            /*
             * We have to write the changed xmin to disk *before* we change
             * the in-memory value, otherwise after a crash we wouldn't know
             * that some catalog tuples might have been removed already.
             *
             * Ensure that by first writing to ->xmin and only update
             * ->effective_xmin once the new state is synced to disk. After a
             * crash ->effective_xmin is set to ->xmin.
             */
            if (TransactionIdIsValid(MyReplicationSlot->candidate_catalog_xmin) &&
                MyReplicationSlot->data.catalog_xmin != MyReplicationSlot->candidate_catalog_xmin)
            {
                MyReplicationSlot->data.catalog_xmin = MyReplicationSlot->candidate_catalog_xmin;
                MyReplicationSlot->candidate_catalog_xmin = InvalidTransactionId;
                MyReplicationSlot->candidate_xmin_lsn = InvalidXLogRecPtr;
                updated_xmin = true;
            }
        }
 
        if (MyReplicationSlot->candidate_restart_valid != InvalidXLogRecPtr &&
            MyReplicationSlot->candidate_restart_valid <= lsn)
        {
            Assert(MyReplicationSlot->candidate_restart_lsn != InvalidXLogRecPtr);
 
            MyReplicationSlot->data.restart_lsn = MyReplicationSlot->candidate_restart_lsn;
            MyReplicationSlot->candidate_restart_lsn = InvalidXLogRecPtr;
            MyReplicationSlot->candidate_restart_valid = InvalidXLogRecPtr;
            updated_restart = true;
        }
 
        SpinLockRelease(&MyReplicationSlot->mutex);
 
        /* first write new xmin to disk, so we know what's up after a crash */
        if (updated_xmin || updated_restart)
        {
#ifdef USE_INJECTION_POINTS
            XLogSegNo   seg1,
                        seg2;
 
            XLByteToSeg(restart_lsn, seg1, wal_segment_size);
            XLByteToSeg(MyReplicationSlot->data.restart_lsn, seg2, wal_segment_size);
 
            /* trigger injection point, but only if segment changes */
            if (seg1 != seg2)
                INJECTION_POINT("logical-replication-slot-advance-segment", NULL);
#endif
 
            ReplicationSlotMarkDirty();
            ReplicationSlotSave();
            elog(DEBUG1, "updated xmin: %u restart: %u", updated_xmin, updated_restart);
        }
 
        /*
         * Now the new xmin is safely on disk, we can let the global value
         * advance. We do not take ProcArrayLock or similar since we only
         * advance xmin here and there's not much harm done by a concurrent
         * computation missing that.
         */
        if (updated_xmin)
        {
            SpinLockAcquire(&MyReplicationSlot->mutex);
            MyReplicationSlot->effective_catalog_xmin = MyReplicationSlot->data.catalog_xmin;
            SpinLockRelease(&MyReplicationSlot->mutex);
 
            ReplicationSlotsComputeRequiredXmin(false);
            ReplicationSlotsComputeRequiredLSN();
        }
    }
    else
    {
        SpinLockAcquire(&MyReplicationSlot->mutex);
 
        /*
         * Prevent moving the confirmed_flush backwards. See comments above
         * for the details.
         */
        if (lsn > MyReplicationSlot->data.confirmed_flush)
            MyReplicationSlot->data.confirmed_flush = lsn;
 
        SpinLockRelease(&MyReplicationSlot->mutex);
    }
}
 
/*
 * Clear logical streaming state during (sub)transaction abort.
 */
void
ResetLogicalStreamingState(void)
{
    CheckXidAlive = InvalidTransactionId;
    bsysscan = false;
}
 
/*
 * Report stats for a slot.
 */
void
UpdateDecodingStats(LogicalDecodingContext *ctx)
{
    ReorderBuffer *rb = ctx->reorder;
    PgStat_StatReplSlotEntry repSlotStat;
 
    /* Nothing to do if we don't have any replication stats to be sent. */
    if (rb->spillBytes <= 0 && rb->streamBytes <= 0 && rb->totalBytes <= 0)
        return;
 
    elog(DEBUG2, "UpdateDecodingStats: updating stats %p %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64 " %" PRId64,
         rb,
         rb->spillTxns,
         rb->spillCount,
         rb->spillBytes,
         rb->streamTxns,
         rb->streamCount,
         rb->streamBytes,
         rb->totalTxns,
         rb->totalBytes);
 
    repSlotStat.spill_txns = rb->spillTxns;
    repSlotStat.spill_count = rb->spillCount;
    repSlotStat.spill_bytes = rb->spillBytes;
    repSlotStat.stream_txns = rb->streamTxns;
    repSlotStat.stream_count = rb->streamCount;
    repSlotStat.stream_bytes = rb->streamBytes;
    repSlotStat.total_txns = rb->totalTxns;
    repSlotStat.total_bytes = rb->totalBytes;
 
    pgstat_report_replslot(ctx->slot, &repSlotStat);
 
    rb->spillTxns = 0;
    rb->spillCount = 0;
    rb->spillBytes = 0;
    rb->streamTxns = 0;
    rb->streamCount = 0;
    rb->streamBytes = 0;
    rb->totalTxns = 0;
    rb->totalBytes = 0;
}
 
/*
 * Read up to the end of WAL starting from the decoding slot's restart_lsn.
 * Return true if any meaningful/decodable WAL records are encountered,
 * otherwise false.
 */
bool
LogicalReplicationSlotHasPendingWal(XLogRecPtr end_of_wal)
{
    bool        has_pending_wal = false;
 
    Assert(MyReplicationSlot);
 
    PG_TRY();
    {
        LogicalDecodingContext *ctx;
 
        /*
         * Create our decoding context in fast_forward mode, passing start_lsn
         * as InvalidXLogRecPtr, so that we start processing from the 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 points to a
         * valid record.
         */
        XLogBeginRead(ctx->reader, MyReplicationSlot->data.restart_lsn);
 
        /* Invalidate non-timetravel entries */
        InvalidateSystemCaches();
 
        /* Loop until the end of WAL or some changes are processed */
        while (!has_pending_wal && ctx->reader->EndRecPtr < end_of_wal)
        {
            XLogRecord *record;
            char       *errm = NULL;
 
            record = XLogReadRecord(ctx->reader, &errm);
 
            if (errm)
                elog(ERROR, "could not find record for logical decoding: %s", errm);
 
            if (record != NULL)
                LogicalDecodingProcessRecord(ctx, ctx->reader);
 
            has_pending_wal = ctx->processing_required;
 
            CHECK_FOR_INTERRUPTS();
        }
 
        /* Clean up */
        FreeDecodingContext(ctx);
        InvalidateSystemCaches();
    }
    PG_CATCH();
    {
        /* clear all timetravel entries */
        InvalidateSystemCaches();
 
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    return has_pending_wal;
}
 
/*
 * 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.
 *
 * *found_consistent_snapshot will be true if the initial decoding snapshot has
 * been built; Otherwise, it will be false.
 */
XLogRecPtr
LogicalSlotAdvanceAndCheckSnapState(XLogRecPtr moveto,
                                    bool *found_consistent_snapshot)
{
    LogicalDecodingContext *ctx;
    ResourceOwner old_resowner PG_USED_FOR_ASSERTS_ONLY = CurrentResourceOwner;
    XLogRecPtr  retlsn;
 
    Assert(moveto != InvalidXLogRecPtr);
 
    if (found_consistent_snapshot)
        *found_consistent_snapshot = false;
 
    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);
 
        /*
         * Wait for specified streaming replication standby servers (if any)
         * to confirm receipt of WAL up to moveto lsn.
         */
        WaitForStandbyConfirmation(moveto);
 
        /*
         * 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 records until we reach the requested target */
        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, "could not find record while advancing replication slot: %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);
 
                /*
                 * We used to have bugs where logical decoding would fail to
                 * preserve the resource owner.  That's important here, so
                 * verify that that doesn't happen anymore.  XXX this could be
                 * removed once it's been battle-tested.
                 */
                Assert(CurrentResourceOwner == old_resowner);
            }
 
            CHECK_FOR_INTERRUPTS();
        }
 
        if (found_consistent_snapshot && DecodingContextReady(ctx))
            *found_consistent_snapshot = true;
 
        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;
}