logical.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 * logical.c
 *     PostgreSQL logical decoding coordination
 *
 * Copyright (c) 2012-2016, 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 "miscadmin.h"

#include "access/xact.h"
#include "access/xlog_internal.h"

#include "replication/decode.h"
#include "replication/logical.h"
#include "replication/reorderbuffer.h"
#include "replication/origin.h"
#include "replication/snapbuild.h"

#include "storage/proc.h"
#include "storage/procarray.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 change_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
                  Relation relation, 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);

static void LoadOutputPlugin(OutputPluginCallbacks *callbacks, char *plugin);

/*
 * Make sure the current settings & environment are capable of doing logical
 * decoding.
 */
void
CheckLogicalDecodingRequirements(void)
{
    CheckSlotRequirements();

    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")));

    /* ----
     * TODO: We got to change that someday soon...
     *
     * There's basically three things missing to allow this:
     * 1) We need to be able to correctly and quickly identify the timeline a
     *    LSN belongs to
     * 2) We need to force hot_standby_feedback to be enabled at all times so
     *    the primary cannot remove rows we need.
     * 3) support dropping replication slots referring to a database, in
     *    dbase_redo. There can't be any active ones due to HS recovery
     *    conflicts, so that should be relatively easy.
     * ----
     */
    if (RecoveryInProgress())
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
               errmsg("logical decoding cannot be used while in recovery")));
}

/*
 * Helper function for CreateInitialDecodingContext() and
 * CreateDecodingContext() performing common tasks.
 */
static LogicalDecodingContext *
StartupDecodingContext(List *output_plugin_options,
                       XLogRecPtr start_lsn,
                       TransactionId xmin_horizon,
                       XLogPageReadCB read_page,
                       LogicalOutputPluginWriterPrepareWrite prepare_write,
                       LogicalOutputPluginWriterWrite do_write)
{
    ReplicationSlot *slot;
    MemoryContext context,
                old_context;
    LogicalDecodingContext *ctx;

    /* shorter lines... */
    slot = MyReplicationSlot;

    context = AllocSetContextCreate(CurrentMemoryContext,
                                    "Logical Decoding Context",
                                    ALLOCSET_DEFAULT_MINSIZE,
                                    ALLOCSET_DEFAULT_INITSIZE,
                                    ALLOCSET_DEFAULT_MAXSIZE);
    old_context = MemoryContextSwitchTo(context);
    ctx = palloc0(sizeof(LogicalDecodingContext));

    ctx->context = context;

    /*
     * (re-)load output plugins, so we detect a bad (removed) output plugin
     * now.
     */
    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);
        MyPgXact->vacuumFlags |= PROC_IN_LOGICAL_DECODING;
        LWLockRelease(ProcArrayLock);
    }

    ctx->slot = slot;

    ctx->reader = XLogReaderAllocate(read_page, ctx);
    if (!ctx->reader)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory")));

    ctx->reader->private_data = ctx;

    ctx->reorder = ReorderBufferAllocate();
    ctx->snapshot_builder =
        AllocateSnapshotBuilder(ctx->reorder, xmin_horizon, start_lsn);

    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->commit = commit_cb_wrapper;
    ctx->reorder->message = message_cb_wrapper;

    ctx->out = makeStringInfo();
    ctx->prepare_write = prepare_write;
    ctx->write = do_write;

    ctx->output_plugin_options = output_plugin_options;

    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
 * read_page, prepare_write, do_write are 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 *
CreateInitDecodingContext(char *plugin,
                          List *output_plugin_options,
                          XLogPageReadCB read_page,
                          LogicalOutputPluginWriterPrepareWrite prepare_write,
                          LogicalOutputPluginWriterWrite do_write)
{
    TransactionId xmin_horizon = InvalidTransactionId;
    ReplicationSlot *slot;
    LogicalDecodingContext *ctx;
    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");

    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 */
    SpinLockAcquire(&slot->mutex);
    StrNCpy(NameStr(slot->data.plugin), plugin, NAMEDATALEN);
    SpinLockRelease(&slot->mutex);

    ReplicationSlotReserveWal();

    /* ----
     * 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.
     * ----
     */
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

    slot->effective_catalog_xmin = GetOldestSafeDecodingTransactionId();
    slot->data.catalog_xmin = slot->effective_catalog_xmin;

    ReplicationSlotsComputeRequiredXmin(true);

    LWLockRelease(ProcArrayLock);

    /*
     * tell the snapshot builder to only assemble snapshot once reaching the a
     * running_xact's record with the respective xmin.
     */
    xmin_horizon = slot->data.catalog_xmin;

    ReplicationSlotMarkDirty();
    ReplicationSlotSave();

    ctx = StartupDecodingContext(NIL, InvalidXLogRecPtr, xmin_horizon,
                                 read_page, prepare_write, do_write);

    /* 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);

    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
 *      contains options passed to the output plugin.
 *
 * read_page, prepare_write, do_write
 *      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,
                      XLogPageReadCB read_page,
                      LogicalOutputPluginWriterPrepareWrite prepare_write,
                      LogicalOutputPluginWriterWrite do_write)
{
    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"))));

    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 (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.
         */
        elog(DEBUG1, "cannot stream from %X/%X, minimum is %X/%X, forwarding",
             (uint32) (start_lsn >> 32), (uint32) start_lsn,
             (uint32) (slot->data.confirmed_flush >> 32),
             (uint32) slot->data.confirmed_flush);

        start_lsn = slot->data.confirmed_flush;
    }

    ctx = StartupDecodingContext(output_plugin_options,
                                 start_lsn, InvalidTransactionId,
                                 read_page, prepare_write, do_write);

    /* 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);

    ereport(LOG,
            (errmsg("starting logical decoding for slot \"%s\"",
                    NameStr(slot->data.name)),
             errdetail("streaming transactions committing after %X/%X, reading WAL from %X/%X",
                       (uint32) (slot->data.confirmed_flush >> 32),
                       (uint32) slot->data.confirmed_flush,
                       (uint32) (slot->data.restart_lsn >> 32),
                       (uint32) 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)
{
    XLogRecPtr  startptr;

    /* Initialize from where to start reading WAL. */
    startptr = ctx->slot->data.restart_lsn;

    elog(DEBUG1, "searching for logical decoding starting point, starting at %X/%X",
         (uint32) (ctx->slot->data.restart_lsn >> 32),
         (uint32) ctx->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, startptr, &err);
        if (err)
            elog(ERROR, "%s", err);
        if (!record)
            elog(ERROR, "no record found");     /* shouldn't happen */

        startptr = InvalidXLogRecPtr;

        LogicalDecodingProcessRecord(ctx, ctx->reader);

        /* only continue till we found a consistent spot */
        if (DecodingContextReady(ctx))
            break;

        CHECK_FOR_INTERRUPTS();
    }

    ctx->slot->data.confirmed_flush = ctx->reader->EndRecPtr;
}

/*
 * 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;
}

/*
 * Load the output plugin, lookup its output plugin init function, and check
 * that it provides the required callbacks.
 */
static void
LoadOutputPlugin(OutputPluginCallbacks *callbacks, 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/%X",
                   NameStr(state->ctx->slot->data.name),
                   NameStr(state->ctx->slot->data.plugin),
                   state->callback_name,
                   (uint32) (state->report_location >> 32),
                   (uint32) 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;

    /* 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 = (void *) &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;

    /* set output state */
    ctx->accept_writes = 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;

    /* 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 = (void *) &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;

    /* set output state */
    ctx->accept_writes = 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;

    /* 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 = (void *) &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;

    /* 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;

    /* 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 = (void *) &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 */

    /* do the actual work: call callback */
    ctx->callbacks.commit_cb(ctx, txn, commit_lsn);

    /* 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;

    /* 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 = (void *) &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 up2date
     * 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->callbacks.change_cb(ctx, txn, relation, change);

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

bool
filter_by_origin_cb_wrapper(LogicalDecodingContext *ctx, RepOriginId origin_id)
{
    LogicalErrorCallbackState state;
    ErrorContextCallback errcallback;
    bool        ret;

    /* 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 = (void *) &state;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;

    /* set output state */
    ctx->accept_writes = 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;

    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 = (void *) &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;

    /* 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;
}

/*
 * 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;

    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;
    }
    SpinLockRelease(&slot->mutex);

    /* 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 IncreaseRestartDecodingForSlot 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)
    {
    }

    /*
     * 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;

        /* 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.
     */
    if (slot->candidate_restart_valid == InvalidXLogRecPtr)
    {
        slot->candidate_restart_valid = current_lsn;
        slot->candidate_restart_lsn = restart_lsn;

        elog(DEBUG1, "got new restart lsn %X/%X at %X/%X",
             (uint32) (restart_lsn >> 32), (uint32) restart_lsn,
             (uint32) (current_lsn >> 32), (uint32) current_lsn);
    }
    else
    {
        elog(DEBUG1, "failed to increase restart lsn: proposed %X/%X, after %X/%X, current candidate %X/%X, current after %X/%X, flushed up to %X/%X",
             (uint32) (restart_lsn >> 32), (uint32) restart_lsn,
             (uint32) (current_lsn >> 32), (uint32) current_lsn,
             (uint32) (slot->candidate_restart_lsn >> 32),
             (uint32) slot->candidate_restart_lsn,
             (uint32) (slot->candidate_restart_valid >> 32),
             (uint32) slot->candidate_restart_valid,
             (uint32) (slot->data.confirmed_flush >> 32),
             (uint32) slot->data.confirmed_flush
            );
    }
    SpinLockRelease(&slot->mutex);

    /* candidates are already valid with the current flush position, apply */
    if (updated_lsn)
        LogicalConfirmReceivedLocation(slot->data.confirmed_flush);
}

/*
 * Handle a consumer's conformation 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;

        SpinLockAcquire(&MyReplicationSlot->mutex);

        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)
        {
            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);
        MyReplicationSlot->data.confirmed_flush = lsn;
        SpinLockRelease(&MyReplicationSlot->mutex);
    }
}