logicalfuncs.c

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/*-------------------------------------------------------------------------
 *
 * logicalfuncs.c
 *
 *     Support functions for using logical decoding and management of
 *     logical replication slots via SQL.
 *
 *
 * Copyright (c) 2012-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/logicalfuncs.c
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <unistd.h>

#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogutils.h"
#include "catalog/pg_type.h"
#include "fmgr.h"
#include "funcapi.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "replication/decode.h"
#include "replication/logical.h"
#include "replication/message.h"
#include "storage/fd.h"
#include "utils/array.h"
#include "utils/builtins.h"
#include "utils/inval.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/pg_lsn.h"
#include "utils/regproc.h"
#include "utils/resowner.h"

/* private date for writing out data */
typedef struct DecodingOutputState
{
    Tuplestorestate *tupstore;
    TupleDesc   tupdesc;
    bool        binary_output;
    int64       returned_rows;
} DecodingOutputState;

/*
 * Prepare for an output plugin write.
 */
static void
LogicalOutputPrepareWrite(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
                          bool last_write)
{
    resetStringInfo(ctx->out);
}

/*
 * Perform output plugin write into tuplestore.
 */
static void
LogicalOutputWrite(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
                   bool last_write)
{
    Datum       values[3];
    bool        nulls[3];
    DecodingOutputState *p;

    /* SQL Datums can only be of a limited length... */
    if (ctx->out->len > MaxAllocSize - VARHDRSZ)
        elog(ERROR, "too much output for sql interface");

    p = (DecodingOutputState *) ctx->output_writer_private;

    memset(nulls, 0, sizeof(nulls));
    values[0] = LSNGetDatum(lsn);
    values[1] = TransactionIdGetDatum(xid);

    /*
     * Assert ctx->out is in database encoding when we're writing textual
     * output.
     */
    if (!p->binary_output)
        Assert(pg_verify_mbstr(GetDatabaseEncoding(),
                               ctx->out->data, ctx->out->len,
                               false));

    /* ick, but cstring_to_text_with_len works for bytea perfectly fine */
    values[2] = PointerGetDatum(cstring_to_text_with_len(ctx->out->data, ctx->out->len));

    tuplestore_putvalues(p->tupstore, p->tupdesc, values, nulls);
    p->returned_rows++;
}

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

/*
 * Helper function for the various SQL callable logical decoding functions.
 */
static Datum
pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool binary)
{
    Name        name;
    XLogRecPtr  upto_lsn;
    int32       upto_nchanges;
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    MemoryContext per_query_ctx;
    MemoryContext oldcontext;
    XLogRecPtr  end_of_wal;
    LogicalDecodingContext *ctx;
    ResourceOwner old_resowner = CurrentResourceOwner;
    ArrayType  *arr;
    Size        ndim;
    List       *options = NIL;
    DecodingOutputState *p;

    check_permissions();

    CheckLogicalDecodingRequirements();

    if (PG_ARGISNULL(0))
        ereport(ERROR,
                (errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
                 errmsg("slot name must not be null")));
    name = PG_GETARG_NAME(0);

    if (PG_ARGISNULL(1))
        upto_lsn = InvalidXLogRecPtr;
    else
        upto_lsn = PG_GETARG_LSN(1);

    if (PG_ARGISNULL(2))
        upto_nchanges = InvalidXLogRecPtr;
    else
        upto_nchanges = PG_GETARG_INT32(2);

    if (PG_ARGISNULL(3))
        ereport(ERROR,
                (errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
                 errmsg("options array must not be null")));
    arr = PG_GETARG_ARRAYTYPE_P(3);

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

    /* state to write output to */
    p = palloc0(sizeof(DecodingOutputState));

    p->binary_output = binary;

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

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

    /* Deconstruct options array */
    ndim = ARR_NDIM(arr);
    if (ndim > 1)
    {
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("array must be one-dimensional")));
    }
    else if (array_contains_nulls(arr))
    {
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("array must not contain nulls")));
    }
    else if (ndim == 1)
    {
        int         nelems;
        Datum      *datum_opts;
        int         i;

        Assert(ARR_ELEMTYPE(arr) == TEXTOID);

        deconstruct_array(arr, TEXTOID, -1, false, TYPALIGN_INT,
                          &datum_opts, NULL, &nelems);

        if (nelems % 2 != 0)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("array must have even number of elements")));

        for (i = 0; i < nelems; i += 2)
        {
            char       *name = TextDatumGetCString(datum_opts[i]);
            char       *opt = TextDatumGetCString(datum_opts[i + 1]);

            options = lappend(options, makeDefElem(name, (Node *) makeString(opt), -1));
        }
    }

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

    /*
     * Compute the current end-of-wal and maintain ThisTimeLineID.
     * RecoveryInProgress() will update ThisTimeLineID on promotion.
     */
    if (!RecoveryInProgress())
        end_of_wal = GetFlushRecPtr();
    else
        end_of_wal = GetXLogReplayRecPtr(&ThisTimeLineID);

    (void) ReplicationSlotAcquire(NameStr(*name), SAB_Error);

    PG_TRY();
    {
        /* restart at slot's confirmed_flush */
        ctx = CreateDecodingContext(InvalidXLogRecPtr,
                                    options,
                                    false,
                                    read_local_xlog_page,
                                    wal_segment_close,
                                    LogicalOutputPrepareWrite,
                                    LogicalOutputWrite, NULL);

        /*
         * After the sanity checks in CreateDecodingContext, make sure the
         * restart_lsn is valid.  Avoid "cannot get changes" wording in this
         * errmsg because that'd be confusingly ambiguous about no changes
         * being available.
         */
        if (XLogRecPtrIsInvalid(MyReplicationSlot->data.restart_lsn))
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("can no longer get changes from replication slot \"%s\"",
                            NameStr(*name)),
                     errdetail("This slot has never previously reserved WAL, or it has been invalidated.")));

        MemoryContextSwitchTo(oldcontext);

        /*
         * Check whether the output plugin writes textual output if that's
         * what we need.
         */
        if (!binary &&
            ctx->options.output_type !=OUTPUT_PLUGIN_TEXTUAL_OUTPUT)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("logical decoding output plugin \"%s\" produces binary output, but function \"%s\" expects textual data",
                            NameStr(MyReplicationSlot->data.plugin),
                            format_procedure(fcinfo->flinfo->fn_oid))));

        ctx->output_writer_private = p;

        /*
         * Decoding of WAL must start at restart_lsn so that the entirety of
         * xacts that committed after the slot's confirmed_flush can be
         * accumulated into reorder buffers.
         */
        XLogBeginRead(ctx->reader, MyReplicationSlot->data.restart_lsn);

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

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

            while (XLogReadRecord(ctx->reader, &record, &errm) ==
                   XLREAD_NEED_DATA)
            {
                if (!ctx->page_read(ctx->reader))
                    break;
            }

            if (errm)
                elog(ERROR, "%s", errm);

            /*
             * The {begin_txn,change,commit_txn}_wrapper callbacks above will
             * store the description into our tuplestore.
             */
            if (record != NULL)
                LogicalDecodingProcessRecord(ctx, ctx->reader);

            /* check limits */
            if (upto_lsn != InvalidXLogRecPtr &&
                upto_lsn <= ctx->reader->EndRecPtr)
                break;
            if (upto_nchanges != 0 &&
                upto_nchanges <= p->returned_rows)
                break;
            CHECK_FOR_INTERRUPTS();
        }

        tuplestore_donestoring(tupstore);

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

        /*
         * Next time, start where we left off. (Hunting things, the family
         * business..)
         */
        if (ctx->reader->EndRecPtr != InvalidXLogRecPtr && confirm)
        {
            LogicalConfirmReceivedLocation(ctx->reader->EndRecPtr);

            /*
             * 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's written out at the next checkpoint.
             * We'll still lose its position on crash, as documented, but it's
             * better than always losing the position even on clean restart.
             */
            ReplicationSlotMarkDirty();
        }

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

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

        PG_RE_THROW();
    }
    PG_END_TRY();

    return (Datum) 0;
}

/*
 * SQL function returning the changestream as text, consuming the data.
 */
Datum
pg_logical_slot_get_changes(PG_FUNCTION_ARGS)
{
    return pg_logical_slot_get_changes_guts(fcinfo, true, false);
}

/*
 * SQL function returning the changestream as text, only peeking ahead.
 */
Datum
pg_logical_slot_peek_changes(PG_FUNCTION_ARGS)
{
    return pg_logical_slot_get_changes_guts(fcinfo, false, false);
}

/*
 * SQL function returning the changestream in binary, consuming the data.
 */
Datum
pg_logical_slot_get_binary_changes(PG_FUNCTION_ARGS)
{
    return pg_logical_slot_get_changes_guts(fcinfo, true, true);
}

/*
 * SQL function returning the changestream in binary, only peeking ahead.
 */
Datum
pg_logical_slot_peek_binary_changes(PG_FUNCTION_ARGS)
{
    return pg_logical_slot_get_changes_guts(fcinfo, false, true);
}


/*
 * SQL function for writing logical decoding message into WAL.
 */
Datum
pg_logical_emit_message_bytea(PG_FUNCTION_ARGS)
{
    bool        transactional = PG_GETARG_BOOL(0);
    char       *prefix = text_to_cstring(PG_GETARG_TEXT_PP(1));
    bytea      *data = PG_GETARG_BYTEA_PP(2);
    XLogRecPtr  lsn;

    lsn = LogLogicalMessage(prefix, VARDATA_ANY(data), VARSIZE_ANY_EXHDR(data),
                            transactional);
    PG_RETURN_LSN(lsn);
}

Datum
pg_logical_emit_message_text(PG_FUNCTION_ARGS)
{
    /* bytea and text are compatible */
    return pg_logical_emit_message_bytea(fcinfo);
}