pqmq.c

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/*-------------------------------------------------------------------------
 *
 * pqmq.c
 *    Use the frontend/backend protocol for communication over a shm_mq
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *  src/backend/libpq/pqmq.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/parallel.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "libpq/pqmq.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "replication/logicalworker.h"
#include "storage/latch.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/wait_event.h"
 
static shm_mq_handle *pq_mq_handle = NULL;
static bool pq_mq_busy = false;
static pid_t pq_mq_parallel_leader_pid = 0;
static ProcNumber pq_mq_parallel_leader_proc_number = INVALID_PROC_NUMBER;
 
static void pq_cleanup_redirect_to_shm_mq(dsm_segment *seg, Datum arg);
static void mq_comm_reset(void);
static int  mq_flush(void);
static int  mq_flush_if_writable(void);
static bool mq_is_send_pending(void);
static int  mq_putmessage(char msgtype, const char *s, size_t len);
static void mq_putmessage_noblock(char msgtype, const char *s, size_t len);
 
static const PQcommMethods PqCommMqMethods = {
    .comm_reset = mq_comm_reset,
    .flush = mq_flush,
    .flush_if_writable = mq_flush_if_writable,
    .is_send_pending = mq_is_send_pending,
    .putmessage = mq_putmessage,
    .putmessage_noblock = mq_putmessage_noblock
};
 
/*
 * Arrange to redirect frontend/backend protocol messages to a shared-memory
 * message queue.
 */
void
pq_redirect_to_shm_mq(dsm_segment *seg, shm_mq_handle *mqh)
{
    PqCommMethods = &PqCommMqMethods;
    pq_mq_handle = mqh;
    whereToSendOutput = DestRemote;
    FrontendProtocol = PG_PROTOCOL_LATEST;
    on_dsm_detach(seg, pq_cleanup_redirect_to_shm_mq, (Datum) 0);
}
 
/*
 * When the DSM that contains our shm_mq goes away, we need to stop sending
 * messages to it.
 */
static void
pq_cleanup_redirect_to_shm_mq(dsm_segment *seg, Datum arg)
{
    if (pq_mq_handle != NULL)
    {
        pfree(pq_mq_handle);
        pq_mq_handle = NULL;
    }
    whereToSendOutput = DestNone;
}
 
/*
 * Arrange to SendProcSignal() to the parallel leader each time we transmit
 * message data via the shm_mq.
 */
void
pq_set_parallel_leader(pid_t pid, ProcNumber procNumber)
{
    Assert(PqCommMethods == &PqCommMqMethods);
    pq_mq_parallel_leader_pid = pid;
    pq_mq_parallel_leader_proc_number = procNumber;
}
 
static void
mq_comm_reset(void)
{
    /* Nothing to do. */
}
 
static int
mq_flush(void)
{
    /* Nothing to do. */
    return 0;
}
 
static int
mq_flush_if_writable(void)
{
    /* Nothing to do. */
    return 0;
}
 
static bool
mq_is_send_pending(void)
{
    /* There's never anything pending. */
    return 0;
}
 
/*
 * Transmit a libpq protocol message to the shared memory message queue
 * selected via pq_mq_handle.  We don't include a length word, because the
 * receiver will know the length of the message from shm_mq_receive().
 */
static int
mq_putmessage(char msgtype, const char *s, size_t len)
{
    shm_mq_iovec iov[2];
    shm_mq_result result;
 
    /*
     * If we're sending a message, and we have to wait because the queue is
     * full, and then we get interrupted, and that interrupt results in trying
     * to send another message, we respond by detaching the queue.  There's no
     * way to return to the original context, but even if there were, just
     * queueing the message would amount to indefinitely postponing the
     * response to the interrupt.  So we do this instead.
     */
    if (pq_mq_busy)
    {
        if (pq_mq_handle != NULL)
        {
            shm_mq_detach(pq_mq_handle);
            pfree(pq_mq_handle);
            pq_mq_handle = NULL;
        }
        return EOF;
    }
 
    /*
     * If the message queue is already gone, just ignore the message. This
     * doesn't necessarily indicate a problem; for example, DEBUG messages can
     * be generated late in the shutdown sequence, after all DSMs have already
     * been detached.
     */
    if (pq_mq_handle == NULL)
        return 0;
 
    pq_mq_busy = true;
 
    iov[0].data = &msgtype;
    iov[0].len = 1;
    iov[1].data = s;
    iov[1].len = len;
 
    for (;;)
    {
        /*
         * Immediately notify the receiver by passing force_flush as true so
         * that the shared memory value is updated before we send the parallel
         * message signal right after this.
         */
        Assert(pq_mq_handle != NULL);
        result = shm_mq_sendv(pq_mq_handle, iov, 2, true, true);
 
        if (pq_mq_parallel_leader_pid != 0)
        {
            if (IsLogicalParallelApplyWorker())
                SendProcSignal(pq_mq_parallel_leader_pid,
                               PROCSIG_PARALLEL_APPLY_MESSAGE,
                               pq_mq_parallel_leader_proc_number);
            else
            {
                Assert(IsParallelWorker());
                SendProcSignal(pq_mq_parallel_leader_pid,
                               PROCSIG_PARALLEL_MESSAGE,
                               pq_mq_parallel_leader_proc_number);
            }
        }
 
        if (result != SHM_MQ_WOULD_BLOCK)
            break;
 
        (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
                         WAIT_EVENT_MESSAGE_QUEUE_PUT_MESSAGE);
        ResetLatch(MyLatch);
        CHECK_FOR_INTERRUPTS();
    }
 
    pq_mq_busy = false;
 
    Assert(result == SHM_MQ_SUCCESS || result == SHM_MQ_DETACHED);
    if (result != SHM_MQ_SUCCESS)
        return EOF;
    return 0;
}
 
static void
mq_putmessage_noblock(char msgtype, const char *s, size_t len)
{
    /*
     * While the shm_mq machinery does support sending a message in
     * non-blocking mode, there's currently no way to try sending beginning to
     * send the message that doesn't also commit us to completing the
     * transmission.  This could be improved in the future, but for now we
     * don't need it.
     */
    elog(ERROR, "not currently supported");
}
 
/*
 * Parse an ErrorResponse or NoticeResponse payload and populate an ErrorData
 * structure with the results.
 */
void
pq_parse_errornotice(StringInfo msg, ErrorData *edata)
{
    /* Initialize edata with reasonable defaults. */
    MemSet(edata, 0, sizeof(ErrorData));
    edata->elevel = ERROR;
    edata->assoc_context = CurrentMemoryContext;
 
    /* Loop over fields and extract each one. */
    for (;;)
    {
        char        code = pq_getmsgbyte(msg);
        const char *value;
 
        if (code == '\0')
        {
            pq_getmsgend(msg);
            break;
        }
        value = pq_getmsgrawstring(msg);
 
        switch (code)
        {
            case PG_DIAG_SEVERITY:
                /* ignore, trusting we'll get a nonlocalized version */
                break;
            case PG_DIAG_SEVERITY_NONLOCALIZED:
                if (strcmp(value, "DEBUG") == 0)
                {
                    /*
                     * We can't reconstruct the exact DEBUG level, but
                     * presumably it was >= client_min_messages, so select
                     * DEBUG1 to ensure we'll pass it on to the client.
                     */
                    edata->elevel = DEBUG1;
                }
                else if (strcmp(value, "LOG") == 0)
                {
                    /*
                     * It can't be LOG_SERVER_ONLY, or the worker wouldn't
                     * have sent it to us; so LOG is the correct value.
                     */
                    edata->elevel = LOG;
                }
                else if (strcmp(value, "INFO") == 0)
                    edata->elevel = INFO;
                else if (strcmp(value, "NOTICE") == 0)
                    edata->elevel = NOTICE;
                else if (strcmp(value, "WARNING") == 0)
                    edata->elevel = WARNING;
                else if (strcmp(value, "ERROR") == 0)
                    edata->elevel = ERROR;
                else if (strcmp(value, "FATAL") == 0)
                    edata->elevel = FATAL;
                else if (strcmp(value, "PANIC") == 0)
                    edata->elevel = PANIC;
                else
                    elog(ERROR, "unrecognized error severity: \"%s\"", value);
                break;
            case PG_DIAG_SQLSTATE:
                if (strlen(value) != 5)
                    elog(ERROR, "invalid SQLSTATE: \"%s\"", value);
                edata->sqlerrcode = MAKE_SQLSTATE(value[0], value[1], value[2],
                                                  value[3], value[4]);
                break;
            case PG_DIAG_MESSAGE_PRIMARY:
                edata->message = pstrdup(value);
                break;
            case PG_DIAG_MESSAGE_DETAIL:
                edata->detail = pstrdup(value);
                break;
            case PG_DIAG_MESSAGE_HINT:
                edata->hint = pstrdup(value);
                break;
            case PG_DIAG_STATEMENT_POSITION:
                edata->cursorpos = pg_strtoint32(value);
                break;
            case PG_DIAG_INTERNAL_POSITION:
                edata->internalpos = pg_strtoint32(value);
                break;
            case PG_DIAG_INTERNAL_QUERY:
                edata->internalquery = pstrdup(value);
                break;
            case PG_DIAG_CONTEXT:
                edata->context = pstrdup(value);
                break;
            case PG_DIAG_SCHEMA_NAME:
                edata->schema_name = pstrdup(value);
                break;
            case PG_DIAG_TABLE_NAME:
                edata->table_name = pstrdup(value);
                break;
            case PG_DIAG_COLUMN_NAME:
                edata->column_name = pstrdup(value);
                break;
            case PG_DIAG_DATATYPE_NAME:
                edata->datatype_name = pstrdup(value);
                break;
            case PG_DIAG_CONSTRAINT_NAME:
                edata->constraint_name = pstrdup(value);
                break;
            case PG_DIAG_SOURCE_FILE:
                edata->filename = pstrdup(value);
                break;
            case PG_DIAG_SOURCE_LINE:
                edata->lineno = pg_strtoint32(value);
                break;
            case PG_DIAG_SOURCE_FUNCTION:
                edata->funcname = pstrdup(value);
                break;
            default:
                elog(ERROR, "unrecognized error field code: %d", code);
                break;
        }
    }
}