sinval.c

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/*-------------------------------------------------------------------------
 *
 * sinval.c
 *    POSTGRES shared cache invalidation communication code.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/sinval.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/xact.h"
#include "miscadmin.h"
#include "storage/latch.h"
#include "storage/sinvaladt.h"
#include "utils/inval.h"
 
 
uint64      SharedInvalidMessageCounter;
 
 
/*
 * Because backends sitting idle will not be reading sinval events, we
 * need a way to give an idle backend a swift kick in the rear and make
 * it catch up before the sinval queue overflows and forces it to go
 * through a cache reset exercise.  This is done by sending
 * PROCSIG_CATCHUP_INTERRUPT to any backend that gets too far behind.
 *
 * The signal handler will set an interrupt pending flag and will set the
 * processes latch. Whenever starting to read from the client, or when
 * interrupted while doing so, ProcessClientReadInterrupt() will call
 * ProcessCatchupEvent().
 */
volatile sig_atomic_t catchupInterruptPending = false;
 
 
/*
 * SendSharedInvalidMessages
 *  Add shared-cache-invalidation message(s) to the global SI message queue.
 */
void
SendSharedInvalidMessages(const SharedInvalidationMessage *msgs, int n)
{
    SIInsertDataEntries(msgs, n);
}
 
/*
 * ReceiveSharedInvalidMessages
 *      Process shared-cache-invalidation messages waiting for this backend
 *
 * We guarantee to process all messages that had been queued before the
 * routine was entered.  It is of course possible for more messages to get
 * queued right after our last SIGetDataEntries call.
 *
 * NOTE: it is entirely possible for this routine to be invoked recursively
 * as a consequence of processing inside the invalFunction or resetFunction.
 * Furthermore, such a recursive call must guarantee that all outstanding
 * inval messages have been processed before it exits.  This is the reason
 * for the strange-looking choice to use a statically allocated buffer array
 * and counters; it's so that a recursive call can process messages already
 * sucked out of sinvaladt.c.
 */
void
ReceiveSharedInvalidMessages(void (*invalFunction) (SharedInvalidationMessage *msg),
                             void (*resetFunction) (void))
{
#define MAXINVALMSGS 32
    static SharedInvalidationMessage messages[MAXINVALMSGS];
 
    /*
     * We use volatile here to prevent bugs if a compiler doesn't realize that
     * recursion is a possibility ...
     */
    static volatile int nextmsg = 0;
    static volatile int nummsgs = 0;
 
    /* Deal with any messages still pending from an outer recursion */
    while (nextmsg < nummsgs)
    {
        SharedInvalidationMessage msg = messages[nextmsg++];
 
        SharedInvalidMessageCounter++;
        invalFunction(&msg);
    }
 
    do
    {
        int         getResult;
 
        nextmsg = nummsgs = 0;
 
        /* Try to get some more messages */
        getResult = SIGetDataEntries(messages, MAXINVALMSGS);
 
        if (getResult < 0)
        {
            /* got a reset message */
            elog(DEBUG4, "cache state reset");
            SharedInvalidMessageCounter++;
            resetFunction();
            break;              /* nothing more to do */
        }
 
        /* Process them, being wary that a recursive call might eat some */
        nextmsg = 0;
        nummsgs = getResult;
 
        while (nextmsg < nummsgs)
        {
            SharedInvalidationMessage msg = messages[nextmsg++];
 
            SharedInvalidMessageCounter++;
            invalFunction(&msg);
        }
 
        /*
         * We only need to loop if the last SIGetDataEntries call (which might
         * have been within a recursive call) returned a full buffer.
         */
    } while (nummsgs == MAXINVALMSGS);
 
    /*
     * We are now caught up.  If we received a catchup signal, reset that
     * flag, and call SICleanupQueue().  This is not so much because we need
     * to flush dead messages right now, as that we want to pass on the
     * catchup signal to the next slowest backend.  "Daisy chaining" the
     * catchup signal this way avoids creating spikes in system load for what
     * should be just a background maintenance activity.
     */
    if (catchupInterruptPending)
    {
        catchupInterruptPending = false;
        elog(DEBUG4, "sinval catchup complete, cleaning queue");
        SICleanupQueue(false, 0);
    }
}
 
 
/*
 * HandleCatchupInterrupt
 *
 * This is called when PROCSIG_CATCHUP_INTERRUPT is received.
 *
 * We used to directly call ProcessCatchupEvent directly when idle. These days
 * we just set a flag to do it later and notify the process of that fact by
 * setting the process's latch.
 */
void
HandleCatchupInterrupt(void)
{
    /*
     * Note: this is called by a SIGNAL HANDLER. You must be very wary what
     * you do here.
     */
 
    catchupInterruptPending = true;
 
    /* make sure the event is processed in due course */
    SetLatch(MyLatch);
}
 
/*
 * ProcessCatchupInterrupt
 *
 * The portion of catchup interrupt handling that runs outside of the signal
 * handler, which allows it to actually process pending invalidations.
 */
void
ProcessCatchupInterrupt(void)
{
    while (catchupInterruptPending)
    {
        /*
         * What we need to do here is cause ReceiveSharedInvalidMessages() to
         * run, which will do the necessary work and also reset the
         * catchupInterruptPending flag.  If we are inside a transaction we
         * can just call AcceptInvalidationMessages() to do this.  If we
         * aren't, we start and immediately end a transaction; the call to
         * AcceptInvalidationMessages() happens down inside transaction start.
         *
         * It is awfully tempting to just call AcceptInvalidationMessages()
         * without the rest of the xact start/stop overhead, and I think that
         * would actually work in the normal case; but I am not sure that
         * things would clean up nicely if we got an error partway through.
         */
        if (IsTransactionOrTransactionBlock())
        {
            elog(DEBUG4, "ProcessCatchupEvent inside transaction");
            AcceptInvalidationMessages();
        }
        else
        {
            elog(DEBUG4, "ProcessCatchupEvent outside transaction");
            StartTransactionCommand();
            CommitTransactionCommand();
        }
    }
}