procsignal.c

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/*-------------------------------------------------------------------------
 *
 * procsignal.c
 *    Routines for interprocess signaling
 *
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/procsignal.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <signal.h>
#include <unistd.h>
 
#include "access/parallel.h"
#include "commands/async.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "port/pg_bitutils.h"
#include "replication/logicalctl.h"
#include "replication/logicalworker.h"
#include "replication/walsender.h"
#include "storage/condition_variable.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/proc.h"
#include "storage/shmem.h"
#include "storage/sinval.h"
#include "storage/smgr.h"
#include "tcop/tcopprot.h"
#include "utils/memutils.h"
#include "utils/wait_event.h"
 
/*
 * The SIGUSR1 signal is multiplexed to support signaling multiple event
 * types. The specific reason is communicated via flags in shared memory.
 * We keep a boolean flag for each possible "reason", so that different
 * reasons can be signaled to a process concurrently.  (However, if the same
 * reason is signaled more than once nearly simultaneously, the process may
 * observe it only once.)
 *
 * Each process that wants to receive signals registers its process ID
 * in the ProcSignalSlots array. The array is indexed by ProcNumber to make
 * slot allocation simple, and to avoid having to search the array when you
 * know the ProcNumber of the process you're signaling.  (We do support
 * signaling without ProcNumber, but it's a bit less efficient.)
 *
 * The fields in each slot are protected by a spinlock, pss_mutex. pss_pid can
 * also be read without holding the spinlock, as a quick preliminary check
 * when searching for a particular PID in the array.
 *
 * pss_signalFlags are intended to be set in cases where we don't need to
 * keep track of whether or not the target process has handled the signal,
 * but sometimes we need confirmation, as when making a global state change
 * that cannot be considered complete until all backends have taken notice
 * of it. For such use cases, we set a bit in pss_barrierCheckMask and then
 * increment the current "barrier generation"; when the new barrier generation
 * (or greater) appears in the pss_barrierGeneration flag of every process,
 * we know that the message has been received everywhere.
 */
typedef struct
{
    pg_atomic_uint32 pss_pid;
    int         pss_cancel_key_len; /* 0 means no cancellation is possible */
    uint8       pss_cancel_key[MAX_CANCEL_KEY_LENGTH];
    volatile sig_atomic_t pss_signalFlags[NUM_PROCSIGNALS];
    slock_t     pss_mutex;      /* protects the above fields */
 
    /* Barrier-related fields (not protected by pss_mutex) */
    pg_atomic_uint64 pss_barrierGeneration;
    pg_atomic_uint32 pss_barrierCheckMask;
    ConditionVariable pss_barrierCV;
} ProcSignalSlot;
 
/*
 * Information that is global to the entire ProcSignal system can be stored
 * here.
 *
 * psh_barrierGeneration is the highest barrier generation in existence.
 */
struct ProcSignalHeader
{
    pg_atomic_uint64 psh_barrierGeneration;
    ProcSignalSlot psh_slot[FLEXIBLE_ARRAY_MEMBER];
};
 
/*
 * We reserve a slot for each possible ProcNumber, plus one for each
 * possible auxiliary process type.  (This scheme assumes there is not
 * more than one of any auxiliary process type at a time, except for
 * IO workers.)
 */
#define NumProcSignalSlots  (MaxBackends + NUM_AUXILIARY_PROCS)
 
/* Check whether the relevant type bit is set in the flags. */
#define BARRIER_SHOULD_CHECK(flags, type) \
    (((flags) & (((uint32) 1) << (uint32) (type))) != 0)
 
/* Clear the relevant type bit from the flags. */
#define BARRIER_CLEAR_BIT(flags, type) \
    ((flags) &= ~(((uint32) 1) << (uint32) (type)))
 
NON_EXEC_STATIC ProcSignalHeader *ProcSignal = NULL;
static ProcSignalSlot *MyProcSignalSlot = NULL;
 
static bool CheckProcSignal(ProcSignalReason reason);
static void CleanupProcSignalState(int status, Datum arg);
static void ResetProcSignalBarrierBits(uint32 flags);
 
/*
 * ProcSignalShmemSize
 *      Compute space needed for ProcSignal's shared memory
 */
Size
ProcSignalShmemSize(void)
{
    Size        size;
 
    size = mul_size(NumProcSignalSlots, sizeof(ProcSignalSlot));
    size = add_size(size, offsetof(ProcSignalHeader, psh_slot));
    return size;
}
 
/*
 * ProcSignalShmemInit
 *      Allocate and initialize ProcSignal's shared memory
 */
void
ProcSignalShmemInit(void)
{
    Size        size = ProcSignalShmemSize();
    bool        found;
 
    ProcSignal = (ProcSignalHeader *)
        ShmemInitStruct("ProcSignal", size, &found);
 
    /* If we're first, initialize. */
    if (!found)
    {
        int         i;
 
        pg_atomic_init_u64(&ProcSignal->psh_barrierGeneration, 0);
 
        for (i = 0; i < NumProcSignalSlots; ++i)
        {
            ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
 
            SpinLockInit(&slot->pss_mutex);
            pg_atomic_init_u32(&slot->pss_pid, 0);
            slot->pss_cancel_key_len = 0;
            MemSet(slot->pss_signalFlags, 0, sizeof(slot->pss_signalFlags));
            pg_atomic_init_u64(&slot->pss_barrierGeneration, PG_UINT64_MAX);
            pg_atomic_init_u32(&slot->pss_barrierCheckMask, 0);
            ConditionVariableInit(&slot->pss_barrierCV);
        }
    }
}
 
/*
 * ProcSignalInit
 *      Register the current process in the ProcSignal array
 */
void
ProcSignalInit(const uint8 *cancel_key, int cancel_key_len)
{
    ProcSignalSlot *slot;
    uint64      barrier_generation;
    uint32      old_pss_pid;
 
    Assert(cancel_key_len >= 0 && cancel_key_len <= MAX_CANCEL_KEY_LENGTH);
    if (MyProcNumber < 0)
        elog(ERROR, "MyProcNumber not set");
    if (MyProcNumber >= NumProcSignalSlots)
        elog(ERROR, "unexpected MyProcNumber %d in ProcSignalInit (max %d)", MyProcNumber, NumProcSignalSlots);
    slot = &ProcSignal->psh_slot[MyProcNumber];
 
    SpinLockAcquire(&slot->pss_mutex);
 
    /* Value used for sanity check below */
    old_pss_pid = pg_atomic_read_u32(&slot->pss_pid);
 
    /* Clear out any leftover signal reasons */
    MemSet(slot->pss_signalFlags, 0, NUM_PROCSIGNALS * sizeof(sig_atomic_t));
 
    /*
     * Initialize barrier state. Since we're a brand-new process, there
     * shouldn't be any leftover backend-private state that needs to be
     * updated. Therefore, we can broadcast the latest barrier generation and
     * disregard any previously-set check bits.
     *
     * NB: This only works if this initialization happens early enough in the
     * startup sequence that we haven't yet cached any state that might need
     * to be invalidated. That's also why we have a memory barrier here, to be
     * sure that any later reads of memory happen strictly after this.
     */
    pg_atomic_write_u32(&slot->pss_barrierCheckMask, 0);
    barrier_generation =
        pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration);
    pg_atomic_write_u64(&slot->pss_barrierGeneration, barrier_generation);
 
    if (cancel_key_len > 0)
        memcpy(slot->pss_cancel_key, cancel_key, cancel_key_len);
    slot->pss_cancel_key_len = cancel_key_len;
    pg_atomic_write_u32(&slot->pss_pid, MyProcPid);
 
    SpinLockRelease(&slot->pss_mutex);
 
    /* Spinlock is released, do the check */
    if (old_pss_pid != 0)
        elog(LOG, "process %d taking over ProcSignal slot %d, but it's not empty",
             MyProcPid, MyProcNumber);
 
    /* Remember slot location for CheckProcSignal */
    MyProcSignalSlot = slot;
 
    /* Set up to release the slot on process exit */
    on_shmem_exit(CleanupProcSignalState, (Datum) 0);
}
 
/*
 * CleanupProcSignalState
 *      Remove current process from ProcSignal mechanism
 *
 * This function is called via on_shmem_exit() during backend shutdown.
 */
static void
CleanupProcSignalState(int status, Datum arg)
{
    pid_t       old_pid;
    ProcSignalSlot *slot = MyProcSignalSlot;
 
    /*
     * Clear MyProcSignalSlot, so that a SIGUSR1 received after this point
     * won't try to access it after it's no longer ours (and perhaps even
     * after we've unmapped the shared memory segment).
     */
    Assert(MyProcSignalSlot != NULL);
    MyProcSignalSlot = NULL;
 
    /* sanity check */
    SpinLockAcquire(&slot->pss_mutex);
    old_pid = pg_atomic_read_u32(&slot->pss_pid);
    if (old_pid != MyProcPid)
    {
        /*
         * don't ERROR here. We're exiting anyway, and don't want to get into
         * infinite loop trying to exit
         */
        SpinLockRelease(&slot->pss_mutex);
        elog(LOG, "process %d releasing ProcSignal slot %d, but it contains %d",
             MyProcPid, (int) (slot - ProcSignal->psh_slot), (int) old_pid);
        return;                 /* XXX better to zero the slot anyway? */
    }
 
    /* Mark the slot as unused */
    pg_atomic_write_u32(&slot->pss_pid, 0);
    slot->pss_cancel_key_len = 0;
 
    /*
     * Make this slot look like it's absorbed all possible barriers, so that
     * no barrier waits block on it.
     */
    pg_atomic_write_u64(&slot->pss_barrierGeneration, PG_UINT64_MAX);
 
    SpinLockRelease(&slot->pss_mutex);
 
    ConditionVariableBroadcast(&slot->pss_barrierCV);
}
 
/*
 * SendProcSignal
 *      Send a signal to a Postgres process
 *
 * Providing procNumber is optional, but it will speed up the operation.
 *
 * On success (a signal was sent), zero is returned.
 * On error, -1 is returned, and errno is set (typically to ESRCH or EPERM).
 *
 * Not to be confused with ProcSendSignal
 */
int
SendProcSignal(pid_t pid, ProcSignalReason reason, ProcNumber procNumber)
{
    volatile ProcSignalSlot *slot;
 
    if (procNumber != INVALID_PROC_NUMBER)
    {
        Assert(procNumber < NumProcSignalSlots);
        slot = &ProcSignal->psh_slot[procNumber];
 
        SpinLockAcquire(&slot->pss_mutex);
        if (pg_atomic_read_u32(&slot->pss_pid) == pid)
        {
            /* Atomically set the proper flag */
            slot->pss_signalFlags[reason] = true;
            SpinLockRelease(&slot->pss_mutex);
            /* Send signal */
            return kill(pid, SIGUSR1);
        }
        SpinLockRelease(&slot->pss_mutex);
    }
    else
    {
        /*
         * procNumber not provided, so search the array using pid.  We search
         * the array back to front so as to reduce search overhead.  Passing
         * INVALID_PROC_NUMBER means that the target is most likely an
         * auxiliary process, which will have a slot near the end of the
         * array.
         */
        int         i;
 
        for (i = NumProcSignalSlots - 1; i >= 0; i--)
        {
            slot = &ProcSignal->psh_slot[i];
 
            if (pg_atomic_read_u32(&slot->pss_pid) == pid)
            {
                SpinLockAcquire(&slot->pss_mutex);
                if (pg_atomic_read_u32(&slot->pss_pid) == pid)
                {
                    /* Atomically set the proper flag */
                    slot->pss_signalFlags[reason] = true;
                    SpinLockRelease(&slot->pss_mutex);
                    /* Send signal */
                    return kill(pid, SIGUSR1);
                }
                SpinLockRelease(&slot->pss_mutex);
            }
        }
    }
 
    errno = ESRCH;
    return -1;
}
 
/*
 * EmitProcSignalBarrier
 *      Send a signal to every Postgres process
 *
 * The return value of this function is the barrier "generation" created
 * by this operation. This value can be passed to WaitForProcSignalBarrier
 * to wait until it is known that every participant in the ProcSignal
 * mechanism has absorbed the signal (or started afterwards).
 *
 * Note that it would be a bad idea to use this for anything that happens
 * frequently, as interrupting every backend could cause a noticeable
 * performance hit.
 *
 * Callers are entitled to assume that this function will not throw ERROR
 * or FATAL.
 */
uint64
EmitProcSignalBarrier(ProcSignalBarrierType type)
{
    uint32      flagbit = 1 << (uint32) type;
    uint64      generation;
 
    /*
     * Set all the flags.
     *
     * Note that pg_atomic_fetch_or_u32 has full barrier semantics, so this is
     * totally ordered with respect to anything the caller did before, and
     * anything that we do afterwards. (This is also true of the later call to
     * pg_atomic_add_fetch_u64.)
     */
    for (int i = 0; i < NumProcSignalSlots; i++)
    {
        volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
 
        pg_atomic_fetch_or_u32(&slot->pss_barrierCheckMask, flagbit);
    }
 
    /*
     * Increment the generation counter.
     */
    generation =
        pg_atomic_add_fetch_u64(&ProcSignal->psh_barrierGeneration, 1);
 
    /*
     * Signal all the processes, so that they update their advertised barrier
     * generation.
     *
     * Concurrency is not a problem here. Backends that have exited don't
     * matter, and new backends that have joined since we entered this
     * function must already have current state, since the caller is
     * responsible for making sure that the relevant state is entirely visible
     * before calling this function in the first place. We still have to wake
     * them up - because we can't distinguish between such backends and older
     * backends that need to update state - but they won't actually need to
     * change any state.
     */
    for (int i = NumProcSignalSlots - 1; i >= 0; i--)
    {
        volatile ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
        pid_t       pid = pg_atomic_read_u32(&slot->pss_pid);
 
        if (pid != 0)
        {
            SpinLockAcquire(&slot->pss_mutex);
            pid = pg_atomic_read_u32(&slot->pss_pid);
            if (pid != 0)
            {
                /* see SendProcSignal for details */
                slot->pss_signalFlags[PROCSIG_BARRIER] = true;
                SpinLockRelease(&slot->pss_mutex);
                kill(pid, SIGUSR1);
            }
            else
                SpinLockRelease(&slot->pss_mutex);
        }
    }
 
    return generation;
}
 
/*
 * WaitForProcSignalBarrier - wait until it is guaranteed that all changes
 * requested by a specific call to EmitProcSignalBarrier() have taken effect.
 */
void
WaitForProcSignalBarrier(uint64 generation)
{
    Assert(generation <= pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration));
 
    elog(DEBUG1,
         "waiting for all backends to process ProcSignalBarrier generation "
         UINT64_FORMAT,
         generation);
 
    for (int i = NumProcSignalSlots - 1; i >= 0; i--)
    {
        ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
        uint64      oldval;
 
        /*
         * It's important that we check only pss_barrierGeneration here and
         * not pss_barrierCheckMask. Bits in pss_barrierCheckMask get cleared
         * before the barrier is actually absorbed, but pss_barrierGeneration
         * is updated only afterward.
         */
        oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
        while (oldval < generation)
        {
            if (ConditionVariableTimedSleep(&slot->pss_barrierCV,
                                            5000,
                                            WAIT_EVENT_PROC_SIGNAL_BARRIER))
                ereport(LOG,
                        (errmsg("still waiting for backend with PID %d to accept ProcSignalBarrier",
                                (int) pg_atomic_read_u32(&slot->pss_pid))));
            oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
        }
        ConditionVariableCancelSleep();
    }
 
    elog(DEBUG1,
         "finished waiting for all backends to process ProcSignalBarrier generation "
         UINT64_FORMAT,
         generation);
 
    /*
     * The caller is probably calling this function because it wants to read
     * the shared state or perform further writes to shared state once all
     * backends are known to have absorbed the barrier. However, the read of
     * pss_barrierGeneration was performed unlocked; insert a memory barrier
     * to separate it from whatever follows.
     */
    pg_memory_barrier();
}
 
/*
 * Handle receipt of an interrupt indicating a global barrier event.
 *
 * All the actual work is deferred to ProcessProcSignalBarrier(), because we
 * cannot safely access the barrier generation inside the signal handler as
 * 64bit atomics might use spinlock based emulation, even for reads. As this
 * routine only gets called when PROCSIG_BARRIER is sent that won't cause a
 * lot of unnecessary work.
 */
static void
HandleProcSignalBarrierInterrupt(void)
{
    InterruptPending = true;
    ProcSignalBarrierPending = true;
    /* latch will be set by procsignal_sigusr1_handler */
}
 
/*
 * Perform global barrier related interrupt checking.
 *
 * Any backend that participates in ProcSignal signaling must arrange to
 * call this function periodically. It is called from CHECK_FOR_INTERRUPTS(),
 * which is enough for normal backends, but not necessarily for all types of
 * background processes.
 */
void
ProcessProcSignalBarrier(void)
{
    uint64      local_gen;
    uint64      shared_gen;
    volatile uint32 flags;
 
    Assert(MyProcSignalSlot);
 
    /* Exit quickly if there's no work to do. */
    if (!ProcSignalBarrierPending)
        return;
    ProcSignalBarrierPending = false;
 
    /*
     * It's not unlikely to process multiple barriers at once, before the
     * signals for all the barriers have arrived. To avoid unnecessary work in
     * response to subsequent signals, exit early if we already have processed
     * all of them.
     */
    local_gen = pg_atomic_read_u64(&MyProcSignalSlot->pss_barrierGeneration);
    shared_gen = pg_atomic_read_u64(&ProcSignal->psh_barrierGeneration);
 
    Assert(local_gen <= shared_gen);
 
    if (local_gen == shared_gen)
        return;
 
    /*
     * Get and clear the flags that are set for this backend. Note that
     * pg_atomic_exchange_u32 is a full barrier, so we're guaranteed that the
     * read of the barrier generation above happens before we atomically
     * extract the flags, and that any subsequent state changes happen
     * afterward.
     *
     * NB: In order to avoid race conditions, we must zero
     * pss_barrierCheckMask first and only afterwards try to do barrier
     * processing. If we did it in the other order, someone could send us
     * another barrier of some type right after we called the
     * barrier-processing function but before we cleared the bit. We would
     * have no way of knowing that the bit needs to stay set in that case, so
     * the need to call the barrier-processing function again would just get
     * forgotten. So instead, we tentatively clear all the bits and then put
     * back any for which we don't manage to successfully absorb the barrier.
     */
    flags = pg_atomic_exchange_u32(&MyProcSignalSlot->pss_barrierCheckMask, 0);
 
    /*
     * If there are no flags set, then we can skip doing any real work.
     * Otherwise, establish a PG_TRY block, so that we don't lose track of
     * which types of barrier processing are needed if an ERROR occurs.
     */
    if (flags != 0)
    {
        bool        success = true;
 
        PG_TRY();
        {
            /*
             * Process each type of barrier. The barrier-processing functions
             * should normally return true, but may return false if the
             * barrier can't be absorbed at the current time. This should be
             * rare, because it's pretty expensive.  Every single
             * CHECK_FOR_INTERRUPTS() will return here until we manage to
             * absorb the barrier, and that cost will add up in a hurry.
             *
             * NB: It ought to be OK to call the barrier-processing functions
             * unconditionally, but it's more efficient to call only the ones
             * that might need us to do something based on the flags.
             */
            while (flags != 0)
            {
                ProcSignalBarrierType type;
                bool        processed = true;
 
                type = (ProcSignalBarrierType) pg_rightmost_one_pos32(flags);
                switch (type)
                {
                    case PROCSIGNAL_BARRIER_SMGRRELEASE:
                        processed = ProcessBarrierSmgrRelease();
                        break;
                    case PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO:
                        processed = ProcessBarrierUpdateXLogLogicalInfo();
                        break;
                }
 
                /*
                 * To avoid an infinite loop, we must always unset the bit in
                 * flags.
                 */
                BARRIER_CLEAR_BIT(flags, type);
 
                /*
                 * If we failed to process the barrier, reset the shared bit
                 * so we try again later, and set a flag so that we don't bump
                 * our generation.
                 */
                if (!processed)
                {
                    ResetProcSignalBarrierBits(((uint32) 1) << type);
                    success = false;
                }
            }
        }
        PG_CATCH();
        {
            /*
             * If an ERROR occurred, we'll need to try again later to handle
             * that barrier type and any others that haven't been handled yet
             * or weren't successfully absorbed.
             */
            ResetProcSignalBarrierBits(flags);
            PG_RE_THROW();
        }
        PG_END_TRY();
 
        /*
         * If some barrier types were not successfully absorbed, we will have
         * to try again later.
         */
        if (!success)
            return;
    }
 
    /*
     * State changes related to all types of barriers that might have been
     * emitted have now been handled, so we can update our notion of the
     * generation to the one we observed before beginning the updates. If
     * things have changed further, it'll get fixed up when this function is
     * next called.
     */
    pg_atomic_write_u64(&MyProcSignalSlot->pss_barrierGeneration, shared_gen);
    ConditionVariableBroadcast(&MyProcSignalSlot->pss_barrierCV);
}
 
/*
 * If it turns out that we couldn't absorb one or more barrier types, either
 * because the barrier-processing functions returned false or due to an error,
 * arrange for processing to be retried later.
 */
static void
ResetProcSignalBarrierBits(uint32 flags)
{
    pg_atomic_fetch_or_u32(&MyProcSignalSlot->pss_barrierCheckMask, flags);
    ProcSignalBarrierPending = true;
    InterruptPending = true;
}
 
/*
 * CheckProcSignal - check to see if a particular reason has been
 * signaled, and clear the signal flag.  Should be called after receiving
 * SIGUSR1.
 */
static bool
CheckProcSignal(ProcSignalReason reason)
{
    volatile ProcSignalSlot *slot = MyProcSignalSlot;
 
    if (slot != NULL)
    {
        /*
         * Careful here --- don't clear flag if we haven't seen it set.
         * pss_signalFlags is of type "volatile sig_atomic_t" to allow us to
         * read it here safely, without holding the spinlock.
         */
        if (slot->pss_signalFlags[reason])
        {
            slot->pss_signalFlags[reason] = false;
            return true;
        }
    }
 
    return false;
}
 
/*
 * procsignal_sigusr1_handler - handle SIGUSR1 signal.
 */
void
procsignal_sigusr1_handler(SIGNAL_ARGS)
{
    if (CheckProcSignal(PROCSIG_CATCHUP_INTERRUPT))
        HandleCatchupInterrupt();
 
    if (CheckProcSignal(PROCSIG_NOTIFY_INTERRUPT))
        HandleNotifyInterrupt();
 
    if (CheckProcSignal(PROCSIG_PARALLEL_MESSAGE))
        HandleParallelMessageInterrupt();
 
    if (CheckProcSignal(PROCSIG_WALSND_INIT_STOPPING))
        HandleWalSndInitStopping();
 
    if (CheckProcSignal(PROCSIG_BARRIER))
        HandleProcSignalBarrierInterrupt();
 
    if (CheckProcSignal(PROCSIG_LOG_MEMORY_CONTEXT))
        HandleLogMemoryContextInterrupt();
 
    if (CheckProcSignal(PROCSIG_PARALLEL_APPLY_MESSAGE))
        HandleParallelApplyMessageInterrupt();
 
    if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT))
        HandleRecoveryConflictInterrupt();
 
    SetLatch(MyLatch);
}
 
/*
 * Send a query cancellation signal to backend.
 *
 * Note: This is called from a backend process before authentication.  We
 * cannot take LWLocks yet, but that's OK; we rely on atomic reads of the
 * fields in the ProcSignal slots.
 */
void
SendCancelRequest(int backendPID, const uint8 *cancel_key, int cancel_key_len)
{
    if (backendPID == 0)
    {
        ereport(LOG, (errmsg("invalid cancel request with PID 0")));
        return;
    }
 
    /*
     * See if we have a matching backend. Reading the pss_pid and
     * pss_cancel_key fields is racy, a backend might die and remove itself
     * from the array at any time.  The probability of the cancellation key
     * matching wrong process is miniscule, however, so we can live with that.
     * PIDs are reused too, so sending the signal based on PID is inherently
     * racy anyway, although OS's avoid reusing PIDs too soon.
     */
    for (int i = 0; i < NumProcSignalSlots; i++)
    {
        ProcSignalSlot *slot = &ProcSignal->psh_slot[i];
        bool        match;
 
        if (pg_atomic_read_u32(&slot->pss_pid) != backendPID)
            continue;
 
        /* Acquire the spinlock and re-check */
        SpinLockAcquire(&slot->pss_mutex);
        if (pg_atomic_read_u32(&slot->pss_pid) != backendPID)
        {
            SpinLockRelease(&slot->pss_mutex);
            continue;
        }
        else
        {
            match = slot->pss_cancel_key_len == cancel_key_len &&
                timingsafe_bcmp(slot->pss_cancel_key, cancel_key, cancel_key_len) == 0;
 
            SpinLockRelease(&slot->pss_mutex);
 
            if (match)
            {
                /* Found a match; signal that backend to cancel current op */
                ereport(DEBUG2,
                        (errmsg_internal("processing cancel request: sending SIGINT to process %d",
                                         backendPID)));
 
                /*
                 * If we have setsid(), signal the backend's whole process
                 * group
                 */
#ifdef HAVE_SETSID
                kill(-backendPID, SIGINT);
#else
                kill(backendPID, SIGINT);
#endif
            }
            else
            {
                /* Right PID, wrong key: no way, Jose */
                ereport(LOG,
                        (errmsg("wrong key in cancel request for process %d",
                                backendPID)));
            }
            return;
        }
    }
 
    /* No matching backend */
    ereport(LOG,
            (errmsg("PID %d in cancel request did not match any process",
                    backendPID)));
}