procsignal.c

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/*-------------------------------------------------------------------------
 *
 * procsignal.c
 *    Routines for interprocess signaling
 *
 *
 * Portions Copyright (c) 1996-2021, 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 "port/pg_bitutils.h"
#include "commands/async.h"
#include "miscadmin.h"
#include "pgstat.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 "tcop/tcopprot.h"
#include "utils/memutils.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 backend ID to make
 * slot allocation simple, and to avoid having to search the array when you
 * know the backend ID of the process you're signaling.  (We do support
 * signaling without backend ID, but it's a bit less efficient.)
 *
 * The flags are actually declared as "volatile sig_atomic_t" for maximum
 * portability.  This should ensure that loads and stores of the flag
 * values are atomic, allowing us to dispense with any explicit locking.
 *
 * 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
{
    volatile pid_t pss_pid;
    volatile sig_atomic_t pss_signalFlags[NUM_PROCSIGNALS];
    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.
 */
typedef struct
{
    pg_atomic_uint64 psh_barrierGeneration;
    ProcSignalSlot psh_slot[FLEXIBLE_ARRAY_MEMBER];
} ProcSignalHeader;

/*
 * We reserve a slot for each possible BackendId, 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.)
 */
#define NumProcSignalSlots  (MaxBackends + NUM_AUXPROCTYPES)

/* 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)))

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);
static bool ProcessBarrierPlaceholder(void);

/*
 * 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];

            slot->pss_pid = 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
 *
 * The passed index should be my BackendId if the process has one,
 * or MaxBackends + aux process type if not.
 */
void
ProcSignalInit(int pss_idx)
{
    ProcSignalSlot *slot;
    uint64      barrier_generation;

    Assert(pss_idx >= 1 && pss_idx <= NumProcSignalSlots);

    slot = &ProcSignal->psh_slot[pss_idx - 1];

    /* sanity check */
    if (slot->pss_pid != 0)
        elog(LOG, "process %d taking over ProcSignal slot %d, but it's not empty",
             MyProcPid, pss_idx);

    /* 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);
    pg_memory_barrier();

    /* Mark slot with my PID */
    slot->pss_pid = MyProcPid;

    /* Remember slot location for CheckProcSignal */
    MyProcSignalSlot = slot;

    /* Set up to release the slot on process exit */
    on_shmem_exit(CleanupProcSignalState, Int32GetDatum(pss_idx));
}

/*
 * 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)
{
    int         pss_idx = DatumGetInt32(arg);
    ProcSignalSlot *slot;

    slot = &ProcSignal->psh_slot[pss_idx - 1];
    Assert(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).
     */
    MyProcSignalSlot = NULL;

    /* sanity check */
    if (slot->pss_pid != MyProcPid)
    {
        /*
         * don't ERROR here. We're exiting anyway, and don't want to get into
         * infinite loop trying to exit
         */
        elog(LOG, "process %d releasing ProcSignal slot %d, but it contains %d",
             MyProcPid, pss_idx, (int) slot->pss_pid);
        return;                 /* XXX better to zero the slot anyway? */
    }

    /*
     * 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);
    ConditionVariableBroadcast(&slot->pss_barrierCV);

    slot->pss_pid = 0;
}

/*
 * SendProcSignal
 *      Send a signal to a Postgres process
 *
 * Providing backendId 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, BackendId backendId)
{
    volatile ProcSignalSlot *slot;

    if (backendId != InvalidBackendId)
    {
        slot = &ProcSignal->psh_slot[backendId - 1];

        /*
         * Note: Since there's no locking, it's possible that the target
         * process detaches from shared memory and exits right after this
         * test, before we set the flag and send signal. And the signal slot
         * might even be recycled by a new process, so it's remotely possible
         * that we set a flag for a wrong process. That's OK, all the signals
         * are such that no harm is done if they're mistakenly fired.
         */
        if (slot->pss_pid == pid)
        {
            /* Atomically set the proper flag */
            slot->pss_signalFlags[reason] = true;
            /* Send signal */
            return kill(pid, SIGUSR1);
        }
    }
    else
    {
        /*
         * BackendId not provided, so search the array using pid.  We search
         * the array back to front so as to reduce search overhead.  Passing
         * InvalidBackendId 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 (slot->pss_pid == pid)
            {
                /* the above note about race conditions applies here too */

                /* Atomically set the proper flag */
                slot->pss_signalFlags[reason] = true;
                /* Send signal */
                return kill(pid, SIGUSR1);
            }
        }
    }

    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 = slot->pss_pid;

        if (pid != 0)
        {
            /* see SendProcSignal for details */
            slot->pss_signalFlags[PROCSIG_BARRIER] = true;
            kill(pid, SIGUSR1);
        }
    }

    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));

    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)
        {
            ConditionVariableSleep(&slot->pss_barrierCV,
                                   WAIT_EVENT_PROC_SIGNAL_BARRIER);
            oldval = pg_atomic_read_u64(&slot->pss_barrierGeneration);
        }
        ConditionVariableCancelSleep();
    }

    /*
     * 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_PLACEHOLDER:
                        processed = ProcessBarrierPlaceholder();
                        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;
}

static bool
ProcessBarrierPlaceholder(void)
{
    /*
     * XXX. This is just a placeholder until the first real user of this
     * machinery gets committed. Rename PROCSIGNAL_BARRIER_PLACEHOLDER to
     * PROCSIGNAL_BARRIER_SOMETHING_ELSE where SOMETHING_ELSE is something
     * appropriately descriptive. Get rid of this function and instead have
     * ProcessBarrierSomethingElse. Most likely, that function should live in
     * the file pertaining to that subsystem, rather than here.
     *
     * The return value should be 'true' if the barrier was successfully
     * absorbed and 'false' if not. Note that returning 'false' can lead to
     * very frequent retries, so try hard to make that an uncommon case.
     */
    return 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 */
        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)
{
    int         save_errno = errno;

    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_RECOVERY_CONFLICT_DATABASE))
        RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_DATABASE);

    if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_TABLESPACE))
        RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_TABLESPACE);

    if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_LOCK))
        RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_LOCK);

    if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_SNAPSHOT))
        RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_SNAPSHOT);

    if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_STARTUP_DEADLOCK))
        RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_STARTUP_DEADLOCK);

    if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN))
        RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);

    SetLatch(MyLatch);

    errno = save_errno;
}