slot.c

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/*-------------------------------------------------------------------------
 *
 * slot.c
 *     Replication slot management.
 *
 *
 * Copyright (c) 2012-2026, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *    src/backend/replication/slot.c
 *
 * NOTES
 *
 * Replication slots are used to keep state about replication streams
 * originating from this cluster.  Their primary purpose is to prevent the
 * premature removal of WAL or of old tuple versions in a manner that would
 * interfere with replication; they are also useful for monitoring purposes.
 * Slots need to be permanent (to allow restarts), crash-safe, and allocatable
 * on standbys (to support cascading setups).  The requirement that slots be
 * usable on standbys precludes storing them in the system catalogs.
 *
 * Each replication slot gets its own directory inside the directory
 * $PGDATA / PG_REPLSLOT_DIR.  Inside that directory the state file will
 * contain the slot's own data.  Additional data can be stored alongside that
 * file if required.  While the server is running, the state data is also
 * cached in memory for efficiency.
 *
 * ReplicationSlotAllocationLock must be taken in exclusive mode to allocate
 * or free a slot. ReplicationSlotControlLock must be taken in shared mode
 * to iterate over the slots, and in exclusive mode to change the in_use flag
 * of a slot.  The remaining data in each slot is protected by its mutex.
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include <unistd.h>
#include <sys/stat.h>
 
#include "access/transam.h"
#include "access/xlog_internal.h"
#include "access/xlogrecovery.h"
#include "common/file_utils.h"
#include "common/string.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/interrupt.h"
#include "replication/logicallauncher.h"
#include "replication/slotsync.h"
#include "replication/slot.h"
#include "replication/walsender_private.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/subsystems.h"
#include "utils/builtins.h"
#include "utils/guc_hooks.h"
#include "utils/injection_point.h"
#include "utils/varlena.h"
#include "utils/wait_event.h"
 
/*
 * Replication slot on-disk data structure.
 */
typedef struct ReplicationSlotOnDisk
{
    /* first part of this struct needs to be version independent */
 
    /* data not covered by checksum */
    uint32      magic;
    pg_crc32c   checksum;
 
    /* data covered by checksum */
    uint32      version;
    uint32      length;
 
    /*
     * The actual data in the slot that follows can differ based on the above
     * 'version'.
     */
 
    ReplicationSlotPersistentData slotdata;
} ReplicationSlotOnDisk;
 
/*
 * Struct for the configuration of synchronized_standby_slots.
 *
 * Note: this must be a flat representation that can be held in a single chunk
 * of guc_malloc'd memory, so that it can be stored as the "extra" data for the
 * synchronized_standby_slots GUC.
 */
typedef struct
{
    /* Number of slot names in the slot_names[] */
    int         nslotnames;
 
    /*
     * slot_names contains 'nslotnames' consecutive null-terminated C strings.
     */
    char        slot_names[FLEXIBLE_ARRAY_MEMBER];
} SyncStandbySlotsConfigData;
 
/*
 * Lookup table for slot invalidation causes.
 */
typedef struct SlotInvalidationCauseMap
{
    ReplicationSlotInvalidationCause cause;
    const char *cause_name;
} SlotInvalidationCauseMap;
 
static const SlotInvalidationCauseMap SlotInvalidationCauses[] = {
    {RS_INVAL_NONE, "none"},
    {RS_INVAL_WAL_REMOVED, "wal_removed"},
    {RS_INVAL_HORIZON, "rows_removed"},
    {RS_INVAL_WAL_LEVEL, "wal_level_insufficient"},
    {RS_INVAL_IDLE_TIMEOUT, "idle_timeout"},
};
 
/*
 * Ensure that the lookup table is up-to-date with the enums defined in
 * ReplicationSlotInvalidationCause.
 */
StaticAssertDecl(lengthof(SlotInvalidationCauses) == (RS_INVAL_MAX_CAUSES + 1),
                 "array length mismatch");
 
/* size of version independent data */
#define ReplicationSlotOnDiskConstantSize \
    offsetof(ReplicationSlotOnDisk, slotdata)
/* size of the part of the slot not covered by the checksum */
#define ReplicationSlotOnDiskNotChecksummedSize  \
    offsetof(ReplicationSlotOnDisk, version)
/* size of the part covered by the checksum */
#define ReplicationSlotOnDiskChecksummedSize \
    sizeof(ReplicationSlotOnDisk) - ReplicationSlotOnDiskNotChecksummedSize
/* size of the slot data that is version dependent */
#define ReplicationSlotOnDiskV2Size \
    sizeof(ReplicationSlotOnDisk) - ReplicationSlotOnDiskConstantSize
 
#define SLOT_MAGIC      0x1051CA1   /* format identifier */
#define SLOT_VERSION    5       /* version for new files */
 
/* Control array for replication slot management */
ReplicationSlotCtlData *ReplicationSlotCtl = NULL;
 
static void ReplicationSlotsShmemRequest(void *arg);
static void ReplicationSlotsShmemInit(void *arg);
 
const ShmemCallbacks ReplicationSlotsShmemCallbacks = {
    .request_fn = ReplicationSlotsShmemRequest,
    .init_fn = ReplicationSlotsShmemInit,
};
 
/* My backend's replication slot in the shared memory array */
ReplicationSlot *MyReplicationSlot = NULL;
 
/* GUC variables */
int         max_replication_slots = 10; /* the maximum number of replication
                                         * slots */
int         max_repack_replication_slots = 5;   /* the maximum number of slots
                                                 * for REPACK */
 
/*
 * Invalidate replication slots that have remained idle longer than this
 * duration; '0' disables it.
 */
int         idle_replication_slot_timeout_secs = 0;
 
/*
 * This GUC lists streaming replication standby server slot names that
 * logical WAL sender processes will wait for.
 */
char       *synchronized_standby_slots;
 
/* This is the parsed and cached configuration for synchronized_standby_slots */
static SyncStandbySlotsConfigData *synchronized_standby_slots_config;
 
/*
 * Oldest LSN that has been confirmed to be flushed to the standbys
 * corresponding to the physical slots specified in the synchronized_standby_slots GUC.
 */
static XLogRecPtr ss_oldest_flush_lsn = InvalidXLogRecPtr;
 
static void ReplicationSlotShmemExit(int code, Datum arg);
static bool IsSlotForConflictCheck(const char *name);
static void ReplicationSlotDropPtr(ReplicationSlot *slot);
 
/* internal persistency functions */
static void RestoreSlotFromDisk(const char *name);
static void CreateSlotOnDisk(ReplicationSlot *slot);
static void SaveSlotToPath(ReplicationSlot *slot, const char *dir, int elevel);
 
/*
 * Register shared memory space needed for replication slots.
 */
static void
ReplicationSlotsShmemRequest(void *arg)
{
    Size        size;
 
    if (max_replication_slots + max_repack_replication_slots == 0)
        return;
 
    size = offsetof(ReplicationSlotCtlData, replication_slots);
    size = add_size(size,
                    mul_size(max_replication_slots + max_repack_replication_slots,
                             sizeof(ReplicationSlot)));
    ShmemRequestStruct(.name = "ReplicationSlot Ctl",
                       .size = size,
                       .ptr = (void **) &ReplicationSlotCtl,
        );
}
 
/*
 * Initialize shared memory for replication slots.
 */
static void
ReplicationSlotsShmemInit(void *arg)
{
    for (int i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *slot = &ReplicationSlotCtl->replication_slots[i];
 
        /* everything else is zeroed by the memset above */
        slot->active_proc = INVALID_PROC_NUMBER;
        SpinLockInit(&slot->mutex);
        LWLockInitialize(&slot->io_in_progress_lock,
                         LWTRANCHE_REPLICATION_SLOT_IO);
        ConditionVariableInit(&slot->active_cv);
    }
}
 
/*
 * Register the callback for replication slot cleanup and releasing.
 */
void
ReplicationSlotInitialize(void)
{
    before_shmem_exit(ReplicationSlotShmemExit, 0);
}
 
/*
 * Release and cleanup replication slots.
 */
static void
ReplicationSlotShmemExit(int code, Datum arg)
{
    /* Make sure active replication slots are released */
    if (MyReplicationSlot != NULL)
        ReplicationSlotRelease();
 
    /* Also cleanup all the temporary slots. */
    ReplicationSlotCleanup(false);
}
 
/*
 * Check whether the passed slot name is valid and report errors at elevel.
 *
 * See comments for ReplicationSlotValidateNameInternal().
 */
bool
ReplicationSlotValidateName(const char *name, bool allow_reserved_name,
                            int elevel)
{
    int         err_code;
    char       *err_msg = NULL;
    char       *err_hint = NULL;
 
    if (!ReplicationSlotValidateNameInternal(name, allow_reserved_name,
                                             &err_code, &err_msg, &err_hint))
    {
        /*
         * Use errmsg_internal() and errhint_internal() instead of errmsg()
         * and errhint(), since the messages from
         * ReplicationSlotValidateNameInternal() are already translated. This
         * avoids double translation.
         */
        ereport(elevel,
                errcode(err_code),
                errmsg_internal("%s", err_msg),
                (err_hint != NULL) ? errhint_internal("%s", err_hint) : 0);
 
        pfree(err_msg);
        if (err_hint != NULL)
            pfree(err_hint);
        return false;
    }
 
    return true;
}
 
/*
 * Check whether the passed slot name is valid.
 *
 * An error will be reported for a reserved replication slot name if
 * allow_reserved_name is set to false.
 *
 * Slot names may consist out of [a-z0-9_]{1,NAMEDATALEN-1} which should allow
 * the name to be used as a directory name on every supported OS.
 *
 * Returns true if the slot name is valid. Otherwise, returns false and stores
 * the error code, error message, and optional hint in err_code, err_msg, and
 * err_hint, respectively. The caller is responsible for freeing err_msg and
 * err_hint, which are palloc'd.
 */
bool
ReplicationSlotValidateNameInternal(const char *name, bool allow_reserved_name,
                                    int *err_code, char **err_msg, char **err_hint)
{
    const char *cp;
 
    if (strlen(name) == 0)
    {
        *err_code = ERRCODE_INVALID_NAME;
        *err_msg = psprintf(_("replication slot name \"%s\" is too short"), name);
        *err_hint = NULL;
        return false;
    }
 
    if (strlen(name) >= NAMEDATALEN)
    {
        *err_code = ERRCODE_NAME_TOO_LONG;
        *err_msg = psprintf(_("replication slot name \"%s\" is too long"), name);
        *err_hint = NULL;
        return false;
    }
 
    for (cp = name; *cp; cp++)
    {
        if (!((*cp >= 'a' && *cp <= 'z')
              || (*cp >= '0' && *cp <= '9')
              || (*cp == '_')))
        {
            *err_code = ERRCODE_INVALID_NAME;
            *err_msg = psprintf(_("replication slot name \"%s\" contains invalid character"), name);
            *err_hint = psprintf(_("Replication slot names may only contain lower case letters, numbers, and the underscore character."));
            return false;
        }
    }
 
    if (!allow_reserved_name && IsSlotForConflictCheck(name))
    {
        *err_code = ERRCODE_RESERVED_NAME;
        *err_msg = psprintf(_("replication slot name \"%s\" is reserved"), name);
        *err_hint = psprintf(_("The name \"%s\" is reserved for the conflict detection slot."),
                             CONFLICT_DETECTION_SLOT);
        return false;
    }
 
    return true;
}
 
/*
 * Return true if the replication slot name is "pg_conflict_detection".
 */
static bool
IsSlotForConflictCheck(const char *name)
{
    return (strcmp(name, CONFLICT_DETECTION_SLOT) == 0);
}
 
/*
 * Create a new replication slot and mark it as used by this backend.
 *
 * name: Name of the slot
 * db_specific: logical decoding is db specific; if the slot is going to
 *     be used for that pass true, otherwise false.
 * two_phase: If enabled, allows decoding of prepared transactions.
 * repack: If true, use a slot from the pool for REPACK.
 * failover: If enabled, allows the slot to be synced to standbys so
 *     that logical replication can be resumed after failover.
 * synced: True if the slot is synchronized from the primary server.
 */
void
ReplicationSlotCreate(const char *name, bool db_specific,
                      ReplicationSlotPersistency persistency,
                      bool two_phase, bool repack, bool failover, bool synced)
{
    ReplicationSlot *slot = NULL;
    int         startpoint,
                endpoint;
 
    Assert(MyReplicationSlot == NULL);
 
    /*
     * The logical launcher or pg_upgrade may create or migrate an internal
     * slot, so using a reserved name is allowed in these cases.
     */
    ReplicationSlotValidateName(name, IsBinaryUpgrade || IsLogicalLauncher(),
                                ERROR);
 
    if (failover)
    {
        /*
         * Do not allow users to create the failover enabled slots on the
         * standby as we do not support sync to the cascading standby.
         *
         * However, failover enabled slots can be created during slot
         * synchronization because we need to retain the same values as the
         * remote slot.
         */
        if (RecoveryInProgress() && !IsSyncingReplicationSlots())
            ereport(ERROR,
                    errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                    errmsg("cannot enable failover for a replication slot created on the standby"));
 
        /*
         * Do not allow users to create failover enabled temporary slots,
         * because temporary slots will not be synced to the standby.
         *
         * However, failover enabled temporary slots can be created during
         * slot synchronization. See the comments atop slotsync.c for details.
         */
        if (persistency == RS_TEMPORARY && !IsSyncingReplicationSlots())
            ereport(ERROR,
                    errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                    errmsg("cannot enable failover for a temporary replication slot"));
    }
 
    /*
     * If some other backend ran this code concurrently with us, we'd likely
     * both allocate the same slot, and that would be bad.  We'd also be at
     * risk of missing a name collision.  Also, we don't want to try to create
     * a new slot while somebody's busy cleaning up an old one, because we
     * might both be monkeying with the same directory.
     */
    LWLockAcquire(ReplicationSlotAllocationLock, LW_EXCLUSIVE);
 
    /*
     * Check for name collision (across the whole array), and identify an
     * allocatable slot (in the array slice specific to our current use case:
     * either general, or REPACK only).  We need to hold
     * ReplicationSlotControlLock in shared mode for this, so that nobody else
     * can change the in_use flags while we're looking at them.
     */
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    startpoint = !repack ? 0 : max_replication_slots;
    endpoint = max_replication_slots + (repack ? max_repack_replication_slots : 0);
    for (int i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
 
        if (s->in_use && strcmp(name, NameStr(s->data.name)) == 0)
            ereport(ERROR,
                    (errcode(ERRCODE_DUPLICATE_OBJECT),
                     errmsg("replication slot \"%s\" already exists", name)));
 
        if (i >= startpoint && i < endpoint &&
            !s->in_use && slot == NULL)
            slot = s;
    }
    LWLockRelease(ReplicationSlotControlLock);
 
    /* If all slots are in use, we're out of luck. */
    if (slot == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
                 errmsg("all replication slots are in use"),
                 errhint("Free one or increase \"%s\".",
                         repack ? "max_repack_replication_slots" : "max_replication_slots")));
 
    /*
     * Since this slot is not in use, nobody should be looking at any part of
     * it other than the in_use field unless they're trying to allocate it.
     * And since we hold ReplicationSlotAllocationLock, nobody except us can
     * be doing that.  So it's safe to initialize the slot.
     */
    Assert(!slot->in_use);
    Assert(slot->active_proc == INVALID_PROC_NUMBER);
 
    /* first initialize persistent data */
    memset(&slot->data, 0, sizeof(ReplicationSlotPersistentData));
    namestrcpy(&slot->data.name, name);
    slot->data.database = db_specific ? MyDatabaseId : InvalidOid;
    slot->data.persistency = persistency;
    slot->data.two_phase = two_phase;
    slot->data.two_phase_at = InvalidXLogRecPtr;
    slot->data.failover = failover;
    slot->data.synced = synced;
 
    /* and then data only present in shared memory */
    slot->just_dirtied = false;
    slot->dirty = false;
    slot->effective_xmin = InvalidTransactionId;
    slot->effective_catalog_xmin = InvalidTransactionId;
    slot->candidate_catalog_xmin = InvalidTransactionId;
    slot->candidate_xmin_lsn = InvalidXLogRecPtr;
    slot->candidate_restart_valid = InvalidXLogRecPtr;
    slot->candidate_restart_lsn = InvalidXLogRecPtr;
    slot->last_saved_confirmed_flush = InvalidXLogRecPtr;
    slot->last_saved_restart_lsn = InvalidXLogRecPtr;
    slot->inactive_since = 0;
    slot->slotsync_skip_reason = SS_SKIP_NONE;
 
    /*
     * Create the slot on disk.  We haven't actually marked the slot allocated
     * yet, so no special cleanup is required if this errors out.
     */
    CreateSlotOnDisk(slot);
 
    /*
     * We need to briefly prevent any other backend from iterating over the
     * slots while we flip the in_use flag. We also need to set the active
     * flag while holding the ControlLock as otherwise a concurrent
     * ReplicationSlotAcquire() could acquire the slot as well.
     */
    LWLockAcquire(ReplicationSlotControlLock, LW_EXCLUSIVE);
 
    slot->in_use = true;
 
    /* We can now mark the slot active, and that makes it our slot. */
    SpinLockAcquire(&slot->mutex);
    Assert(slot->active_proc == INVALID_PROC_NUMBER);
    slot->active_proc = MyProcNumber;
    SpinLockRelease(&slot->mutex);
    MyReplicationSlot = slot;
 
    LWLockRelease(ReplicationSlotControlLock);
 
    /*
     * Create statistics entry for the new logical slot. We don't collect any
     * stats for physical slots, so no need to create an entry for the same.
     * See ReplicationSlotDropPtr for why we need to do this before releasing
     * ReplicationSlotAllocationLock.
     */
    if (SlotIsLogical(slot))
        pgstat_create_replslot(slot);
 
    /*
     * Now that the slot has been marked as in_use and active, it's safe to
     * let somebody else try to allocate a slot.
     */
    LWLockRelease(ReplicationSlotAllocationLock);
 
    /* Let everybody know we've modified this slot */
    ConditionVariableBroadcast(&slot->active_cv);
}
 
/*
 * Search for the named replication slot.
 *
 * Return the replication slot if found, otherwise NULL.
 */
ReplicationSlot *
SearchNamedReplicationSlot(const char *name, bool need_lock)
{
    int         i;
    ReplicationSlot *slot = NULL;
 
    if (need_lock)
        LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
    for (i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
 
        if (s->in_use && strcmp(name, NameStr(s->data.name)) == 0)
        {
            slot = s;
            break;
        }
    }
 
    if (need_lock)
        LWLockRelease(ReplicationSlotControlLock);
 
    return slot;
}
 
/*
 * Return the index of the replication slot in
 * ReplicationSlotCtl->replication_slots.
 *
 * This is mainly useful to have an efficient key for storing replication slot
 * stats.
 */
int
ReplicationSlotIndex(ReplicationSlot *slot)
{
    Assert(slot >= ReplicationSlotCtl->replication_slots &&
           slot < ReplicationSlotCtl->replication_slots +
           (max_replication_slots + max_repack_replication_slots));
 
    return slot - ReplicationSlotCtl->replication_slots;
}
 
/*
 * If the slot at 'index' is unused, return false. Otherwise 'name' is set to
 * the slot's name and true is returned.
 *
 * This likely is only useful for pgstat_replslot.c during shutdown, in other
 * cases there are obvious TOCTOU issues.
 */
bool
ReplicationSlotName(int index, Name name)
{
    ReplicationSlot *slot;
    bool        found;
 
    slot = &ReplicationSlotCtl->replication_slots[index];
 
    /*
     * Ensure that the slot cannot be dropped while we copy the name. Don't
     * need the spinlock as the name of an existing slot cannot change.
     */
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    found = slot->in_use;
    if (slot->in_use)
        namestrcpy(name, NameStr(slot->data.name));
    LWLockRelease(ReplicationSlotControlLock);
 
    return found;
}
 
/*
 * Find a previously created slot and mark it as used by this process.
 *
 * An error is raised if nowait is true and the slot is currently in use. If
 * nowait is false, we sleep until the slot is released by the owning process.
 *
 * An error is raised if error_if_invalid is true and the slot is found to
 * be invalid. It should always be set to true, except when we are temporarily
 * acquiring the slot and don't intend to change it.
 */
void
ReplicationSlotAcquire(const char *name, bool nowait, bool error_if_invalid)
{
    ReplicationSlot *s;
    ProcNumber  active_proc;
    int         active_pid;
 
    Assert(name != NULL);
 
retry:
    Assert(MyReplicationSlot == NULL);
 
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
    /* Check if the slot exists with the given name. */
    s = SearchNamedReplicationSlot(name, false);
    if (s == NULL || !s->in_use)
    {
        LWLockRelease(ReplicationSlotControlLock);
 
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("replication slot \"%s\" does not exist",
                        name)));
    }
 
    /*
     * Do not allow users to acquire the reserved slot. This scenario may
     * occur if the launcher that owns the slot has terminated unexpectedly
     * due to an error, and a backend process attempts to reuse the slot.
     */
    if (!IsLogicalLauncher() && IsSlotForConflictCheck(name))
        ereport(ERROR,
                errcode(ERRCODE_UNDEFINED_OBJECT),
                errmsg("cannot acquire replication slot \"%s\"", name),
                errdetail("The slot is reserved for conflict detection and can only be acquired by logical replication launcher."));
 
    /*
     * This is the slot we want; check if it's active under some other
     * process.  In single user mode, we don't need this check.
     */
    if (IsUnderPostmaster)
    {
        /*
         * Get ready to sleep on the slot in case it is active.  (We may end
         * up not sleeping, but we don't want to do this while holding the
         * spinlock.)
         */
        if (!nowait)
            ConditionVariablePrepareToSleep(&s->active_cv);
 
        /*
         * It is important to reset the inactive_since under spinlock here to
         * avoid race conditions with slot invalidation. See comments related
         * to inactive_since in InvalidatePossiblyObsoleteSlot.
         */
        SpinLockAcquire(&s->mutex);
        if (s->active_proc == INVALID_PROC_NUMBER)
            s->active_proc = MyProcNumber;
        active_proc = s->active_proc;
        ReplicationSlotSetInactiveSince(s, 0, false);
        SpinLockRelease(&s->mutex);
    }
    else
    {
        s->active_proc = active_proc = MyProcNumber;
        ReplicationSlotSetInactiveSince(s, 0, true);
    }
    active_pid = GetPGProcByNumber(active_proc)->pid;
    LWLockRelease(ReplicationSlotControlLock);
 
    /*
     * If we found the slot but it's already active in another process, we
     * wait until the owning process signals us that it's been released, or
     * error out.
     */
    if (active_proc != MyProcNumber)
    {
        if (!nowait)
        {
            /* Wait here until we get signaled, and then restart */
            ConditionVariableSleep(&s->active_cv,
                                   WAIT_EVENT_REPLICATION_SLOT_DROP);
            ConditionVariableCancelSleep();
            goto retry;
        }
 
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_IN_USE),
                 errmsg("replication slot \"%s\" is active for PID %d",
                        NameStr(s->data.name), active_pid)));
    }
    else if (!nowait)
        ConditionVariableCancelSleep(); /* no sleep needed after all */
 
    /* We made this slot active, so it's ours now. */
    MyReplicationSlot = s;
 
    /*
     * We need to check for invalidation after making the slot ours to avoid
     * the possible race condition with the checkpointer that can otherwise
     * invalidate the slot immediately after the check.
     */
    if (error_if_invalid && s->data.invalidated != RS_INVAL_NONE)
        ereport(ERROR,
                errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                errmsg("can no longer access replication slot \"%s\"",
                       NameStr(s->data.name)),
                errdetail("This replication slot has been invalidated due to \"%s\".",
                          GetSlotInvalidationCauseName(s->data.invalidated)));
 
    /* Let everybody know we've modified this slot */
    ConditionVariableBroadcast(&s->active_cv);
 
    /*
     * The call to pgstat_acquire_replslot() protects against stats for a
     * different slot, from before a restart or such, being present during
     * pgstat_report_replslot().
     */
    if (SlotIsLogical(s))
        pgstat_acquire_replslot(s);
 
 
    if (am_walsender)
    {
        ereport(log_replication_commands ? LOG : DEBUG1,
                SlotIsLogical(s)
                ? errmsg("acquired logical replication slot \"%s\"",
                         NameStr(s->data.name))
                : errmsg("acquired physical replication slot \"%s\"",
                         NameStr(s->data.name)));
    }
}
 
/*
 * Release the replication slot that this backend considers to own.
 *
 * This or another backend can re-acquire the slot later.
 * Resources this slot requires will be preserved.
 */
void
ReplicationSlotRelease(void)
{
    ReplicationSlot *slot = MyReplicationSlot;
    char       *slotname = NULL;    /* keep compiler quiet */
    bool        is_logical;
    TimestampTz now = 0;
 
    Assert(slot != NULL && slot->active_proc != INVALID_PROC_NUMBER);
 
    is_logical = SlotIsLogical(slot);
 
    if (am_walsender)
        slotname = pstrdup(NameStr(slot->data.name));
 
    if (slot->data.persistency == RS_EPHEMERAL)
    {
        /*
         * If slot is ephemeral, we drop it upon release, and request logical
         * decoding be disabled.
         */
        ReplicationSlotDropAcquired(is_logical);
    }
 
    /*
     * If slot needed to temporarily restrain both data and catalog xmin to
     * create the catalog snapshot, remove that temporary constraint.
     * Snapshots can only be exported while the initial snapshot is still
     * acquired.
     */
    if (!TransactionIdIsValid(slot->data.xmin) &&
        TransactionIdIsValid(slot->effective_xmin))
    {
        SpinLockAcquire(&slot->mutex);
        slot->effective_xmin = InvalidTransactionId;
        SpinLockRelease(&slot->mutex);
        ReplicationSlotsComputeRequiredXmin(false);
    }
 
    /*
     * Set the time since the slot has become inactive. We get the current
     * time beforehand to avoid system call while holding the spinlock.
     */
    now = GetCurrentTimestamp();
 
    if (slot->data.persistency == RS_PERSISTENT)
    {
        /*
         * Mark persistent slot inactive.  We're not freeing it, just
         * disconnecting, but wake up others that may be waiting for it.
         */
        SpinLockAcquire(&slot->mutex);
        slot->active_proc = INVALID_PROC_NUMBER;
        ReplicationSlotSetInactiveSince(slot, now, false);
        SpinLockRelease(&slot->mutex);
        ConditionVariableBroadcast(&slot->active_cv);
    }
    else
        ReplicationSlotSetInactiveSince(slot, now, true);
 
    MyReplicationSlot = NULL;
 
    /* might not have been set when we've been a plain slot */
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    MyProc->statusFlags &= ~PROC_IN_LOGICAL_DECODING;
    ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
    LWLockRelease(ProcArrayLock);
 
    if (am_walsender)
    {
        ereport(log_replication_commands ? LOG : DEBUG1,
                is_logical
                ? errmsg("released logical replication slot \"%s\"",
                         slotname)
                : errmsg("released physical replication slot \"%s\"",
                         slotname));
 
        pfree(slotname);
    }
}
 
/*
 * Cleanup temporary slots created in current session.
 *
 * Cleanup only synced temporary slots if 'synced_only' is true, else
 * cleanup all temporary slots.
 *
 * If it drops the last logical slot in the cluster, requests to disable
 * logical decoding.
 */
void
ReplicationSlotCleanup(bool synced_only)
{
    int         i;
    bool        found_valid_logicalslot;
    bool        dropped_logical = false;
 
    Assert(MyReplicationSlot == NULL);
 
restart:
    found_valid_logicalslot = false;
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    for (i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
 
        if (!s->in_use)
            continue;
 
        SpinLockAcquire(&s->mutex);
 
        found_valid_logicalslot |=
            (SlotIsLogical(s) && s->data.invalidated == RS_INVAL_NONE);
 
        if ((s->active_proc == MyProcNumber &&
             (!synced_only || s->data.synced)))
        {
            Assert(s->data.persistency == RS_TEMPORARY);
            SpinLockRelease(&s->mutex);
            LWLockRelease(ReplicationSlotControlLock);  /* avoid deadlock */
 
            if (SlotIsLogical(s))
                dropped_logical = true;
 
            ReplicationSlotDropPtr(s);
 
            ConditionVariableBroadcast(&s->active_cv);
            goto restart;
        }
        else
            SpinLockRelease(&s->mutex);
    }
 
    LWLockRelease(ReplicationSlotControlLock);
 
    if (dropped_logical && !found_valid_logicalslot)
        RequestDisableLogicalDecoding();
}
 
/*
 * Permanently drop the replication slot identified by the passed-in name.
 *
 * If this is a logical slot, request that logical decoding be disabled.
 */
void
ReplicationSlotDrop(const char *name, bool nowait)
{
    ReplicationSlotAcquire(name, nowait, false);
 
    /*
     * Do not allow users to drop the slots which are currently being synced
     * from the primary to the standby.
     */
    if (RecoveryInProgress() && MyReplicationSlot->data.synced)
        ereport(ERROR,
                errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                errmsg("cannot drop replication slot \"%s\"", name),
                errdetail("This replication slot is being synchronized from the primary server."));
 
    ReplicationSlotDropAcquired(SlotIsLogical(MyReplicationSlot));
}
 
/*
 * Change the definition of the slot identified by the specified name.
 *
 * Altering the two_phase property of a slot requires caution on the
 * client-side. Enabling it at any random point during decoding has the
 * risk that transactions prepared before this change may be skipped by
 * the decoder, leading to missing prepare records on the client. So, we
 * enable it for subscription related slots only once the initial tablesync
 * is finished. See comments atop worker.c. Disabling it is safe only when
 * there are no pending prepared transaction, otherwise, the changes of
 * already prepared transactions can be replicated again along with their
 * corresponding commit leading to duplicate data or errors.
 */
void
ReplicationSlotAlter(const char *name, const bool *failover,
                     const bool *two_phase)
{
    bool        update_slot = false;
 
    Assert(MyReplicationSlot == NULL);
    Assert(failover || two_phase);
 
    ReplicationSlotAcquire(name, false, true);
 
    if (SlotIsPhysical(MyReplicationSlot))
        ereport(ERROR,
                errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                errmsg("cannot use %s with a physical replication slot",
                       "ALTER_REPLICATION_SLOT"));
 
    if (RecoveryInProgress())
    {
        /*
         * Do not allow users to alter the slots which are currently being
         * synced from the primary to the standby.
         */
        if (MyReplicationSlot->data.synced)
            ereport(ERROR,
                    errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                    errmsg("cannot alter replication slot \"%s\"", name),
                    errdetail("This replication slot is being synchronized from the primary server."));
 
        /*
         * Do not allow users to enable failover on the standby as we do not
         * support sync to the cascading standby.
         */
        if (failover && *failover)
            ereport(ERROR,
                    errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                    errmsg("cannot enable failover for a replication slot"
                           " on the standby"));
    }
 
    if (failover)
    {
        /*
         * Do not allow users to enable failover for temporary slots as we do
         * not support syncing temporary slots to the standby.
         */
        if (*failover && MyReplicationSlot->data.persistency == RS_TEMPORARY)
            ereport(ERROR,
                    errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                    errmsg("cannot enable failover for a temporary replication slot"));
 
        if (MyReplicationSlot->data.failover != *failover)
        {
            SpinLockAcquire(&MyReplicationSlot->mutex);
            MyReplicationSlot->data.failover = *failover;
            SpinLockRelease(&MyReplicationSlot->mutex);
 
            update_slot = true;
        }
    }
 
    if (two_phase && MyReplicationSlot->data.two_phase != *two_phase)
    {
        SpinLockAcquire(&MyReplicationSlot->mutex);
        MyReplicationSlot->data.two_phase = *two_phase;
        SpinLockRelease(&MyReplicationSlot->mutex);
 
        update_slot = true;
    }
 
    if (update_slot)
    {
        ReplicationSlotMarkDirty();
        ReplicationSlotSave();
    }
 
    ReplicationSlotRelease();
}
 
/*
 * Permanently drop the currently acquired replication slot.
 *
 * If caller requests it, have checkpointer attempt to disable logical
 * decoding.  Obviously, this should only be done if the slot is logical.
 */
void
ReplicationSlotDropAcquired(bool try_disable)
{
    ReplicationSlot *slot;
 
    Assert(MyReplicationSlot != NULL);
    slot = MyReplicationSlot;
 
    /* Can only disable logical decoding if slot is logical */
    Assert(!try_disable || SlotIsLogical(slot));
 
    /* slot isn't acquired anymore */
    MyReplicationSlot = NULL;
 
    ReplicationSlotDropPtr(slot);
 
    if (try_disable)
        RequestDisableLogicalDecoding();
}
 
/*
 * Permanently drop the replication slot which will be released by the point
 * this function returns.
 */
static void
ReplicationSlotDropPtr(ReplicationSlot *slot)
{
    char        path[MAXPGPATH];
    char        tmppath[MAXPGPATH];
 
    /*
     * If some other backend ran this code concurrently with us, we might try
     * to delete a slot with a certain name while someone else was trying to
     * create a slot with the same name.
     */
    LWLockAcquire(ReplicationSlotAllocationLock, LW_EXCLUSIVE);
 
    /* Generate pathnames. */
    sprintf(path, "%s/%s", PG_REPLSLOT_DIR, NameStr(slot->data.name));
    sprintf(tmppath, "%s/%s.tmp", PG_REPLSLOT_DIR, NameStr(slot->data.name));
 
    /*
     * Rename the slot directory on disk, so that we'll no longer recognize
     * this as a valid slot.  Note that if this fails, we've got to mark the
     * slot inactive before bailing out.  If we're dropping an ephemeral or a
     * temporary slot, we better never fail hard as the caller won't expect
     * the slot to survive and this might get called during error handling.
     */
    if (rename(path, tmppath) == 0)
    {
        /*
         * We need to fsync() the directory we just renamed and its parent to
         * make sure that our changes are on disk in a crash-safe fashion.  If
         * fsync() fails, we can't be sure whether the changes are on disk or
         * not.  For now, we handle that by panicking;
         * StartupReplicationSlots() will try to straighten it out after
         * restart.
         */
        START_CRIT_SECTION();
        fsync_fname(tmppath, true);
        fsync_fname(PG_REPLSLOT_DIR, true);
        END_CRIT_SECTION();
    }
    else
    {
        bool        fail_softly = slot->data.persistency != RS_PERSISTENT;
 
        SpinLockAcquire(&slot->mutex);
        slot->active_proc = INVALID_PROC_NUMBER;
        SpinLockRelease(&slot->mutex);
 
        /* wake up anyone waiting on this slot */
        ConditionVariableBroadcast(&slot->active_cv);
 
        ereport(fail_softly ? WARNING : ERROR,
                (errcode_for_file_access(),
                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                        path, tmppath)));
    }
 
    /*
     * The slot is definitely gone.  Lock out concurrent scans of the array
     * long enough to kill it.  It's OK to clear the active PID here without
     * grabbing the mutex because nobody else can be scanning the array here,
     * and nobody can be attached to this slot and thus access it without
     * scanning the array.
     *
     * Also wake up processes waiting for it.
     */
    LWLockAcquire(ReplicationSlotControlLock, LW_EXCLUSIVE);
    slot->active_proc = INVALID_PROC_NUMBER;
    slot->in_use = false;
    LWLockRelease(ReplicationSlotControlLock);
    ConditionVariableBroadcast(&slot->active_cv);
 
    /*
     * Slot is dead and doesn't prevent resource removal anymore, recompute
     * limits.
     */
    ReplicationSlotsComputeRequiredXmin(false);
    ReplicationSlotsComputeRequiredLSN();
 
    /*
     * If removing the directory fails, the worst thing that will happen is
     * that the user won't be able to create a new slot with the same name
     * until the next server restart.  We warn about it, but that's all.
     */
    if (!rmtree(tmppath, true))
        ereport(WARNING,
                (errmsg("could not remove directory \"%s\"", tmppath)));
 
    /*
     * Drop the statistics entry for the replication slot.  Do this while
     * holding ReplicationSlotAllocationLock so that we don't drop a
     * statistics entry for another slot with the same name just created in
     * another session.
     */
    if (SlotIsLogical(slot))
        pgstat_drop_replslot(slot);
 
    /*
     * We release this at the very end, so that nobody starts trying to create
     * a slot while we're still cleaning up the detritus of the old one.
     */
    LWLockRelease(ReplicationSlotAllocationLock);
}
 
/*
 * Serialize the currently acquired slot's state from memory to disk, thereby
 * guaranteeing the current state will survive a crash.
 */
void
ReplicationSlotSave(void)
{
    char        path[MAXPGPATH];
 
    Assert(MyReplicationSlot != NULL);
 
    sprintf(path, "%s/%s", PG_REPLSLOT_DIR, NameStr(MyReplicationSlot->data.name));
    SaveSlotToPath(MyReplicationSlot, path, ERROR);
}
 
/*
 * Signal that it would be useful if the currently acquired slot would be
 * flushed out to disk.
 *
 * Note that the actual flush to disk can be delayed for a long time, if
 * required for correctness explicitly do a ReplicationSlotSave().
 */
void
ReplicationSlotMarkDirty(void)
{
    ReplicationSlot *slot = MyReplicationSlot;
 
    Assert(MyReplicationSlot != NULL);
 
    SpinLockAcquire(&slot->mutex);
    MyReplicationSlot->just_dirtied = true;
    MyReplicationSlot->dirty = true;
    SpinLockRelease(&slot->mutex);
}
 
/*
 * Convert a slot that's marked as RS_EPHEMERAL or RS_TEMPORARY to a
 * RS_PERSISTENT slot, guaranteeing it will be there after an eventual crash.
 */
void
ReplicationSlotPersist(void)
{
    ReplicationSlot *slot = MyReplicationSlot;
 
    Assert(slot != NULL);
    Assert(slot->data.persistency != RS_PERSISTENT);
 
    SpinLockAcquire(&slot->mutex);
    slot->data.persistency = RS_PERSISTENT;
    SpinLockRelease(&slot->mutex);
 
    ReplicationSlotMarkDirty();
    ReplicationSlotSave();
}
 
/*
 * Compute the oldest xmin across all slots and store it in the ProcArray.
 *
 * If already_locked is true, both the ReplicationSlotControlLock and the
 * ProcArrayLock have already been acquired exclusively. It is crucial that the
 * caller first acquires the ReplicationSlotControlLock, followed by the
 * ProcArrayLock, to prevent any undetectable deadlocks since this function
 * acquires them in that order.
 */
void
ReplicationSlotsComputeRequiredXmin(bool already_locked)
{
    int         i;
    TransactionId agg_xmin = InvalidTransactionId;
    TransactionId agg_catalog_xmin = InvalidTransactionId;
 
    Assert(ReplicationSlotCtl != NULL);
    Assert(!already_locked ||
           (LWLockHeldByMeInMode(ReplicationSlotControlLock, LW_EXCLUSIVE) &&
            LWLockHeldByMeInMode(ProcArrayLock, LW_EXCLUSIVE)));
 
    /*
     * Hold the ReplicationSlotControlLock until after updating the slot xmin
     * values, so no backend updates the initial xmin for newly created slot
     * concurrently. A shared lock is used here to minimize lock contention,
     * especially when many slots exist and advancements occur frequently.
     * This is safe since an exclusive lock is taken during initial slot xmin
     * update in slot creation.
     *
     * One might think that we can hold the ProcArrayLock exclusively and
     * update the slot xmin values, but it could increase lock contention on
     * the ProcArrayLock, which is not great since this function can be called
     * at non-negligible frequency.
     *
     * Concurrent invocation of this function may cause the computed slot xmin
     * to regress. However, this is harmless because tuples prior to the most
     * recent xmin are no longer useful once advancement occurs (see
     * LogicalConfirmReceivedLocation where the slot's xmin value is flushed
     * before updating the effective_xmin). Thus, such regression merely
     * prevents VACUUM from prematurely removing tuples without causing the
     * early deletion of required data.
     */
    if (!already_locked)
        LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
    for (i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
        TransactionId effective_xmin;
        TransactionId effective_catalog_xmin;
        bool        invalidated;
 
        if (!s->in_use)
            continue;
 
        SpinLockAcquire(&s->mutex);
        effective_xmin = s->effective_xmin;
        effective_catalog_xmin = s->effective_catalog_xmin;
        invalidated = s->data.invalidated != RS_INVAL_NONE;
        SpinLockRelease(&s->mutex);
 
        /* invalidated slots need not apply */
        if (invalidated)
            continue;
 
        /* check the data xmin */
        if (TransactionIdIsValid(effective_xmin) &&
            (!TransactionIdIsValid(agg_xmin) ||
             TransactionIdPrecedes(effective_xmin, agg_xmin)))
            agg_xmin = effective_xmin;
 
        /* check the catalog xmin */
        if (TransactionIdIsValid(effective_catalog_xmin) &&
            (!TransactionIdIsValid(agg_catalog_xmin) ||
             TransactionIdPrecedes(effective_catalog_xmin, agg_catalog_xmin)))
            agg_catalog_xmin = effective_catalog_xmin;
    }
 
    ProcArraySetReplicationSlotXmin(agg_xmin, agg_catalog_xmin, already_locked);
 
    if (!already_locked)
        LWLockRelease(ReplicationSlotControlLock);
}
 
/*
 * Compute the oldest restart LSN across all slots and inform xlog module.
 *
 * Note: while max_slot_wal_keep_size is theoretically relevant for this
 * purpose, we don't try to account for that, because this module doesn't
 * know what to compare against.
 */
void
ReplicationSlotsComputeRequiredLSN(void)
{
    int         i;
    XLogRecPtr  min_required = InvalidXLogRecPtr;
 
    Assert(ReplicationSlotCtl != NULL);
 
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    for (i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
        XLogRecPtr  restart_lsn;
        XLogRecPtr  last_saved_restart_lsn;
        bool        invalidated;
        ReplicationSlotPersistency persistency;
 
        if (!s->in_use)
            continue;
 
        SpinLockAcquire(&s->mutex);
        persistency = s->data.persistency;
        restart_lsn = s->data.restart_lsn;
        invalidated = s->data.invalidated != RS_INVAL_NONE;
        last_saved_restart_lsn = s->last_saved_restart_lsn;
        SpinLockRelease(&s->mutex);
 
        /* invalidated slots need not apply */
        if (invalidated)
            continue;
 
        /*
         * For persistent slot use last_saved_restart_lsn to compute the
         * oldest LSN for removal of WAL segments.  The segments between
         * last_saved_restart_lsn and restart_lsn might be needed by a
         * persistent slot in the case of database crash.  Non-persistent
         * slots can't survive the database crash, so we don't care about
         * last_saved_restart_lsn for them.
         */
        if (persistency == RS_PERSISTENT)
        {
            if (XLogRecPtrIsValid(last_saved_restart_lsn) &&
                restart_lsn > last_saved_restart_lsn)
            {
                restart_lsn = last_saved_restart_lsn;
            }
        }
 
        if (XLogRecPtrIsValid(restart_lsn) &&
            (!XLogRecPtrIsValid(min_required) ||
             restart_lsn < min_required))
            min_required = restart_lsn;
    }
    LWLockRelease(ReplicationSlotControlLock);
 
    XLogSetReplicationSlotMinimumLSN(min_required);
}
 
/*
 * Compute the oldest WAL LSN required by *logical* decoding slots..
 *
 * Returns InvalidXLogRecPtr if logical decoding is disabled or no logical
 * slots exist.
 *
 * NB: this returns a value >= ReplicationSlotsComputeRequiredLSN(), since it
 * ignores physical replication slots.
 *
 * The results aren't required frequently, so we don't maintain a precomputed
 * value like we do for ComputeRequiredLSN() and ComputeRequiredXmin().
 */
XLogRecPtr
ReplicationSlotsComputeLogicalRestartLSN(void)
{
    XLogRecPtr  result = InvalidXLogRecPtr;
    int         i;
 
    if (max_replication_slots + max_repack_replication_slots <= 0)
        return InvalidXLogRecPtr;
 
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
    for (i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s;
        XLogRecPtr  restart_lsn;
        XLogRecPtr  last_saved_restart_lsn;
        bool        invalidated;
        ReplicationSlotPersistency persistency;
 
        s = &ReplicationSlotCtl->replication_slots[i];
 
        /* cannot change while ReplicationSlotCtlLock is held */
        if (!s->in_use)
            continue;
 
        /* we're only interested in logical slots */
        if (!SlotIsLogical(s))
            continue;
 
        /* read once, it's ok if it increases while we're checking */
        SpinLockAcquire(&s->mutex);
        persistency = s->data.persistency;
        restart_lsn = s->data.restart_lsn;
        invalidated = s->data.invalidated != RS_INVAL_NONE;
        last_saved_restart_lsn = s->last_saved_restart_lsn;
        SpinLockRelease(&s->mutex);
 
        /* invalidated slots need not apply */
        if (invalidated)
            continue;
 
        /*
         * For persistent slot use last_saved_restart_lsn to compute the
         * oldest LSN for removal of WAL segments.  The segments between
         * last_saved_restart_lsn and restart_lsn might be needed by a
         * persistent slot in the case of database crash.  Non-persistent
         * slots can't survive the database crash, so we don't care about
         * last_saved_restart_lsn for them.
         */
        if (persistency == RS_PERSISTENT)
        {
            if (XLogRecPtrIsValid(last_saved_restart_lsn) &&
                restart_lsn > last_saved_restart_lsn)
            {
                restart_lsn = last_saved_restart_lsn;
            }
        }
 
        if (!XLogRecPtrIsValid(restart_lsn))
            continue;
 
        if (!XLogRecPtrIsValid(result) ||
            restart_lsn < result)
            result = restart_lsn;
    }
 
    LWLockRelease(ReplicationSlotControlLock);
 
    return result;
}
 
/*
 * ReplicationSlotsCountDBSlots -- count the number of slots that refer to the
 * passed database oid.
 *
 * Returns true if there are any slots referencing the database. *nslots will
 * be set to the absolute number of slots in the database, *nactive to ones
 * currently active.
 */
bool
ReplicationSlotsCountDBSlots(Oid dboid, int *nslots, int *nactive)
{
    int         i;
 
    *nslots = *nactive = 0;
 
    if (max_replication_slots + max_repack_replication_slots <= 0)
        return false;
 
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    for (i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s;
 
        s = &ReplicationSlotCtl->replication_slots[i];
 
        /* cannot change while ReplicationSlotCtlLock is held */
        if (!s->in_use)
            continue;
 
        /* only logical slots are database specific, skip */
        if (!SlotIsLogical(s))
            continue;
 
        /* not our database, skip */
        if (s->data.database != dboid)
            continue;
 
        /* NB: intentionally counting invalidated slots */
 
        /* count slots with spinlock held */
        SpinLockAcquire(&s->mutex);
        (*nslots)++;
        if (s->active_proc != INVALID_PROC_NUMBER)
            (*nactive)++;
        SpinLockRelease(&s->mutex);
    }
    LWLockRelease(ReplicationSlotControlLock);
 
    if (*nslots > 0)
        return true;
    return false;
}
 
/*
 * ReplicationSlotsDropDBSlots -- Drop all db-specific slots relating to the
 * passed database oid. The caller should hold an exclusive lock on the
 * pg_database oid for the database to prevent creation of new slots on the db
 * or replay from existing slots.
 *
 * Another session that concurrently acquires an existing slot on the target DB
 * (most likely to drop it) may cause this function to ERROR. If that happens
 * it may have dropped some but not all slots.
 *
 * This routine isn't as efficient as it could be - but we don't drop
 * databases often, especially databases with lots of slots.
 *
 * If the last logical slot in the cluster is dropped, request to disable
 * logical decoding.
 */
void
ReplicationSlotsDropDBSlots(Oid dboid)
{
    int         i;
    bool        found_valid_logicalslot;
    bool        dropped = false;
 
    if (max_replication_slots + max_repack_replication_slots <= 0)
        return;
 
restart:
    found_valid_logicalslot = false;
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    for (i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s;
        char       *slotname;
        ProcNumber  active_proc;
 
        s = &ReplicationSlotCtl->replication_slots[i];
 
        /* cannot change while ReplicationSlotCtlLock is held */
        if (!s->in_use)
            continue;
 
        /* only logical slots are database specific, skip */
        if (!SlotIsLogical(s))
            continue;
 
        /*
         * Check logical slots on other databases too so we can disable
         * logical decoding only if no slots in the cluster.
         */
        SpinLockAcquire(&s->mutex);
        found_valid_logicalslot |= (s->data.invalidated == RS_INVAL_NONE);
        SpinLockRelease(&s->mutex);
 
        /* not our database, skip */
        if (s->data.database != dboid)
            continue;
 
        /* NB: intentionally including invalidated slots to drop */
 
        /* acquire slot, so ReplicationSlotDropAcquired can be reused  */
        SpinLockAcquire(&s->mutex);
        /* can't change while ReplicationSlotControlLock is held */
        slotname = NameStr(s->data.name);
        active_proc = s->active_proc;
        if (active_proc == INVALID_PROC_NUMBER)
        {
            MyReplicationSlot = s;
            s->active_proc = MyProcNumber;
        }
        SpinLockRelease(&s->mutex);
 
        /*
         * Even though we hold an exclusive lock on the database object a
         * logical slot for that DB can still be active, e.g. if it's
         * concurrently being dropped by a backend connected to another DB.
         *
         * That's fairly unlikely in practice, so we'll just bail out.
         *
         * The slot sync worker holds a shared lock on the database before
         * operating on synced logical slots to avoid conflict with the drop
         * happening here. The persistent synced slots are thus safe but there
         * is a possibility that the slot sync worker has created a temporary
         * slot (which stays active even on release) and we are trying to drop
         * that here. In practice, the chances of hitting this scenario are
         * less as during slot synchronization, the temporary slot is
         * immediately converted to persistent and thus is safe due to the
         * shared lock taken on the database. So, we'll just bail out in such
         * a case.
         *
         * XXX: We can consider shutting down the slot sync worker before
         * trying to drop synced temporary slots here.
         */
        if (active_proc != INVALID_PROC_NUMBER)
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_IN_USE),
                     errmsg("replication slot \"%s\" is active for PID %d",
                            slotname, GetPGProcByNumber(active_proc)->pid)));
 
        /*
         * To avoid duplicating ReplicationSlotDropAcquired() and to avoid
         * holding ReplicationSlotControlLock over filesystem operations,
         * release ReplicationSlotControlLock and use
         * ReplicationSlotDropAcquired.
         *
         * As that means the set of slots could change, restart scan from the
         * beginning each time we release the lock.
         */
        LWLockRelease(ReplicationSlotControlLock);
        ReplicationSlotDropAcquired(false);
        dropped = true;
        goto restart;
    }
    LWLockRelease(ReplicationSlotControlLock);
 
    if (dropped && !found_valid_logicalslot)
        RequestDisableLogicalDecoding();
}
 
/*
 * Returns true if there is at least one in-use valid logical replication slot.
 */
bool
CheckLogicalSlotExists(void)
{
    bool        found = false;
 
    if (max_replication_slots + max_repack_replication_slots <= 0)
        return false;
 
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    for (int i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s;
        bool        invalidated;
 
        s = &ReplicationSlotCtl->replication_slots[i];
 
        /* cannot change while ReplicationSlotCtlLock is held */
        if (!s->in_use)
            continue;
 
        if (SlotIsPhysical(s))
            continue;
 
        SpinLockAcquire(&s->mutex);
        invalidated = s->data.invalidated != RS_INVAL_NONE;
        SpinLockRelease(&s->mutex);
 
        if (invalidated)
            continue;
 
        found = true;
        break;
    }
    LWLockRelease(ReplicationSlotControlLock);
 
    return found;
}
 
/*
 * Check whether the server's configuration supports using replication
 * slots.
 */
void
CheckSlotRequirements(bool repack)
{
    /*
     * NB: Adding a new requirement likely means that RestoreSlotFromDisk()
     * needs the same check.
     */
 
    if (!repack && max_replication_slots == 0)
        ereport(ERROR,
                errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                errmsg("replication slots can only be used if \"%s\" > 0",
                       "max_replication_slots"));
 
    if (repack && max_repack_replication_slots == 0)
        ereport(ERROR,
                errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                errmsg("REPACK can only be used if \"%s\" > 0",
                       "max_repack_replication_slots"));
 
    if (wal_level < WAL_LEVEL_REPLICA)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("replication slots can only be used if \"wal_level\" >= \"replica\"")));
}
 
/*
 * Check whether the user has privilege to use replication slots.
 */
void
CheckSlotPermissions(void)
{
    if (!has_rolreplication(GetUserId()))
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("permission denied to use replication slots"),
                 errdetail("Only roles with the %s attribute may use replication slots.",
                           "REPLICATION")));
}
 
/*
 * Reserve WAL for the currently active slot.
 *
 * Compute and set restart_lsn in a manner that's appropriate for the type of
 * the slot and concurrency safe.
 */
void
ReplicationSlotReserveWal(void)
{
    ReplicationSlot *slot = MyReplicationSlot;
    XLogSegNo   segno;
    XLogRecPtr  restart_lsn;
 
    Assert(slot != NULL);
    Assert(!XLogRecPtrIsValid(slot->data.restart_lsn));
    Assert(!XLogRecPtrIsValid(slot->last_saved_restart_lsn));
 
    /*
     * The replication slot mechanism is used to prevent the removal of
     * required WAL.
     *
     * Acquire an exclusive lock to prevent the checkpoint process from
     * concurrently computing the minimum slot LSN (see
     * CheckPointReplicationSlots). This ensures that the WAL reserved for
     * replication cannot be removed during a checkpoint.
     *
     * The mechanism is reliable because if WAL reservation occurs first, the
     * checkpoint must wait for the restart_lsn update before determining the
     * minimum non-removable LSN. On the other hand, if the checkpoint happens
     * first, subsequent WAL reservations will select positions at or beyond
     * the redo pointer of that checkpoint.
     */
    LWLockAcquire(ReplicationSlotAllocationLock, LW_EXCLUSIVE);
 
    /*
     * For logical slots log a standby snapshot and start logical decoding at
     * exactly that position. That allows the slot to start up more quickly.
     * But on a standby we cannot do WAL writes, so just use the replay
     * pointer; effectively, an attempt to create a logical slot on standby
     * will cause it to wait for an xl_running_xact record to be logged
     * independently on the primary, so that a snapshot can be built using the
     * record.
     *
     * None of this is needed (or indeed helpful) for physical slots as
     * they'll start replay at the last logged checkpoint anyway. Instead,
     * return the location of the last redo LSN, where a base backup has to
     * start replay at.
     */
    if (SlotIsPhysical(slot))
        restart_lsn = GetRedoRecPtr();
    else if (RecoveryInProgress())
        restart_lsn = GetXLogReplayRecPtr(NULL);
    else
        restart_lsn = GetXLogInsertRecPtr();
 
    SpinLockAcquire(&slot->mutex);
    slot->data.restart_lsn = restart_lsn;
    SpinLockRelease(&slot->mutex);
 
    /* prevent WAL removal as fast as possible */
    ReplicationSlotsComputeRequiredLSN();
 
    /* Checkpoint shouldn't remove the required WAL. */
    XLByteToSeg(slot->data.restart_lsn, segno, wal_segment_size);
    if (XLogGetLastRemovedSegno() >= segno)
        elog(ERROR, "WAL required by replication slot %s has been removed concurrently",
             NameStr(slot->data.name));
 
    LWLockRelease(ReplicationSlotAllocationLock);
 
    if (!RecoveryInProgress() && SlotIsLogical(slot))
    {
        XLogRecPtr  flushptr;
 
        /* make sure we have enough information to start */
        flushptr = LogStandbySnapshot();
 
        /* and make sure it's fsynced to disk */
        XLogFlush(flushptr);
    }
}
 
/*
 * Report that replication slot needs to be invalidated
 */
static void
ReportSlotInvalidation(ReplicationSlotInvalidationCause cause,
                       bool terminating,
                       int pid,
                       NameData slotname,
                       XLogRecPtr restart_lsn,
                       XLogRecPtr oldestLSN,
                       TransactionId snapshotConflictHorizon,
                       long slot_idle_seconds)
{
    StringInfoData err_detail;
    StringInfoData err_hint;
 
    initStringInfo(&err_detail);
    initStringInfo(&err_hint);
 
    switch (cause)
    {
        case RS_INVAL_WAL_REMOVED:
            {
                uint64      ex = oldestLSN - restart_lsn;
 
                appendStringInfo(&err_detail,
                                 ngettext("The slot's restart_lsn %X/%08X exceeds the limit by %" PRIu64 " byte.",
                                          "The slot's restart_lsn %X/%08X exceeds the limit by %" PRIu64 " bytes.",
                                          ex),
                                 LSN_FORMAT_ARGS(restart_lsn),
                                 ex);
                /* translator: %s is a GUC variable name */
                appendStringInfo(&err_hint, _("You might need to increase \"%s\"."),
                                 "max_slot_wal_keep_size");
                break;
            }
        case RS_INVAL_HORIZON:
            appendStringInfo(&err_detail, _("The slot conflicted with xid horizon %u."),
                             snapshotConflictHorizon);
            break;
 
        case RS_INVAL_WAL_LEVEL:
            appendStringInfoString(&err_detail, _("Logical decoding on standby requires the primary server to either set \"wal_level\" >= \"logical\" or have at least one logical slot when \"wal_level\" = \"replica\"."));
            break;
 
        case RS_INVAL_IDLE_TIMEOUT:
            {
                /* translator: %s is a GUC variable name */
                appendStringInfo(&err_detail, _("The slot's idle time of %lds exceeds the configured \"%s\" duration of %ds."),
                                 slot_idle_seconds, "idle_replication_slot_timeout",
                                 idle_replication_slot_timeout_secs);
                /* translator: %s is a GUC variable name */
                appendStringInfo(&err_hint, _("You might need to increase \"%s\"."),
                                 "idle_replication_slot_timeout");
                break;
            }
        case RS_INVAL_NONE:
            pg_unreachable();
    }
 
    ereport(LOG,
            terminating ?
            errmsg("terminating process %d to release replication slot \"%s\"",
                   pid, NameStr(slotname)) :
            errmsg("invalidating obsolete replication slot \"%s\"",
                   NameStr(slotname)),
            errdetail_internal("%s", err_detail.data),
            err_hint.len ? errhint("%s", err_hint.data) : 0);
 
    pfree(err_detail.data);
    pfree(err_hint.data);
}
 
/*
 * Can we invalidate an idle replication slot?
 *
 * Idle timeout invalidation is allowed only when:
 *
 * 1. Idle timeout is set
 * 2. Slot has reserved WAL
 * 3. Slot is inactive
 * 4. The slot is not being synced from the primary while the server is in
 *    recovery. This is because synced slots are always considered to be
 *    inactive because they don't perform logical decoding to produce changes.
 */
static inline bool
CanInvalidateIdleSlot(ReplicationSlot *s)
{
    return (idle_replication_slot_timeout_secs != 0 &&
            XLogRecPtrIsValid(s->data.restart_lsn) &&
            s->inactive_since > 0 &&
            !(RecoveryInProgress() && s->data.synced));
}
 
/*
 * DetermineSlotInvalidationCause - Determine the cause for which a slot
 * becomes invalid among the given possible causes.
 *
 * This function sequentially checks all possible invalidation causes and
 * returns the first one for which the slot is eligible for invalidation.
 */
static ReplicationSlotInvalidationCause
DetermineSlotInvalidationCause(uint32 possible_causes, ReplicationSlot *s,
                               XLogRecPtr oldestLSN, Oid dboid,
                               TransactionId snapshotConflictHorizon,
                               TimestampTz *inactive_since, TimestampTz now)
{
    Assert(possible_causes != RS_INVAL_NONE);
 
    if (possible_causes & RS_INVAL_WAL_REMOVED)
    {
        XLogRecPtr  restart_lsn = s->data.restart_lsn;
 
        if (XLogRecPtrIsValid(restart_lsn) &&
            restart_lsn < oldestLSN)
            return RS_INVAL_WAL_REMOVED;
    }
 
    if (possible_causes & RS_INVAL_HORIZON)
    {
        /* invalid DB oid signals a shared relation */
        if (SlotIsLogical(s) &&
            (dboid == InvalidOid || dboid == s->data.database))
        {
            TransactionId effective_xmin = s->effective_xmin;
            TransactionId catalog_effective_xmin = s->effective_catalog_xmin;
 
            if (TransactionIdIsValid(effective_xmin) &&
                TransactionIdPrecedesOrEquals(effective_xmin,
                                              snapshotConflictHorizon))
                return RS_INVAL_HORIZON;
            else if (TransactionIdIsValid(catalog_effective_xmin) &&
                     TransactionIdPrecedesOrEquals(catalog_effective_xmin,
                                                   snapshotConflictHorizon))
                return RS_INVAL_HORIZON;
        }
    }
 
    if (possible_causes & RS_INVAL_WAL_LEVEL)
    {
        if (SlotIsLogical(s))
            return RS_INVAL_WAL_LEVEL;
    }
 
    if (possible_causes & RS_INVAL_IDLE_TIMEOUT)
    {
        Assert(now > 0);
 
        if (CanInvalidateIdleSlot(s))
        {
            /*
             * Simulate the invalidation due to idle_timeout to test the
             * timeout behavior promptly, without waiting for it to trigger
             * naturally.
             */
#ifdef USE_INJECTION_POINTS
            if (IS_INJECTION_POINT_ATTACHED("slot-timeout-inval"))
            {
                *inactive_since = 0;    /* since the beginning of time */
                return RS_INVAL_IDLE_TIMEOUT;
            }
#endif
 
            /*
             * Check if the slot needs to be invalidated due to
             * idle_replication_slot_timeout GUC.
             */
            if (TimestampDifferenceExceedsSeconds(s->inactive_since, now,
                                                  idle_replication_slot_timeout_secs))
            {
                *inactive_since = s->inactive_since;
                return RS_INVAL_IDLE_TIMEOUT;
            }
        }
    }
 
    return RS_INVAL_NONE;
}
 
/*
 * Helper for InvalidateObsoleteReplicationSlots
 *
 * Acquires the given slot and mark it invalid, if necessary and possible.
 *
 * Returns true if the slot was invalidated.
 *
 * Set *released_lock_out if ReplicationSlotControlLock was released in the
 * interim (and in that case we're not holding the lock at return, otherwise
 * we are).
 *
 * This is inherently racy, because we release the LWLock
 * for syscalls, so caller must restart if we return true.
 */
static bool
InvalidatePossiblyObsoleteSlot(uint32 possible_causes,
                               ReplicationSlot *s,
                               XLogRecPtr oldestLSN,
                               Oid dboid, TransactionId snapshotConflictHorizon,
                               bool *released_lock_out)
{
    int         last_signaled_pid = 0;
    bool        released_lock = false;
    bool        invalidated = false;
    TimestampTz inactive_since = 0;
 
    for (;;)
    {
        XLogRecPtr  restart_lsn;
        NameData    slotname;
        ProcNumber  active_proc;
        int         active_pid = 0;
        ReplicationSlotInvalidationCause invalidation_cause = RS_INVAL_NONE;
        TimestampTz now = 0;
        long        slot_idle_secs = 0;
 
        Assert(LWLockHeldByMeInMode(ReplicationSlotControlLock, LW_SHARED));
 
        if (!s->in_use)
        {
            if (released_lock)
                LWLockRelease(ReplicationSlotControlLock);
            break;
        }
 
        if (possible_causes & RS_INVAL_IDLE_TIMEOUT)
        {
            /*
             * Assign the current time here to avoid system call overhead
             * while holding the spinlock in subsequent code.
             */
            now = GetCurrentTimestamp();
        }
 
        /*
         * Check if the slot needs to be invalidated. If it needs to be
         * invalidated, and is not currently acquired, acquire it and mark it
         * as having been invalidated.  We do this with the spinlock held to
         * avoid race conditions -- for example the restart_lsn could move
         * forward, or the slot could be dropped.
         */
        SpinLockAcquire(&s->mutex);
 
        restart_lsn = s->data.restart_lsn;
 
        /* we do nothing if the slot is already invalid */
        if (s->data.invalidated == RS_INVAL_NONE)
            invalidation_cause = DetermineSlotInvalidationCause(possible_causes,
                                                                s, oldestLSN,
                                                                dboid,
                                                                snapshotConflictHorizon,
                                                                &inactive_since,
                                                                now);
 
        /* if there's no invalidation, we're done */
        if (invalidation_cause == RS_INVAL_NONE)
        {
            SpinLockRelease(&s->mutex);
            if (released_lock)
                LWLockRelease(ReplicationSlotControlLock);
            break;
        }
 
        slotname = s->data.name;
        active_proc = s->active_proc;
 
        /*
         * If the slot can be acquired, do so and mark it invalidated
         * immediately.  Otherwise we'll signal the owning process, below, and
         * retry.
         *
         * Note: Unlike other slot attributes, slot's inactive_since can't be
         * changed until the acquired slot is released or the owning process
         * is terminated. So, the inactive slot can only be invalidated
         * immediately without being terminated.
         */
        if (active_proc == INVALID_PROC_NUMBER)
        {
            MyReplicationSlot = s;
            s->active_proc = MyProcNumber;
            s->data.invalidated = invalidation_cause;
 
            /*
             * XXX: We should consider not overwriting restart_lsn and instead
             * just rely on .invalidated.
             */
            if (invalidation_cause == RS_INVAL_WAL_REMOVED)
            {
                s->data.restart_lsn = InvalidXLogRecPtr;
                s->last_saved_restart_lsn = InvalidXLogRecPtr;
            }
 
            /* Let caller know */
            invalidated = true;
        }
        else
        {
            active_pid = GetPGProcByNumber(active_proc)->pid;
            Assert(active_pid != 0);
        }
 
        SpinLockRelease(&s->mutex);
 
        /*
         * Calculate the idle time duration of the slot if slot is marked
         * invalidated with RS_INVAL_IDLE_TIMEOUT.
         */
        if (invalidation_cause == RS_INVAL_IDLE_TIMEOUT)
        {
            int         slot_idle_usecs;
 
            TimestampDifference(inactive_since, now, &slot_idle_secs,
                                &slot_idle_usecs);
        }
 
        if (active_proc != INVALID_PROC_NUMBER)
        {
            /*
             * Prepare the sleep on the slot's condition variable before
             * releasing the lock, to close a possible race condition if the
             * slot is released before the sleep below.
             */
            ConditionVariablePrepareToSleep(&s->active_cv);
 
            LWLockRelease(ReplicationSlotControlLock);
            released_lock = true;
 
            /*
             * Signal to terminate the process that owns the slot, if we
             * haven't already signalled it.  (Avoidance of repeated
             * signalling is the only reason for there to be a loop in this
             * routine; otherwise we could rely on caller's restart loop.)
             *
             * There is the race condition that other process may own the slot
             * after its current owner process is terminated and before this
             * process owns it. To handle that, we signal only if the PID of
             * the owning process has changed from the previous time. (This
             * logic assumes that the same PID is not reused very quickly.)
             */
            if (last_signaled_pid != active_pid)
            {
                ReportSlotInvalidation(invalidation_cause, true, active_pid,
                                       slotname, restart_lsn,
                                       oldestLSN, snapshotConflictHorizon,
                                       slot_idle_secs);
 
                if (MyBackendType == B_STARTUP)
                    (void) SignalRecoveryConflict(GetPGProcByNumber(active_proc),
                                                  active_pid,
                                                  RECOVERY_CONFLICT_LOGICALSLOT);
                else
                    (void) kill(active_pid, SIGTERM);
 
                last_signaled_pid = active_pid;
            }
 
            /* Wait until the slot is released. */
            ConditionVariableSleep(&s->active_cv,
                                   WAIT_EVENT_REPLICATION_SLOT_DROP);
 
            /*
             * Re-acquire lock and start over; we expect to invalidate the
             * slot next time (unless another process acquires the slot in the
             * meantime).
             *
             * Note: It is possible for a slot to advance its restart_lsn or
             * xmin values sufficiently between when we release the mutex and
             * when we recheck, moving from a conflicting state to a non
             * conflicting state.  This is intentional and safe: if the slot
             * has caught up while we're busy here, the resources we were
             * concerned about (WAL segments or tuples) have not yet been
             * removed, and there's no reason to invalidate the slot.
             */
            LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
            continue;
        }
        else
        {
            /*
             * We hold the slot now and have already invalidated it; flush it
             * to ensure that state persists.
             *
             * Don't want to hold ReplicationSlotControlLock across file
             * system operations, so release it now but be sure to tell caller
             * to restart from scratch.
             */
            LWLockRelease(ReplicationSlotControlLock);
            released_lock = true;
 
            /* Make sure the invalidated state persists across server restart */
            ReplicationSlotMarkDirty();
            ReplicationSlotSave();
            ReplicationSlotRelease();
 
            ReportSlotInvalidation(invalidation_cause, false, active_pid,
                                   slotname, restart_lsn,
                                   oldestLSN, snapshotConflictHorizon,
                                   slot_idle_secs);
 
            /* done with this slot for now */
            break;
        }
    }
 
    Assert(released_lock == !LWLockHeldByMe(ReplicationSlotControlLock));
 
    *released_lock_out = released_lock;
    return invalidated;
}
 
/*
 * Invalidate slots that require resources about to be removed.
 *
 * Returns true when any slot have got invalidated.
 *
 * Whether a slot needs to be invalidated depends on the invalidation cause.
 * A slot is invalidated if it:
 * - RS_INVAL_WAL_REMOVED: requires a LSN older than the given segment
 * - RS_INVAL_HORIZON: requires a snapshot <= the given horizon in the given
 *   db; dboid may be InvalidOid for shared relations
 * - RS_INVAL_WAL_LEVEL: is a logical slot and effective_wal_level is not
 *   logical.
 * - RS_INVAL_IDLE_TIMEOUT: has been idle longer than the configured
 *   "idle_replication_slot_timeout" duration.
 *
 * Note: This function attempts to invalidate the slot for multiple possible
 * causes in a single pass, minimizing redundant iterations. The "cause"
 * parameter can be a MASK representing one or more of the defined causes.
 *
 * If it invalidates the last logical slot in the cluster, it requests to
 * disable logical decoding.
 *
 * NB - this runs as part of checkpoint, so avoid raising errors if possible.
 */
bool
InvalidateObsoleteReplicationSlots(uint32 possible_causes,
                                   XLogSegNo oldestSegno, Oid dboid,
                                   TransactionId snapshotConflictHorizon)
{
    XLogRecPtr  oldestLSN;
    bool        invalidated = false;
    bool        invalidated_logical = false;
    bool        found_valid_logicalslot;
 
    Assert(!(possible_causes & RS_INVAL_HORIZON) || TransactionIdIsValid(snapshotConflictHorizon));
    Assert(!(possible_causes & RS_INVAL_WAL_REMOVED) || oldestSegno > 0);
    Assert(possible_causes != RS_INVAL_NONE);
 
    if (max_replication_slots == 0 && max_repack_replication_slots == 0)
        return invalidated;
 
    XLogSegNoOffsetToRecPtr(oldestSegno, 0, wal_segment_size, oldestLSN);
 
restart:
    found_valid_logicalslot = false;
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    for (int i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
        bool        released_lock = false;
 
        if (!s->in_use)
            continue;
 
        /* Prevent invalidation of logical slots during binary upgrade */
        if (SlotIsLogical(s) && IsBinaryUpgrade)
        {
            SpinLockAcquire(&s->mutex);
            found_valid_logicalslot |= (s->data.invalidated == RS_INVAL_NONE);
            SpinLockRelease(&s->mutex);
 
            continue;
        }
 
        if (InvalidatePossiblyObsoleteSlot(possible_causes, s, oldestLSN,
                                           dboid, snapshotConflictHorizon,
                                           &released_lock))
        {
            Assert(released_lock);
 
            /* Remember we have invalidated a physical or logical slot */
            invalidated = true;
 
            /*
             * Additionally, remember we have invalidated a logical slot as we
             * can request disabling logical decoding later.
             */
            if (SlotIsLogical(s))
                invalidated_logical = true;
        }
        else
        {
            /*
             * We need to check if the slot is invalidated here since
             * InvalidatePossiblyObsoleteSlot() returns false also if the slot
             * is already invalidated.
             */
            SpinLockAcquire(&s->mutex);
            found_valid_logicalslot |=
                (SlotIsLogical(s) && (s->data.invalidated == RS_INVAL_NONE));
            SpinLockRelease(&s->mutex);
        }
 
        /* if the lock was released, start from scratch */
        if (released_lock)
            goto restart;
    }
    LWLockRelease(ReplicationSlotControlLock);
 
    /*
     * If any slots have been invalidated, recalculate the resource limits.
     */
    if (invalidated)
    {
        ReplicationSlotsComputeRequiredXmin(false);
        ReplicationSlotsComputeRequiredLSN();
    }
 
    /*
     * Request the checkpointer to disable logical decoding if no valid
     * logical slots remain. If called by the checkpointer during a
     * checkpoint, only the request is initiated; actual deactivation is
     * deferred until after the checkpoint completes.
     */
    if (invalidated_logical && !found_valid_logicalslot)
        RequestDisableLogicalDecoding();
 
    return invalidated;
}
 
/*
 * Flush all replication slots to disk.
 *
 * It is convenient to flush dirty replication slots at the time of checkpoint.
 * Additionally, in case of a shutdown checkpoint, we also identify the slots
 * for which the confirmed_flush LSN has been updated since the last time it
 * was saved and flush them.
 */
void
CheckPointReplicationSlots(bool is_shutdown)
{
    int         i;
    bool        last_saved_restart_lsn_updated = false;
 
    elog(DEBUG1, "performing replication slot checkpoint");
 
    /*
     * Prevent any slot from being created/dropped while we're active. As we
     * explicitly do *not* want to block iterating over replication_slots or
     * acquiring a slot we cannot take the control lock - but that's OK,
     * because holding ReplicationSlotAllocationLock is strictly stronger, and
     * enough to guarantee that nobody can change the in_use bits on us.
     *
     * Additionally, acquiring the Allocation lock is necessary to serialize
     * the slot flush process with concurrent slot WAL reservation. This
     * ensures that the WAL position being reserved is either flushed to disk
     * or is beyond or equal to the redo pointer of the current checkpoint
     * (See ReplicationSlotReserveWal for details).
     */
    LWLockAcquire(ReplicationSlotAllocationLock, LW_SHARED);
 
    for (i = 0; i < max_replication_slots + max_repack_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
        char        path[MAXPGPATH];
 
        if (!s->in_use)
            continue;
 
        /* save the slot to disk, locking is handled in SaveSlotToPath() */
        sprintf(path, "%s/%s", PG_REPLSLOT_DIR, NameStr(s->data.name));
 
        /*
         * Slot's data is not flushed each time the confirmed_flush LSN is
         * updated as that could lead to frequent writes.  However, we decide
         * to force a flush of all logical slot's data at the time of shutdown
         * if the confirmed_flush LSN is changed since we last flushed it to
         * disk.  This helps in avoiding an unnecessary retreat of the
         * confirmed_flush LSN after restart.
         */
        if (is_shutdown && SlotIsLogical(s))
        {
            SpinLockAcquire(&s->mutex);
 
            if (s->data.invalidated == RS_INVAL_NONE &&
                s->data.confirmed_flush > s->last_saved_confirmed_flush)
            {
                s->just_dirtied = true;
                s->dirty = true;
            }
            SpinLockRelease(&s->mutex);
        }
 
        /*
         * Track if we're going to update slot's last_saved_restart_lsn. We
         * need this to know if we need to recompute the required LSN.
         */
        if (s->last_saved_restart_lsn != s->data.restart_lsn)
            last_saved_restart_lsn_updated = true;
 
        SaveSlotToPath(s, path, LOG);
    }
    LWLockRelease(ReplicationSlotAllocationLock);
 
    /*
     * Recompute the required LSN if SaveSlotToPath() updated
     * last_saved_restart_lsn for any slot.
     */
    if (last_saved_restart_lsn_updated)
        ReplicationSlotsComputeRequiredLSN();
}
 
/*
 * Load all replication slots from disk into memory at server startup. This
 * needs to be run before we start crash recovery.
 */
void
StartupReplicationSlots(void)
{
    DIR        *replication_dir;
    struct dirent *replication_de;
 
    elog(DEBUG1, "starting up replication slots");
 
    /* restore all slots by iterating over all on-disk entries */
    replication_dir = AllocateDir(PG_REPLSLOT_DIR);
    while ((replication_de = ReadDir(replication_dir, PG_REPLSLOT_DIR)) != NULL)
    {
        char        path[MAXPGPATH + sizeof(PG_REPLSLOT_DIR)];
        PGFileType  de_type;
 
        if (strcmp(replication_de->d_name, ".") == 0 ||
            strcmp(replication_de->d_name, "..") == 0)
            continue;
 
        snprintf(path, sizeof(path), "%s/%s", PG_REPLSLOT_DIR, replication_de->d_name);
        de_type = get_dirent_type(path, replication_de, false, DEBUG1);
 
        /* we're only creating directories here, skip if it's not our's */
        if (de_type != PGFILETYPE_ERROR && de_type != PGFILETYPE_DIR)
            continue;
 
        /* we crashed while a slot was being setup or deleted, clean up */
        if (pg_str_endswith(replication_de->d_name, ".tmp"))
        {
            if (!rmtree(path, true))
            {
                ereport(WARNING,
                        (errmsg("could not remove directory \"%s\"",
                                path)));
                continue;
            }
            fsync_fname(PG_REPLSLOT_DIR, true);
            continue;
        }
 
        /* looks like a slot in a normal state, restore */
        RestoreSlotFromDisk(replication_de->d_name);
    }
    FreeDir(replication_dir);
 
    /* currently no slots exist, we're done. */
    if (max_replication_slots + max_repack_replication_slots <= 0)
        return;
 
    /* Now that we have recovered all the data, compute replication xmin */
    ReplicationSlotsComputeRequiredXmin(false);
    ReplicationSlotsComputeRequiredLSN();
}
 
/* ----
 * Manipulation of on-disk state of replication slots
 *
 * NB: none of the routines below should take any notice whether a slot is the
 * current one or not, that's all handled a layer above.
 * ----
 */
static void
CreateSlotOnDisk(ReplicationSlot *slot)
{
    char        tmppath[MAXPGPATH];
    char        path[MAXPGPATH];
    struct stat st;
 
    /*
     * No need to take out the io_in_progress_lock, nobody else can see this
     * slot yet, so nobody else will write. We're reusing SaveSlotToPath which
     * takes out the lock, if we'd take the lock here, we'd deadlock.
     */
 
    sprintf(path, "%s/%s", PG_REPLSLOT_DIR, NameStr(slot->data.name));
    sprintf(tmppath, "%s/%s.tmp", PG_REPLSLOT_DIR, NameStr(slot->data.name));
 
    /*
     * It's just barely possible that some previous effort to create or drop a
     * slot with this name left a temp directory lying around. If that seems
     * to be the case, try to remove it.  If the rmtree() fails, we'll error
     * out at the MakePGDirectory() below, so we don't bother checking
     * success.
     */
    if (stat(tmppath, &st) == 0 && S_ISDIR(st.st_mode))
        rmtree(tmppath, true);
 
    /* Create and fsync the temporary slot directory. */
    if (MakePGDirectory(tmppath) < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not create directory \"%s\": %m",
                        tmppath)));
    fsync_fname(tmppath, true);
 
    /* Write the actual state file. */
    slot->dirty = true;         /* signal that we really need to write */
    SaveSlotToPath(slot, tmppath, ERROR);
 
    /* Rename the directory into place. */
    if (rename(tmppath, path) != 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                        tmppath, path)));
 
    /*
     * If we'd now fail - really unlikely - we wouldn't know whether this slot
     * would persist after an OS crash or not - so, force a restart. The
     * restart would try to fsync this again till it works.
     */
    START_CRIT_SECTION();
 
    fsync_fname(path, true);
    fsync_fname(PG_REPLSLOT_DIR, true);
 
    END_CRIT_SECTION();
}
 
/*
 * Shared functionality between saving and creating a replication slot.
 */
static void
SaveSlotToPath(ReplicationSlot *slot, const char *dir, int elevel)
{
    char        tmppath[MAXPGPATH];
    char        path[MAXPGPATH];
    int         fd;
    ReplicationSlotOnDisk cp;
    bool        was_dirty;
 
    /* first check whether there's something to write out */
    SpinLockAcquire(&slot->mutex);
    was_dirty = slot->dirty;
    slot->just_dirtied = false;
    SpinLockRelease(&slot->mutex);
 
    /* and don't do anything if there's nothing to write */
    if (!was_dirty)
        return;
 
    LWLockAcquire(&slot->io_in_progress_lock, LW_EXCLUSIVE);
 
    /* silence valgrind :( */
    memset(&cp, 0, sizeof(ReplicationSlotOnDisk));
 
    sprintf(tmppath, "%s/state.tmp", dir);
    sprintf(path, "%s/state", dir);
 
    fd = OpenTransientFile(tmppath, O_CREAT | O_EXCL | O_WRONLY | PG_BINARY);
    if (fd < 0)
    {
        /*
         * If not an ERROR, then release the lock before returning.  In case
         * of an ERROR, the error recovery path automatically releases the
         * lock, but no harm in explicitly releasing even in that case.  Note
         * that LWLockRelease() could affect errno.
         */
        int         save_errno = errno;
 
        LWLockRelease(&slot->io_in_progress_lock);
        errno = save_errno;
        ereport(elevel,
                (errcode_for_file_access(),
                 errmsg("could not create file \"%s\": %m",
                        tmppath)));
        return;
    }
 
    cp.magic = SLOT_MAGIC;
    INIT_CRC32C(cp.checksum);
    cp.version = SLOT_VERSION;
    cp.length = ReplicationSlotOnDiskV2Size;
 
    SpinLockAcquire(&slot->mutex);
 
    memcpy(&cp.slotdata, &slot->data, sizeof(ReplicationSlotPersistentData));
 
    SpinLockRelease(&slot->mutex);
 
    COMP_CRC32C(cp.checksum,
                (char *) (&cp) + ReplicationSlotOnDiskNotChecksummedSize,
                ReplicationSlotOnDiskChecksummedSize);
    FIN_CRC32C(cp.checksum);
 
    errno = 0;
    pgstat_report_wait_start(WAIT_EVENT_REPLICATION_SLOT_WRITE);
    if ((write(fd, &cp, sizeof(cp))) != sizeof(cp))
    {
        int         save_errno = errno;
 
        pgstat_report_wait_end();
        CloseTransientFile(fd);
        unlink(tmppath);
        LWLockRelease(&slot->io_in_progress_lock);
 
        /* if write didn't set errno, assume problem is no disk space */
        errno = save_errno ? save_errno : ENOSPC;
        ereport(elevel,
                (errcode_for_file_access(),
                 errmsg("could not write to file \"%s\": %m",
                        tmppath)));
        return;
    }
    pgstat_report_wait_end();
 
    /* fsync the temporary file */
    pgstat_report_wait_start(WAIT_EVENT_REPLICATION_SLOT_SYNC);
    if (pg_fsync(fd) != 0)
    {
        int         save_errno = errno;
 
        pgstat_report_wait_end();
        CloseTransientFile(fd);
        unlink(tmppath);
        LWLockRelease(&slot->io_in_progress_lock);
 
        errno = save_errno;
        ereport(elevel,
                (errcode_for_file_access(),
                 errmsg("could not fsync file \"%s\": %m",
                        tmppath)));
        return;
    }
    pgstat_report_wait_end();
 
    if (CloseTransientFile(fd) != 0)
    {
        int         save_errno = errno;
 
        unlink(tmppath);
        LWLockRelease(&slot->io_in_progress_lock);
 
        errno = save_errno;
        ereport(elevel,
                (errcode_for_file_access(),
                 errmsg("could not close file \"%s\": %m",
                        tmppath)));
        return;
    }
 
    /* rename to permanent file, fsync file and directory */
    if (rename(tmppath, path) != 0)
    {
        int         save_errno = errno;
 
        unlink(tmppath);
        LWLockRelease(&slot->io_in_progress_lock);
 
        errno = save_errno;
        ereport(elevel,
                (errcode_for_file_access(),
                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                        tmppath, path)));
        return;
    }
 
    /*
     * Check CreateSlotOnDisk() for the reasoning of using a critical section.
     */
    START_CRIT_SECTION();
 
    fsync_fname(path, false);
    fsync_fname(dir, true);
    fsync_fname(PG_REPLSLOT_DIR, true);
 
    END_CRIT_SECTION();
 
    /*
     * Successfully wrote, unset dirty bit, unless somebody dirtied again
     * already and remember the confirmed_flush LSN value.
     */
    SpinLockAcquire(&slot->mutex);
    if (!slot->just_dirtied)
        slot->dirty = false;
    slot->last_saved_confirmed_flush = cp.slotdata.confirmed_flush;
    slot->last_saved_restart_lsn = cp.slotdata.restart_lsn;
    SpinLockRelease(&slot->mutex);
 
    LWLockRelease(&slot->io_in_progress_lock);
}
 
/*
 * Load a single slot from disk into memory.
 */
static void
RestoreSlotFromDisk(const char *name)
{
    ReplicationSlotOnDisk cp;
    int         i;
    char        slotdir[MAXPGPATH + sizeof(PG_REPLSLOT_DIR)];
    char        path[MAXPGPATH + sizeof(PG_REPLSLOT_DIR) + 10];
    int         fd;
    bool        restored = false;
    int         readBytes;
    pg_crc32c   checksum;
    TimestampTz now = 0;
 
    /* no need to lock here, no concurrent access allowed yet */
 
    /* delete temp file if it exists */
    sprintf(slotdir, "%s/%s", PG_REPLSLOT_DIR, name);
    sprintf(path, "%s/state.tmp", slotdir);
    if (unlink(path) < 0 && errno != ENOENT)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not remove file \"%s\": %m", path)));
 
    sprintf(path, "%s/state", slotdir);
 
    elog(DEBUG1, "restoring replication slot from \"%s\"", path);
 
    /* on some operating systems fsyncing a file requires O_RDWR */
    fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
 
    /*
     * We do not need to handle this as we are rename()ing the directory into
     * place only after we fsync()ed the state file.
     */
    if (fd < 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m", path)));
 
    /*
     * Sync state file before we're reading from it. We might have crashed
     * while it wasn't synced yet and we shouldn't continue on that basis.
     */
    pgstat_report_wait_start(WAIT_EVENT_REPLICATION_SLOT_RESTORE_SYNC);
    if (pg_fsync(fd) != 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not fsync file \"%s\": %m",
                        path)));
    pgstat_report_wait_end();
 
    /* Also sync the parent directory */
    START_CRIT_SECTION();
    fsync_fname(slotdir, true);
    END_CRIT_SECTION();
 
    /* read part of statefile that's guaranteed to be version independent */
    pgstat_report_wait_start(WAIT_EVENT_REPLICATION_SLOT_READ);
    readBytes = read(fd, &cp, ReplicationSlotOnDiskConstantSize);
    pgstat_report_wait_end();
    if (readBytes != ReplicationSlotOnDiskConstantSize)
    {
        if (readBytes < 0)
            ereport(PANIC,
                    (errcode_for_file_access(),
                     errmsg("could not read file \"%s\": %m", path)));
        else
            ereport(PANIC,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg("could not read file \"%s\": read %d of %zu",
                            path, readBytes,
                            (Size) ReplicationSlotOnDiskConstantSize)));
    }
 
    /* verify magic */
    if (cp.magic != SLOT_MAGIC)
        ereport(PANIC,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("replication slot file \"%s\" has wrong magic number: %u instead of %u",
                        path, cp.magic, SLOT_MAGIC)));
 
    /* verify version */
    if (cp.version != SLOT_VERSION)
        ereport(PANIC,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("replication slot file \"%s\" has unsupported version %u",
                        path, cp.version)));
 
    /* boundary check on length */
    if (cp.length != ReplicationSlotOnDiskV2Size)
        ereport(PANIC,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("replication slot file \"%s\" has corrupted length %u",
                        path, cp.length)));
 
    /* Now that we know the size, read the entire file */
    pgstat_report_wait_start(WAIT_EVENT_REPLICATION_SLOT_READ);
    readBytes = read(fd,
                     (char *) &cp + ReplicationSlotOnDiskConstantSize,
                     cp.length);
    pgstat_report_wait_end();
    if (readBytes != cp.length)
    {
        if (readBytes < 0)
            ereport(PANIC,
                    (errcode_for_file_access(),
                     errmsg("could not read file \"%s\": %m", path)));
        else
            ereport(PANIC,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg("could not read file \"%s\": read %d of %zu",
                            path, readBytes, (Size) cp.length)));
    }
 
    if (CloseTransientFile(fd) != 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not close file \"%s\": %m", path)));
 
    /* now verify the CRC */
    INIT_CRC32C(checksum);
    COMP_CRC32C(checksum,
                (char *) &cp + ReplicationSlotOnDiskNotChecksummedSize,
                ReplicationSlotOnDiskChecksummedSize);
    FIN_CRC32C(checksum);
 
    if (!EQ_CRC32C(checksum, cp.checksum))
        ereport(PANIC,
                (errmsg("checksum mismatch for replication slot file \"%s\": is %u, should be %u",
                        path, checksum, cp.checksum)));
 
    /*
     * If we crashed with an ephemeral slot active, don't restore but delete
     * it.
     */
    if (cp.slotdata.persistency != RS_PERSISTENT)
    {
        if (!rmtree(slotdir, true))
        {
            ereport(WARNING,
                    (errmsg("could not remove directory \"%s\"",
                            slotdir)));
        }
        fsync_fname(PG_REPLSLOT_DIR, true);
        return;
    }
 
    /*
     * Verify that requirements for the specific slot type are met. That's
     * important because if these aren't met we're not guaranteed to retain
     * all the necessary resources for the slot.
     *
     * NB: We have to do so *after* the above checks for ephemeral slots,
     * because otherwise a slot that shouldn't exist anymore could prevent
     * restarts.
     *
     * NB: Changing the requirements here also requires adapting
     * CheckSlotRequirements() and CheckLogicalDecodingRequirements().
     */
    if (cp.slotdata.database != InvalidOid)
    {
        if (wal_level < WAL_LEVEL_REPLICA)
            ereport(FATAL,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("logical replication slot \"%s\" exists, but \"wal_level\" < \"replica\"",
                            NameStr(cp.slotdata.name)),
                     errhint("Change \"wal_level\" to be \"replica\" or higher.")));
 
        /*
         * In standby mode, the hot standby must be enabled. This check is
         * necessary to ensure logical slots are invalidated when they become
         * incompatible due to insufficient wal_level. Otherwise, if the
         * primary reduces effective_wal_level < logical while hot standby is
         * disabled, primary disable logical decoding while hot standby is
         * disabled, logical slots would remain valid even after promotion.
         */
        if (StandbyMode && !EnableHotStandby)
            ereport(FATAL,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("logical replication slot \"%s\" exists on the standby, but \"hot_standby\" = \"off\"",
                            NameStr(cp.slotdata.name)),
                     errhint("Change \"hot_standby\" to be \"on\".")));
    }
    else if (wal_level < WAL_LEVEL_REPLICA)
        ereport(FATAL,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("physical replication slot \"%s\" exists, but \"wal_level\" < \"replica\"",
                        NameStr(cp.slotdata.name)),
                 errhint("Change \"wal_level\" to be \"replica\" or higher.")));
 
    /*
     * Nothing can be active yet, don't lock anything.  Note we iterate up to
     * max_replication_slots instead of adding max_repack_replication_slots as
     * in all other places, because we must enforce the GUC value in case
     * there were more slots before the shutdown than what it is set up to
     * now.
     */
    for (i = 0; i < max_replication_slots; i++)
    {
        ReplicationSlot *slot;
 
        slot = &ReplicationSlotCtl->replication_slots[i];
 
        if (slot->in_use)
            continue;
 
        /* restore the entire set of persistent data */
        memcpy(&slot->data, &cp.slotdata,
               sizeof(ReplicationSlotPersistentData));
 
        /* initialize in memory state */
        slot->effective_xmin = cp.slotdata.xmin;
        slot->effective_catalog_xmin = cp.slotdata.catalog_xmin;
        slot->last_saved_confirmed_flush = cp.slotdata.confirmed_flush;
        slot->last_saved_restart_lsn = cp.slotdata.restart_lsn;
 
        slot->candidate_catalog_xmin = InvalidTransactionId;
        slot->candidate_xmin_lsn = InvalidXLogRecPtr;
        slot->candidate_restart_lsn = InvalidXLogRecPtr;
        slot->candidate_restart_valid = InvalidXLogRecPtr;
 
        slot->in_use = true;
        slot->active_proc = INVALID_PROC_NUMBER;
 
        /*
         * Set the time since the slot has become inactive after loading the
         * slot from the disk into memory. Whoever acquires the slot i.e.
         * makes the slot active will reset it. Use the same inactive_since
         * time for all the slots.
         */
        if (now == 0)
            now = GetCurrentTimestamp();
 
        ReplicationSlotSetInactiveSince(slot, now, false);
 
        restored = true;
        break;
    }
 
    if (!restored)
        ereport(FATAL,
                (errmsg("too many replication slots active before shutdown"),
                 errhint("Increase \"max_replication_slots\" and try again.")));
}
 
/*
 * Maps an invalidation reason for a replication slot to
 * ReplicationSlotInvalidationCause.
 */
ReplicationSlotInvalidationCause
GetSlotInvalidationCause(const char *cause_name)
{
    Assert(cause_name);
 
    /* Search lookup table for the cause having this name */
    for (int i = 0; i <= RS_INVAL_MAX_CAUSES; i++)
    {
        if (strcmp(SlotInvalidationCauses[i].cause_name, cause_name) == 0)
            return SlotInvalidationCauses[i].cause;
    }
 
    Assert(false);
    return RS_INVAL_NONE;       /* to keep compiler quiet */
}
 
/*
 * Maps a ReplicationSlotInvalidationCause to the invalidation
 * reason for a replication slot.
 */
const char *
GetSlotInvalidationCauseName(ReplicationSlotInvalidationCause cause)
{
    /* Search lookup table for the name of this cause */
    for (int i = 0; i <= RS_INVAL_MAX_CAUSES; i++)
    {
        if (SlotInvalidationCauses[i].cause == cause)
            return SlotInvalidationCauses[i].cause_name;
    }
 
    Assert(false);
    return "none";              /* to keep compiler quiet */
}
 
/*
 * A helper function to validate slots specified in GUC synchronized_standby_slots.
 *
 * The rawname will be parsed, and the result will be saved into *elemlist.
 */
static bool
validate_sync_standby_slots(char *rawname, List **elemlist)
{
    /* Verify syntax and parse string into a list of identifiers */
    if (!SplitIdentifierString(rawname, ',', elemlist))
    {
        GUC_check_errdetail("List syntax is invalid.");
        return false;
    }
 
    /* Iterate the list to validate each slot name */
    foreach_ptr(char, name, *elemlist)
    {
        int         err_code;
        char       *err_msg = NULL;
        char       *err_hint = NULL;
 
        if (!ReplicationSlotValidateNameInternal(name, false, &err_code,
                                                 &err_msg, &err_hint))
        {
            GUC_check_errcode(err_code);
            GUC_check_errdetail("%s", err_msg);
            if (err_hint != NULL)
                GUC_check_errhint("%s", err_hint);
            return false;
        }
    }
 
    return true;
}
 
/*
 * GUC check_hook for synchronized_standby_slots
 */
bool
check_synchronized_standby_slots(char **newval, void **extra, GucSource source)
{
    char       *rawname;
    char       *ptr;
    List       *elemlist;
    int         size;
    bool        ok;
    SyncStandbySlotsConfigData *config;
 
    if ((*newval)[0] == '\0')
        return true;
 
    /* Need a modifiable copy of the GUC string */
    rawname = pstrdup(*newval);
 
    /* Now verify if the specified slots exist and have correct type */
    ok = validate_sync_standby_slots(rawname, &elemlist);
 
    if (!ok || elemlist == NIL)
    {
        pfree(rawname);
        list_free(elemlist);
        return ok;
    }
 
    /* Compute the size required for the SyncStandbySlotsConfigData struct */
    size = offsetof(SyncStandbySlotsConfigData, slot_names);
    foreach_ptr(char, slot_name, elemlist)
        size += strlen(slot_name) + 1;
 
    /* GUC extra value must be guc_malloc'd, not palloc'd */
    config = (SyncStandbySlotsConfigData *) guc_malloc(LOG, size);
    if (!config)
        return false;
 
    /* Transform the data into SyncStandbySlotsConfigData */
    config->nslotnames = list_length(elemlist);
 
    ptr = config->slot_names;
    foreach_ptr(char, slot_name, elemlist)
    {
        strcpy(ptr, slot_name);
        ptr += strlen(slot_name) + 1;
    }
 
    *extra = config;
 
    pfree(rawname);
    list_free(elemlist);
    return true;
}
 
/*
 * GUC assign_hook for synchronized_standby_slots
 */
void
assign_synchronized_standby_slots(const char *newval, void *extra)
{
    /*
     * The standby slots may have changed, so we must recompute the oldest
     * LSN.
     */
    ss_oldest_flush_lsn = InvalidXLogRecPtr;
 
    synchronized_standby_slots_config = (SyncStandbySlotsConfigData *) extra;
}
 
/*
 * Check if the passed slot_name is specified in the synchronized_standby_slots GUC.
 */
bool
SlotExistsInSyncStandbySlots(const char *slot_name)
{
    const char *standby_slot_name;
 
    /* Return false if there is no value in synchronized_standby_slots */
    if (synchronized_standby_slots_config == NULL)
        return false;
 
    /*
     * XXX: We are not expecting this list to be long so a linear search
     * shouldn't hurt but if that turns out not to be true then we can cache
     * this information for each WalSender as well.
     */
    standby_slot_name = synchronized_standby_slots_config->slot_names;
    for (int i = 0; i < synchronized_standby_slots_config->nslotnames; i++)
    {
        if (strcmp(standby_slot_name, slot_name) == 0)
            return true;
 
        standby_slot_name += strlen(standby_slot_name) + 1;
    }
 
    return false;
}
 
/*
 * Return true if the slots specified in synchronized_standby_slots have caught up to
 * the given WAL location, false otherwise.
 *
 * The elevel parameter specifies the error level used for logging messages
 * related to slots that do not exist, are invalidated, or are inactive.
 */
bool
StandbySlotsHaveCaughtup(XLogRecPtr wait_for_lsn, int elevel)
{
    const char *name;
    int         caught_up_slot_num = 0;
    XLogRecPtr  min_restart_lsn = InvalidXLogRecPtr;
 
    /*
     * Don't need to wait for the standbys to catch up if there is no value in
     * synchronized_standby_slots.
     */
    if (synchronized_standby_slots_config == NULL)
        return true;
 
    /*
     * Don't need to wait for the standbys to catch up if we are on a standby
     * server, since we do not support syncing slots to cascading standbys.
     */
    if (RecoveryInProgress())
        return true;
 
    /*
     * Don't need to wait for the standbys to catch up if they are already
     * beyond the specified WAL location.
     */
    if (XLogRecPtrIsValid(ss_oldest_flush_lsn) &&
        ss_oldest_flush_lsn >= wait_for_lsn)
        return true;
 
    /*
     * To prevent concurrent slot dropping and creation while filtering the
     * slots, take the ReplicationSlotControlLock outside of the loop.
     */
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
    name = synchronized_standby_slots_config->slot_names;
    for (int i = 0; i < synchronized_standby_slots_config->nslotnames; i++)
    {
        XLogRecPtr  restart_lsn;
        bool        invalidated;
        bool        inactive;
        ReplicationSlot *slot;
 
        slot = SearchNamedReplicationSlot(name, false);
 
        /*
         * If a slot name provided in synchronized_standby_slots does not
         * exist, report a message and exit the loop.
         */
        if (!slot)
        {
            ereport(elevel,
                    errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                    errmsg("replication slot \"%s\" specified in parameter \"%s\" does not exist",
                           name, "synchronized_standby_slots"),
                    errdetail("Logical replication is waiting on the standby associated with replication slot \"%s\".",
                              name),
                    errhint("Create the replication slot \"%s\" or amend parameter \"%s\".",
                            name, "synchronized_standby_slots"));
            break;
        }
 
        /* Same as above: if a slot is not physical, exit the loop. */
        if (SlotIsLogical(slot))
        {
            ereport(elevel,
                    errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                    errmsg("cannot specify logical replication slot \"%s\" in parameter \"%s\"",
                           name, "synchronized_standby_slots"),
                    errdetail("Logical replication is waiting for correction on replication slot \"%s\".",
                              name),
                    errhint("Remove the logical replication slot \"%s\" from parameter \"%s\".",
                            name, "synchronized_standby_slots"));
            break;
        }
 
        SpinLockAcquire(&slot->mutex);
        restart_lsn = slot->data.restart_lsn;
        invalidated = slot->data.invalidated != RS_INVAL_NONE;
        inactive = slot->active_proc == INVALID_PROC_NUMBER;
        SpinLockRelease(&slot->mutex);
 
        if (invalidated)
        {
            /* Specified physical slot has been invalidated */
            ereport(elevel,
                    errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                    errmsg("physical replication slot \"%s\" specified in parameter \"%s\" has been invalidated",
                           name, "synchronized_standby_slots"),
                    errdetail("Logical replication is waiting on the standby associated with replication slot \"%s\".",
                              name),
                    errhint("Drop and recreate the replication slot \"%s\", or amend parameter \"%s\".",
                            name, "synchronized_standby_slots"));
            break;
        }
 
        if (!XLogRecPtrIsValid(restart_lsn) || restart_lsn < wait_for_lsn)
        {
            /* Log a message if no active_pid for this physical slot */
            if (inactive)
                ereport(elevel,
                        errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                        errmsg("replication slot \"%s\" specified in parameter \"%s\" does not have active_pid",
                               name, "synchronized_standby_slots"),
                        errdetail("Logical replication is waiting on the standby associated with replication slot \"%s\".",
                                  name),
                        errhint("Start the standby associated with the replication slot \"%s\", or amend parameter \"%s\".",
                                name, "synchronized_standby_slots"));
 
            /* Continue if the current slot hasn't caught up. */
            break;
        }
 
        Assert(restart_lsn >= wait_for_lsn);
 
        if (!XLogRecPtrIsValid(min_restart_lsn) ||
            min_restart_lsn > restart_lsn)
            min_restart_lsn = restart_lsn;
 
        caught_up_slot_num++;
 
        name += strlen(name) + 1;
    }
 
    LWLockRelease(ReplicationSlotControlLock);
 
    /*
     * Return false if not all the standbys have caught up to the specified
     * WAL location.
     */
    if (caught_up_slot_num != synchronized_standby_slots_config->nslotnames)
        return false;
 
    /* The ss_oldest_flush_lsn must not retreat. */
    Assert(!XLogRecPtrIsValid(ss_oldest_flush_lsn) ||
           min_restart_lsn >= ss_oldest_flush_lsn);
 
    ss_oldest_flush_lsn = min_restart_lsn;
 
    return true;
}
 
/*
 * Wait for physical standbys to confirm receiving the given lsn.
 *
 * Used by logical decoding SQL functions. It waits for physical standbys
 * corresponding to the physical slots specified in the synchronized_standby_slots GUC.
 */
void
WaitForStandbyConfirmation(XLogRecPtr wait_for_lsn)
{
    /*
     * Don't need to wait for the standby to catch up if the current acquired
     * slot is not a logical failover slot, or there is no value in
     * synchronized_standby_slots.
     */
    if (!MyReplicationSlot->data.failover || !synchronized_standby_slots_config)
        return;
 
    ConditionVariablePrepareToSleep(&WalSndCtl->wal_confirm_rcv_cv);
 
    for (;;)
    {
        CHECK_FOR_INTERRUPTS();
 
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
        }
 
        /* Exit if done waiting for every slot. */
        if (StandbySlotsHaveCaughtup(wait_for_lsn, WARNING))
            break;
 
        /*
         * Wait for the slots in the synchronized_standby_slots to catch up,
         * but use a timeout (1s) so we can also check if the
         * synchronized_standby_slots has been changed.
         */
        ConditionVariableTimedSleep(&WalSndCtl->wal_confirm_rcv_cv, 1000,
                                    WAIT_EVENT_WAIT_FOR_STANDBY_CONFIRMATION);
    }
 
    ConditionVariableCancelSleep();
}