logicalctl.c

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/*-------------------------------------------------------------------------
 * logicalctl.c
 *      Functionality to control logical decoding status online.
 *
 * This module enables dynamic control of logical decoding availability.
 * Logical decoding becomes active under two conditions: when the wal_level
 * parameter is set to 'logical', or when at least one valid logical replication
 * slot exists with wal_level set to 'replica'. The system disables logical
 * decoding when neither condition is met. Therefore, the dynamic control
 * of logical decoding availability is required only when wal_level is set
 * to 'replica'. Logical decoding is always enabled when wal_level='logical'
 * and always disabled when wal_level='minimal'.
 *
 * The core concept of dynamically enabling and disabling logical decoding
 * is to separately control two aspects: writing information required for
 * logical decoding to WAL records, and using logical decoding itself. During
 * activation, we first enable logical WAL writing while keeping logical
 * decoding disabled. This change is reflected in the read-only
 * effective_wal_level GUC parameter. Once we ensure that all processes have
 * updated to the latest effective_wal_level value, we then enable logical
 * decoding. Deactivation follows a similar careful, multi-step process
 * in reverse order.
 *
 * While activation occurs synchronously right after creating the first
 * logical slot, deactivation happens asynchronously through the checkpointer
 * process. This design avoids a race condition at the end of recovery; see
 * the comments in UpdateLogicalDecodingStatusEndOfRecovery() for details.
 * Asynchronous deactivation also avoids excessive toggling of the logical
 * decoding status in workloads that repeatedly create and drop a single
 * logical slot. On the other hand, this lazy approach can delay changes
 * to effective_wal_level and the disabling logical decoding, especially
 * when the checkpointer is busy with other tasks. We chose this lazy approach
 * in all deactivation paths to keep the implementation simple, even though
 * laziness is strictly required only for end-of-recovery cases. Future work
 * might address this limitation either by using a dedicated worker instead
 * of the checkpointer, or by implementing synchronous waiting during slot
 * drops if workloads are significantly affected by the lazy deactivation
 * of logical decoding.
 *
 * Standby servers use the primary server's effective_wal_level and logical
 * decoding status. Unlike normal activation and deactivation, these
 * are updated simultaneously without status change coordination, solely by
 * replaying XLOG_LOGICAL_DECODING_STATUS_CHANGE records. The local wal_level
 * setting has no effect during this time. Upon promotion, we update the
 * logical decoding status based on local conditions: the wal_level value and
 * the presence of logical slots.
 *
 * In the future, we could extend support to include automatic transitions
 * of effective_wal_level between 'minimal' and 'logical' WAL levels. However,
 * this enhancement would require additional coordination mechanisms and
 * careful implementation of operations such as terminating walsenders and
 * archiver processes while carefully considering the sequence of operations
 * to ensure system stability during these transitions.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/replication/logical/logicalctl.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/xloginsert.h"
#include "catalog/pg_control.h"
#include "miscadmin.h"
#include "replication/slot.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "storage/subsystems.h"
#include "utils/injection_point.h"
 
/*
 * Struct for controlling the logical decoding status.
 *
 * This struct is protected by LogicalDecodingControlLock.
 */
typedef struct LogicalDecodingCtlData
{
    /*
     * This is the authoritative value used by all processes to determine
     * whether to write additional information required by logical decoding to
     * WAL. Since this information could be checked frequently, each process
     * caches this value in XLogLogicalInfo for better performance.
     */
    bool        xlog_logical_info;
 
    /* True if logical decoding is available in the system */
    bool        logical_decoding_enabled;
 
    /* True if logical decoding might need to be disabled */
    bool        pending_disable;
} LogicalDecodingCtlData;
 
static LogicalDecodingCtlData *LogicalDecodingCtl = NULL;
 
static void LogicalDecodingCtlShmemRequest(void *arg);
 
const ShmemCallbacks LogicalDecodingCtlShmemCallbacks = {
    .request_fn = LogicalDecodingCtlShmemRequest,
};
 
/*
 * A process-local cache of LogicalDecodingCtl->xlog_logical_info. This is
 * initialized at process startup, and updated when processing the process
 * barrier signal in ProcessBarrierUpdateXLogLogicalInfo(). If the process
 * is in an XID-assigned transaction, the cache update is delayed until the
 * transaction ends. See the comments for XLogLogicalInfoUpdatePending for details.
 */
bool        XLogLogicalInfo = false;
 
/*
 * When receiving the PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO signal, if
 * an XID is assigned to the current transaction, the process sets this flag and
 * delays the XLogLogicalInfo update until the transaction ends. This ensures
 * that the XLogLogicalInfo value (typically accessed via XLogLogicalInfoActive)
 * remains consistent throughout the transaction.
 */
static bool XLogLogicalInfoUpdatePending = false;
 
static void update_xlog_logical_info(void);
static void abort_logical_decoding_activation(int code, Datum arg);
static void write_logical_decoding_status_update_record(bool status);
 
static void
LogicalDecodingCtlShmemRequest(void *arg)
{
    ShmemRequestStruct(.name = "Logical decoding control",
                       .size = sizeof(LogicalDecodingCtlData),
                       .ptr = (void **) &LogicalDecodingCtl,
        );
}
 
/*
 * Initialize the logical decoding status in shmem at server startup. This
 * must be called ONCE during postmaster or standalone-backend startup.
 */
void
StartupLogicalDecodingStatus(bool last_status)
{
    /* Logical decoding is always disabled when 'minimal' WAL level */
    if (wal_level == WAL_LEVEL_MINIMAL)
        return;
 
    /*
     * Set the initial logical decoding status based on the last status. If
     * logical decoding was enabled before the last shutdown, it remains
     * enabled as we might have set wal_level='logical' or have at least one
     * logical slot.
     */
    LogicalDecodingCtl->xlog_logical_info = last_status;
    LogicalDecodingCtl->logical_decoding_enabled = last_status;
}
 
/*
 * Update the XLogLogicalInfo cache.
 */
static inline void
update_xlog_logical_info(void)
{
    XLogLogicalInfo = IsXLogLogicalInfoEnabled();
}
 
/*
 * Initialize XLogLogicalInfo backend-private cache. This routine is called
 * during process initialization.
 */
void
InitializeProcessXLogLogicalInfo(void)
{
    update_xlog_logical_info();
}
 
/*
 * This routine is called when we are told to update XLogLogicalInfo
 * by a ProcSignalBarrier.
 */
bool
ProcessBarrierUpdateXLogLogicalInfo(void)
{
    if (GetTopTransactionIdIfAny() != InvalidTransactionId)
    {
        /* Delay updating XLogLogicalInfo until the transaction end */
        XLogLogicalInfoUpdatePending = true;
    }
    else
        update_xlog_logical_info();
 
    return true;
}
 
/*
 * Check the shared memory state and return true if logical decoding is
 * enabled on the system.
 */
bool
IsLogicalDecodingEnabled(void)
{
    bool        enabled;
 
    LWLockAcquire(LogicalDecodingControlLock, LW_SHARED);
    enabled = LogicalDecodingCtl->logical_decoding_enabled;
    LWLockRelease(LogicalDecodingControlLock);
 
    return enabled;
}
 
/*
 * Returns true if logical WAL logging is enabled based on the shared memory
 * status.
 */
bool
IsXLogLogicalInfoEnabled(void)
{
    bool        xlog_logical_info;
 
    LWLockAcquire(LogicalDecodingControlLock, LW_SHARED);
    xlog_logical_info = LogicalDecodingCtl->xlog_logical_info;
    LWLockRelease(LogicalDecodingControlLock);
 
    return xlog_logical_info;
}
 
/*
 * Reset the local cache at end of the transaction.
 */
void
AtEOXact_LogicalCtl(void)
{
    /* Update the local cache if there is a pending update */
    if (XLogLogicalInfoUpdatePending)
    {
        update_xlog_logical_info();
        XLogLogicalInfoUpdatePending = false;
    }
}
 
/*
 * Writes an XLOG_LOGICAL_DECODING_STATUS_CHANGE WAL record with the given
 * status.
 */
static void
write_logical_decoding_status_update_record(bool status)
{
    XLogRecPtr  recptr;
 
    XLogBeginInsert();
    XLogRegisterData(&status, sizeof(bool));
    recptr = XLogInsert(RM_XLOG_ID, XLOG_LOGICAL_DECODING_STATUS_CHANGE);
    XLogFlush(recptr);
}
 
/*
 * A PG_ENSURE_ERROR_CLEANUP callback for activating logical decoding, resetting
 * the shared flags to revert the logical decoding activation process.
 */
static void
abort_logical_decoding_activation(int code, Datum arg)
{
    Assert(MyReplicationSlot);
    Assert(!LogicalDecodingCtl->logical_decoding_enabled);
 
    elog(DEBUG1, "aborting logical decoding activation process");
 
    /*
     * Abort the change to xlog_logical_info. We don't need to check
     * CheckLogicalSlotExists() as we're still holding a logical slot.
     */
    LWLockAcquire(LogicalDecodingControlLock, LW_EXCLUSIVE);
    LogicalDecodingCtl->xlog_logical_info = false;
    LWLockRelease(LogicalDecodingControlLock);
 
    /*
     * Some processes might have already started logical info WAL logging, so
     * tell all running processes to update their caches. We don't need to
     * wait for all processes to disable xlog_logical_info locally as it's
     * always safe to write logical information to WAL records, even when not
     * strictly required.
     */
    EmitProcSignalBarrier(PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO);
}
 
/*
 * Enable logical decoding if disabled.
 *
 * If this function is called during recovery, it simply returns without
 * action since the logical decoding status change is not allowed during
 * this time. The logical decoding status depends on the status on the primary.
 * The caller should use CheckLogicalDecodingRequirements() before calling this
 * function to make sure that the logical decoding status can be modified.
 *
 * Note that there is no interlock between logical decoding activation
 * and slot creation. To ensure enabling logical decoding, the caller
 * needs to call this function after creating a logical slot before
 * initializing the logical decoding context.
 */
void
EnsureLogicalDecodingEnabled(void)
{
    Assert(MyReplicationSlot);
    Assert(wal_level >= WAL_LEVEL_REPLICA);
 
    /* Logical decoding is always enabled */
    if (wal_level >= WAL_LEVEL_LOGICAL)
        return;
 
    if (RecoveryInProgress())
    {
        /*
         * CheckLogicalDecodingRequirements() must have already errored out if
         * logical decoding is not enabled since we cannot enable the logical
         * decoding status during recovery.
         */
        Assert(IsLogicalDecodingEnabled());
        return;
    }
 
    /*
     * Ensure to abort the activation process in cases where there in an
     * interruption during the wait.
     */
    PG_ENSURE_ERROR_CLEANUP(abort_logical_decoding_activation, (Datum) 0);
    {
        EnableLogicalDecoding();
    }
    PG_END_ENSURE_ERROR_CLEANUP(abort_logical_decoding_activation, (Datum) 0);
}
 
/*
 * A workhorse function to enable logical decoding.
 */
void
EnableLogicalDecoding(void)
{
    bool        in_recovery;
 
    LWLockAcquire(LogicalDecodingControlLock, LW_EXCLUSIVE);
 
    /* Return if it is already enabled */
    if (LogicalDecodingCtl->logical_decoding_enabled)
    {
        LogicalDecodingCtl->pending_disable = false;
        LWLockRelease(LogicalDecodingControlLock);
        return;
    }
 
    /*
     * Set logical info WAL logging in shmem. All process starts after this
     * point will include the information required by logical decoding to WAL
     * records.
     */
    LogicalDecodingCtl->xlog_logical_info = true;
 
    LWLockRelease(LogicalDecodingControlLock);
 
    /*
     * Tell all running processes to reflect the xlog_logical_info update, and
     * wait. This ensures that all running processes have enabled logical
     * information WAL logging.
     */
    WaitForProcSignalBarrier(
                             EmitProcSignalBarrier(PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO));
 
    INJECTION_POINT("logical-decoding-activation", NULL);
 
    in_recovery = RecoveryInProgress();
 
    /*
     * There could be some transactions that might have started with the old
     * status, but we don't need to wait for these transactions to complete as
     * long as they have valid XIDs. These transactions will appear in the
     * xl_running_xacts record and therefore the snapshot builder will not try
     * to decode the transaction during the logical decoding initialization.
     *
     * There is a theoretical case where a transaction decides whether to
     * include logical-info to WAL records before getting an XID. In this
     * case, the transaction won't appear in xl_running_xacts.
     *
     * For operations that do not require an XID assignment, the process
     * starts including logical-info immediately upon receiving the signal
     * (barrier). If such an operation checks the effective_wal_level multiple
     * times within a single execution, the resulting WAL records might be
     * inconsistent (i.e., logical-info is included in some records but not in
     * others). However, this is harmless because logical decoding generally
     * ignores WAL records that are not associated with an assigned XID.
     *
     * One might think we need to wait for all running transactions, including
     * those without XIDs and read-only transactions, to finish before
     * enabling logical decoding. However, such a requirement would force the
     * slot creation to wait for a potentially very long time due to
     * long-running read queries, which is practically unacceptable.
     */
 
    START_CRIT_SECTION();
 
    /*
     * We enable logical decoding first, followed by writing the WAL record.
     * This sequence ensures logical decoding becomes available on the primary
     * first.
     */
    LWLockAcquire(LogicalDecodingControlLock, LW_EXCLUSIVE);
 
    LogicalDecodingCtl->logical_decoding_enabled = true;
 
    if (!in_recovery)
        write_logical_decoding_status_update_record(true);
 
    LogicalDecodingCtl->pending_disable = false;
 
    LWLockRelease(LogicalDecodingControlLock);
 
    END_CRIT_SECTION();
 
    if (!in_recovery)
        ereport(LOG,
                errmsg("logical decoding is enabled upon creating a new logical replication slot"));
}
 
/*
 * Initiate a request for disabling logical decoding.
 *
 * Note that this function does not verify whether logical slots exist. The
 * checkpointer will verify if logical decoding should actually be disabled.
 */
void
RequestDisableLogicalDecoding(void)
{
    if (wal_level != WAL_LEVEL_REPLICA)
        return;
 
    /*
     * It's possible that we might not actually need to disable logical
     * decoding if someone creates a new logical slot concurrently. We set the
     * flag anyway and the checkpointer will check it and disable logical
     * decoding if necessary.
     */
    LWLockAcquire(LogicalDecodingControlLock, LW_EXCLUSIVE);
    LogicalDecodingCtl->pending_disable = true;
    LWLockRelease(LogicalDecodingControlLock);
 
    WakeupCheckpointer();
 
    elog(DEBUG1, "requested disabling logical decoding");
}
 
/*
 * Disable logical decoding if necessary.
 *
 * This function disables logical decoding upon a request initiated by
 * RequestDisableLogicalDecoding(). Otherwise, it performs no action.
 */
void
DisableLogicalDecodingIfNecessary(void)
{
    bool        pending_disable;
 
    if (wal_level != WAL_LEVEL_REPLICA)
        return;
 
    /*
     * Sanity check as we cannot disable logical decoding while holding a
     * logical slot.
     */
    Assert(!MyReplicationSlot);
 
    if (RecoveryInProgress())
        return;
 
    LWLockAcquire(LogicalDecodingControlLock, LW_SHARED);
    pending_disable = LogicalDecodingCtl->pending_disable;
    LWLockRelease(LogicalDecodingControlLock);
 
    /* Quick return if no pending disable request */
    if (!pending_disable)
        return;
 
    DisableLogicalDecoding();
}
 
/*
 * A workhorse function to disable logical decoding.
 */
void
DisableLogicalDecoding(void)
{
    bool        in_recovery = RecoveryInProgress();
 
    LWLockAcquire(LogicalDecodingControlLock, LW_EXCLUSIVE);
 
    /*
     * Check if we can disable logical decoding.
     *
     * Skip CheckLogicalSlotExists() check during recovery because the
     * existing slots will be invalidated after disabling logical decoding.
     */
    if (!LogicalDecodingCtl->logical_decoding_enabled ||
        (!in_recovery && CheckLogicalSlotExists()))
    {
        LogicalDecodingCtl->pending_disable = false;
        LWLockRelease(LogicalDecodingControlLock);
        return;
    }
 
    START_CRIT_SECTION();
 
    /*
     * We need to disable logical decoding first and then disable logical
     * information WAL logging in order to ensure that no logical decoding
     * processes WAL records with insufficient information.
     */
    LogicalDecodingCtl->logical_decoding_enabled = false;
 
    /* Write the WAL to disable logical decoding on standbys too */
    if (!in_recovery)
        write_logical_decoding_status_update_record(false);
 
    /* Now disable logical information WAL logging */
    LogicalDecodingCtl->xlog_logical_info = false;
    LogicalDecodingCtl->pending_disable = false;
 
    END_CRIT_SECTION();
 
    if (!in_recovery)
        ereport(LOG,
                errmsg("logical decoding is disabled because there are no valid logical replication slots"));
 
    LWLockRelease(LogicalDecodingControlLock);
 
    /*
     * Tell all running processes to reflect the xlog_logical_info update.
     * Unlike when enabling logical decoding, we don't need to wait for all
     * processes to complete it in this case. We already disabled logical
     * decoding and it's always safe to write logical information to WAL
     * records, even when not strictly required. Therefore, we don't need to
     * wait for all running transactions to finish either.
     */
    EmitProcSignalBarrier(PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO);
}
 
/*
 * Updates the logical decoding status at end of recovery, and ensures that
 * all running processes have the updated XLogLogicalInfo status. This
 * function must be called before accepting writes.
 */
void
UpdateLogicalDecodingStatusEndOfRecovery(void)
{
    bool        new_status = false;
 
    Assert(RecoveryInProgress());
 
    /*
     * With 'minimal' WAL level, there are no logical replication slots during
     * recovery. Logical decoding is always disabled, so there is no need to
     * synchronize XLogLogicalInfo.
     */
    if (wal_level == WAL_LEVEL_MINIMAL)
    {
        Assert(!IsXLogLogicalInfoEnabled() && !IsLogicalDecodingEnabled());
        return;
    }
 
    LWLockAcquire(LogicalDecodingControlLock, LW_EXCLUSIVE);
 
    if (wal_level == WAL_LEVEL_LOGICAL || CheckLogicalSlotExists())
        new_status = true;
 
    /*
     * When recovery ends, we need to either enable or disable logical
     * decoding based on the wal_level setting and the presence of logical
     * slots. We need to note that concurrent slot creation and deletion could
     * happen but WAL writes are still not permitted until recovery fully
     * completes. Here's how we handle concurrent toggling of logical
     * decoding:
     *
     * For 'enable' case, if there's a concurrent disable request before
     * recovery fully completes, the checkpointer will handle it after
     * recovery is done. This means there might be a brief period after
     * recovery where logical decoding remains enabled even with no logical
     * replication slots present. This temporary state is not new - it can
     * already occur due to the checkpointer's asynchronous deactivation
     * process.
     *
     * For 'disable' case, backend cannot create logical replication slots
     * during recovery (see checks in CheckLogicalDecodingRequirements()),
     * which prevents a race condition between disabling logical decoding and
     * concurrent slot creation.
     */
    if (new_status != LogicalDecodingCtl->logical_decoding_enabled)
    {
        /*
         * Update both the logical decoding status and logical WAL logging
         * status. Unlike toggling these status during non-recovery, we don't
         * need to worry about the operation order as WAL writes are still not
         * permitted.
         */
        LogicalDecodingCtl->xlog_logical_info = new_status;
        LogicalDecodingCtl->logical_decoding_enabled = new_status;
 
        elog(DEBUG1,
             "update logical decoding status to %d at the end of recovery",
             new_status);
 
        /*
         * Now that we updated the logical decoding status, clear the pending
         * disable flag. It's possible that a concurrent process drops the
         * last logical slot and initiates the pending disable again. The
         * checkpointer process will check it.
         */
        LogicalDecodingCtl->pending_disable = false;
 
        LWLockRelease(LogicalDecodingControlLock);
 
        write_logical_decoding_status_update_record(new_status);
    }
    else
        LWLockRelease(LogicalDecodingControlLock);
 
    /*
     * Ensure all running processes have the updated status. We don't need to
     * wait for running transactions to finish as we don't accept any writes
     * yet. On the other hand, we need to wait for synchronizing
     * XLogLogicalInfo even if we've not updated the status above as the
     * status have been turned on and off during recovery, having running
     * processes have different status on their local caches.
     */
    if (IsUnderPostmaster)
        WaitForProcSignalBarrier(
                                 EmitProcSignalBarrier(PROCSIGNAL_BARRIER_UPDATE_XLOG_LOGICAL_INFO));
 
    INJECTION_POINT("startup-logical-decoding-status-change-end-of-recovery", NULL);
}