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When the checkpointer writes the shutdown checkpoint, it checks afterwards whether any WAL has been written since it started and throws a PANIC if so. At that point, only walsenders are still active, so one might think this could not happen, but walsenders can also generate WAL, for instance in BASE_BACKUP and logical decoding related commands (e.g. via hint bits). So they can trigger this panic if such a command is run while the shutdown checkpoint is being written. To fix this, divide the walsender shutdown into two phases. First, checkpointer, itself triggered by postmaster, sends a PROCSIG_WALSND_INIT_STOPPING signal to all walsenders. If the backend is idle or runs an SQL query this causes the backend to shutdown, if logical replication is in progress all existing WAL records are processed followed by a shutdown. Otherwise this causes the walsender to switch to the "stopping" state. In this state, the walsender will reject any further replication commands. The checkpointer begins the shutdown checkpoint once all walsenders are confirmed as stopping. When the shutdown checkpoint finishes, the postmaster sends us SIGUSR2. This instructs walsender to send any outstanding WAL, including the shutdown checkpoint record, wait for it to be replicated to the standby, and then exit. Author: Andres Freund, based on an earlier patch by Michael Paquier Reported-By: Fujii Masao, Andres Freund Reviewed-By: Michael Paquier Discussion: https://postgr.es/m/20170602002912.tqlwn4gymzlxpvs2@alap3.anarazel.de Backpatch: 9.4, where logical decoding was introduced
68 lines
1.7 KiB
C
68 lines
1.7 KiB
C
/*-------------------------------------------------------------------------
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*
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* walsender.h
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* Exports from replication/walsender.c.
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*
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* Portions Copyright (c) 2010-2015, PostgreSQL Global Development Group
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*
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* src/include/replication/walsender.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef _WALSENDER_H
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#define _WALSENDER_H
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#include <signal.h>
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#include "fmgr.h"
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/* global state */
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extern bool am_walsender;
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extern bool am_cascading_walsender;
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extern bool am_db_walsender;
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extern bool wake_wal_senders;
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/* user-settable parameters */
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extern int max_wal_senders;
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extern int wal_sender_timeout;
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extern bool log_replication_commands;
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extern void InitWalSender(void);
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extern void exec_replication_command(const char *query_string);
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extern void WalSndErrorCleanup(void);
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extern void WalSndSignals(void);
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extern Size WalSndShmemSize(void);
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extern void WalSndShmemInit(void);
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extern void WalSndWakeup(void);
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extern void WalSndInitStopping(void);
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extern void WalSndWaitStopping(void);
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extern void HandleWalSndInitStopping(void);
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extern void WalSndRqstFileReload(void);
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extern Datum pg_stat_get_wal_senders(PG_FUNCTION_ARGS);
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/*
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* Remember that we want to wakeup walsenders later
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*
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* This is separated from doing the actual wakeup because the writeout is done
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* while holding contended locks.
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*/
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#define WalSndWakeupRequest() \
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do { wake_wal_senders = true; } while (0)
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/*
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* wakeup walsenders if there is work to be done
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*/
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#define WalSndWakeupProcessRequests() \
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do \
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{ \
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if (wake_wal_senders) \
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{ \
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wake_wal_senders = false; \
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if (max_wal_senders > 0) \
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WalSndWakeup(); \
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} \
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} while (0)
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#endif /* _WALSENDER_H */
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