mirror of
https://github.com/postgres/postgres.git
synced 2025-11-06 07:49:08 +03:00
91771b3fbb
Previously LogicalIncreaseRestartDecodingForSlot() accidentally accepted any LSN as the candidate_lsn and candidate_valid after the restart_lsn of the replication slot was updated, so it potentially caused the restart_lsn to move backwards. A scenario where this could happen in logical replication is: after a logical replication restart, based on previous candidate_lsn and candidate_valid values in memory, the restart_lsn advances upon receiving a subscriber acknowledgment. Then, logical decoding restarts from an older point, setting candidate_lsn and candidate_valid based on an old RUNNING_XACTS record. Subsequent subscriber acknowledgments then update the restart_lsn to an LSN older than the current value. In the reported case, after WAL files were removed by a checkpoint, the retreated restart_lsn prevented logical replication from restarting due to missing WAL segments. This change essentially modifies the 'if' condition to 'else if' condition within the function. The previous code had an asymmetry in this regard compared to LogicalIncreaseXminForSlot(), which does almost the same thing for different fields. The WAL removal issue was reported by Hubert Depesz Lubaczewski. Backpatch to all supported versions, since the bug exists since 9.4 where logical decoding was introduced. Reviewed-by: Tomas Vondra, Ashutosh Bapat, Amit Kapila Discussion: https://postgr.es/m/Yz2hivgyjS1RfMKs%40depesz.com Discussion: https://postgr.es/m/85fff40e-148b-4e86-b921-b4b846289132%40vondra.me Backpatch-through: 13
src/backend/replication/README Walreceiver - libpqwalreceiver API ---------------------------------- The transport-specific part of walreceiver, responsible for connecting to the primary server, receiving WAL files and sending messages, is loaded dynamically to avoid having to link the main server binary with libpq. The dynamically loaded module is in libpqwalreceiver subdirectory. The dynamically loaded module implements a set of functions with details about each one of them provided in src/include/replication/walreceiver.h. This API should be considered internal at the moment, but we could open it up for 3rd party replacements of libpqwalreceiver in the future, allowing pluggable methods for receiving WAL. Walreceiver IPC --------------- When the WAL replay in startup process has reached the end of archived WAL, restorable using restore_command, it starts up the walreceiver process to fetch more WAL (if streaming replication is configured). Walreceiver is a postmaster subprocess, so the startup process can't fork it directly. Instead, it sends a signal to postmaster, asking postmaster to launch it. Before that, however, startup process fills in WalRcvData->conninfo and WalRcvData->slotname, and initializes the starting point in WalRcvData->receiveStart. As walreceiver receives WAL from the primary server, and writes and flushes it to disk (in pg_wal), it updates WalRcvData->flushedUpto and signals the startup process to know how far WAL replay can advance. Walreceiver sends information about replication progress to the primary server whenever it either writes or flushes new WAL, or the specified interval elapses. This is used for reporting purpose. Walsender IPC ------------- At shutdown, postmaster handles walsender processes differently from regular backends. It waits for regular backends to die before writing the shutdown checkpoint and terminating pgarch and other auxiliary processes, but that's not desirable for walsenders, because we want the standby servers to receive all the WAL, including the shutdown checkpoint, before the primary is shut down. Therefore postmaster treats walsenders like the pgarch process, and instructs them to terminate at PM_SHUTDOWN_2 phase, after all regular backends have died and checkpointer has issued the shutdown checkpoint. When postmaster accepts a connection, it immediately forks a new process to handle the handshake and authentication, and the process initializes to become a backend. Postmaster doesn't know if the process becomes a regular backend or a walsender process at that time - that's indicated in the connection handshake - so we need some extra signaling to let postmaster identify walsender processes. When walsender process starts up, it marks itself as a walsender process in the PMSignal array. That way postmaster can tell it apart from regular backends. Note that no big harm is done if postmaster thinks that a walsender is a regular backend; it will just terminate the walsender earlier in the shutdown phase. A walsender will look like a regular backend until it's done with the initialization and has marked itself in PMSignal array, and at process termination, after unmarking the PMSignal slot. Each walsender allocates an entry from the WalSndCtl array, and tracks information about replication progress. User can monitor them via statistics views. Walsender - walreceiver protocol -------------------------------- See manual.