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A few places that access system catalogs don't set up an active snapshot before potentially accessing their TOAST tables. To fix, push an active snapshot just before each section of code that might require accessing one of these TOAST tables, and pop it shortly afterwards. While at it, this commit adds some rather strict assertions in an attempt to prevent such issues in the future. Commit16bf24e0e4recently removed pg_replication_origin's TOAST table in order to fix the same problem for that catalog. On the back-branches, those bugs are left in place. We cannot easily remove a catalog's TOAST table on released major versions, and only replication origins with extremely long names are affected. Given the low severity of the issue, fixing older versions doesn't seem worth the trouble of significantly modifying the patch. Also, on v13 and v14, the aforementioned strict assertions have been omitted because commit2776922201, which added HaveRegisteredOrActiveSnapshot(), was not back-patched. While we could probably back-patch it now, I've opted against it because it seems unlikely that new TOAST snapshot issues will be introduced in the oldest supported versions. Reported-by: Alexander Lakhin <exclusion@gmail.com> Reviewed-by: Michael Paquier <michael@paquier.xyz> Discussion: https://postgr.es/m/18127-fe54b6a667f29658%40postgresql.org Discussion: https://postgr.es/m/18309-c0bf914950c46692%40postgresql.org Discussion: https://postgr.es/m/ZvMSUPOqUU-VNADN%40nathan 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.