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Andres Freund 955a684e04 Fix race condition leading to hanging logical slot creation. ... The snapshot assembly during the creation of logical slots relied waiting for transactions in xl_running_xacts to end, by checking for their commit/abort records. Unfortunately, despite locking, it is possible to see an xl_running_xact record listing transactions as ready, that have already WAL-logged an commit/abort record, as the locking just prevents the ProcArray to be adjusted, and the commit record has to be logged first. That lead to either delayed or hanging snapshot creation, because snapbuild.c would wait "forever" to see commit/abort records for some transactions. That hang resolved only if a xl_running_xacts record without any running transactions happened to be logged, far from certain on a busy server. It's impractical to prevent that via more heavyweight locking, the likelihood of deadlocks and significantly increased contention would be too big. Instead change the initial snapshot creation to be solely based on tracking the oldest running transaction via xl_running_xacts->oldestRunningXid - that actually ends up significantly simplifying the code. That has two disadvantages: 1) Because we cannot fully "trust" the contents of xl_running_xacts, we cannot use it to build the initial snapshot. Instead we have to wait twice for all running transactions to finish. 2) Previously a slot, unless the race occurred, could be created when the all transaction perceived as running based on commit/abort records, now we have to wait for the next xl_running_xacts record. To address that, trigger logging new xl_running_xacts record from within snapbuild.c exactly when necessary. Unfortunately snabuild.c's SnapBuild is stored on disk, one of the stupider ideas of a certain Mr Freund, so we can't change it in a minor release. As this is going to be backpatched, we have to hack around a bit to keep on-disk compatibility. A later commit will rejigger that on master. Author: Andres Freund, based on a quite different patch from Petr Jelinek Analyzed-By: Petr Jelinek Reviewed-By: Petr Jelinek Discussion: https://postgr.es/m/f37e975c-908f-858e-707f-058d3b1eb214@2ndquadrant.com Backpatch: 9.4-, where logical decoding has been introduced		9 years ago
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binary.out	Message style improvements	10 years ago
concurrent_ddl_dml.out	…
ddl.out	Rename WAL-related functions and views to use "lsn" not "location".	9 years ago
decoding_in_xact.out	Collection of typo fixes.	11 years ago
decoding_into_rel.out	…
delayed_startup.out	…
messages.out	Add required database and origin filtering for logical messages.	10 years ago
mxact.out	…
ondisk_startup.out	Fix race condition leading to hanging logical slot creation.	9 years ago
permissions.out	Establish conventions about global object names used in regression tests.	10 years ago
prepared.out	…
replorigin.out	Add required database and origin filtering for logical messages.	10 years ago
rewrite.out	…
slot.out	Force synchronous commit in new-ish test_decoding test.	9 years ago
spill.out	Make contrib/test_decoding regression tests safe for CZ locale.	9 years ago
time.out	logical decoding: fix decoding of a commit's commit time.	10 years ago
toast.out	logical decoding: Fix handling of large old tuples with replica identity full.	10 years ago
xact.out	Don't propagate a null subtransaction snapshot up to parent transaction.	10 years ago