2 Commits (11e264517dff7a911d9e6494de86049cab42cde3)

Author	SHA1	Message	Date
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
Andres Freund	ec5896aed3	Fix several weaknesses in slot and logical replication on-disk serialization. ... Heikki noticed in 544E23C0.8090605@vmware.com that slot.c and snapbuild.c were missing the FIN_CRC32 call when computing/checking checksums of on disk files. That doesn't lower the the error detection capabilities of the checksum, but is inconsistent with other usages. In a followup mail Heikki also noticed that, contrary to a comment, the 'version' and 'length' struct fields of replication slot's on disk data where not covered by the checksum. That's not likely to lead to actually missed corruption as those fields are cross checked with the expected version and the actual file length. But it's wrong nonetheless. As fixing these issues makes existing on disk files unreadable, bump the expected versions of on disk files for both slots and logical decoding historic catalog snapshots. This means that loading old files will fail with ERROR: "replication slot file ... has unsupported version 1" and ERROR: "snapbuild state file ... has unsupported version 1 instead of 2" respectively. Given the low likelihood of anybody already using these new features in a production setup that seems acceptable. Fixing these issues made me notice that there's no regression test covering the loading of historic snapshot from disk - so add one. Backpatch to 9.4 where these features were introduced.	11 years ago