2 Commits (02e95a5049f7933cbde1dacf401604ea3fc02aa5)

Author	SHA1	Message	Date
Alvaro Herrera	8d1c1ca70b	Reduce cost of test_decoding's new oldest_xmin test ... Change a whole-database VACUUM into doing just pg_attribute, which is the portion that verifies what we want it to do. The original formulation wastes a lot of CPU time, which leads the test to fail when runtime exceeds isolationtester timeout when it's super-slow, such as under CLOBBER_CACHE_ALWAYS. Per buildfarm member friarbird. It turns out that the previous shape of the test doesn't always detect the condition it is supposed to detect (on unpatched reorderbuffer code): the reason is that there is a good chance of encountering a xl_running_xacts record (logged every 15 seconds) before the checkpoint -- and because we advance the xmin when we receive that WAL record, and we *don't* advance the xmin twice consecutively without receiving a client message in between, that means the xmin is not advanced enough for the tuple to be pruned from pg_attribute by VACUUM. So the test would spuriously pass. The reason this test deficiency wasn't detected earlier is that HOT pruning removes the tuple anyway, even if vacuum leaves it in place, so the test correctly fails (detecting the coding mistake), but for the wrong reason. To fix this mess, run the s0_get_changes step twice before vacuum instead of once: this seems to cause the xmin to be advanced reliably, wreaking havoc with more certainty. Author: Arseny Sher Discussion: https://postgr.es/m/87h8lkuxoa.fsf@ars-thinkpad	8 years ago
Alvaro Herrera	f49a80c481	Fix "base" snapshot handling in logical decoding ... Two closely related bugs are fixed. First, xmin of logical slots was advanced too early. During xl_running_xacts processing, xmin of the slot was set to the oldest running xid in the record, but that's wrong: actually, snapshots which will be used for not-yet-replayed transactions might consider older txns as running too, so we need to keep xmin back for them. The problem wasn't noticed earlier because DDL which allows to delete tuple (set xmax) while some another not-yet-committed transaction looks at it is pretty rare, if not unique: e.g. all forms of ALTER TABLE which change schema acquire ACCESS EXCLUSIVE lock conflicting with any inserts. The included test case (test_decoding's oldest_xmin) uses ALTER of a composite type, which doesn't have such interlocking. To deal with this, we must be able to quickly retrieve oldest xmin (oldest running xid among all assigned snapshots) from ReorderBuffer. To fix, add another list of ReorderBufferTXNs to the reorderbuffer, where transactions are sorted by base-snapshot-LSN. This is slightly different from the existing (sorted by first-LSN) list, because a transaction can have an earlier LSN but a later Xmin, if its first record does not obtain an xmin (eg. xl_xact_assignment). Note this new list doesn't fully replace the existing txn list: we still need that one to prevent WAL recycling. The second issue concerns SnapBuilder snapshots and subtransactions. SnapBuildDistributeNewCatalogSnapshot never assigned a snapshot to a transaction that is known to be a subtxn, which is good in the common case that the top-level transaction already has one (no point in doing so), but a bug otherwise. To fix, arrange to transfer the snapshot from the subtxn to its top-level txn as soon as the kinship gets known. test_decoding's snapshot_transfer verifies this. Also, fix a minor memory leak: refcount of toplevel's old base snapshot was not decremented when the snapshot is transferred from child. Liberally sprinkle code comments, and rewrite a few existing ones. This part is my (Álvaro's) contribution to this commit, as I had to write all those comments in order to understand the existing code and Arseny's patch. Reported-by: Arseny Sher <a.sher@postgrespro.ru> Diagnosed-by: Arseny Sher <a.sher@postgrespro.ru> Co-authored-by: Arseny Sher <a.sher@postgrespro.ru> Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org> Reviewed-by: Antonin Houska <ah@cybertec.at> Discussion: https://postgr.es/m/87lgdyz1wj.fsf@ars-thinkpad	8 years ago