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f49a80c481
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
50 lines
1.9 KiB
Plaintext
50 lines
1.9 KiB
Plaintext
Parsed test spec with 2 sessions
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starting permutation: s0_begin s0_begin_sub0 s0_log_assignment s0_sub_get_base_snap s1_produce_new_snap s0_insert s0_end_sub0 s0_commit s0_get_changes
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step s0_begin: BEGIN;
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step s0_begin_sub0: SAVEPOINT s0;
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step s0_log_assignment: SELECT txid_current() IS NULL;
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?column?
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f
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step s0_sub_get_base_snap: INSERT INTO dummy VALUES (0);
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step s1_produce_new_snap: ALTER TABLE harvest ADD COLUMN mangos int;
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step s0_insert: INSERT INTO harvest VALUES (1, 2, 3);
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step s0_end_sub0: RELEASE SAVEPOINT s0;
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step s0_commit: COMMIT;
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step s0_get_changes: SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
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data
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BEGIN
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table public.dummy: INSERT: i[integer]:0
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table public.harvest: INSERT: apples[integer]:1 pears[integer]:2 mangos[integer]:3
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COMMIT
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?column?
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stop
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starting permutation: s0_begin s0_begin_sub0 s0_log_assignment s0_begin_sub1 s0_sub_get_base_snap s1_produce_new_snap s0_insert s0_end_sub1 s0_end_sub0 s0_commit s0_get_changes
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step s0_begin: BEGIN;
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step s0_begin_sub0: SAVEPOINT s0;
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step s0_log_assignment: SELECT txid_current() IS NULL;
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?column?
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f
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step s0_begin_sub1: SAVEPOINT s1;
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step s0_sub_get_base_snap: INSERT INTO dummy VALUES (0);
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step s1_produce_new_snap: ALTER TABLE harvest ADD COLUMN mangos int;
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step s0_insert: INSERT INTO harvest VALUES (1, 2, 3);
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step s0_end_sub1: RELEASE SAVEPOINT s1;
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step s0_end_sub0: RELEASE SAVEPOINT s0;
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step s0_commit: COMMIT;
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step s0_get_changes: SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
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data
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BEGIN
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table public.dummy: INSERT: i[integer]:0
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table public.harvest: INSERT: apples[integer]:1 pears[integer]:2 mangos[integer]:3
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COMMIT
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?column?
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stop
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