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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
43 lines
1.8 KiB
Ruby
43 lines
1.8 KiB
Ruby
# Test snapshot transfer from subxact to top-level and receival of later snaps.
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setup
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{
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SELECT 'init' FROM pg_create_logical_replication_slot('isolation_slot', 'test_decoding'); -- must be first write in xact
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DROP TABLE IF EXISTS dummy;
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CREATE TABLE dummy(i int);
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DROP TABLE IF EXISTS harvest;
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CREATE TABLE harvest(apples int, pears int);
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}
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teardown
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{
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DROP TABLE IF EXISTS harvest;
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DROP TABLE IF EXISTS dummy;
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SELECT 'stop' FROM pg_drop_replication_slot('isolation_slot');
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}
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session "s0"
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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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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 "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_end_sub1" { RELEASE SAVEPOINT s1; }
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step "s0_insert2" { INSERT INTO harvest VALUES (1, 2, 3, 4); }
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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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session "s1"
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step "s1_produce_new_snap" { ALTER TABLE harvest ADD COLUMN mangos int; }
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# start top-level without base snap, get base snap in subxact, then create new
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# snap and make sure it is queued.
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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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# In previous test, we firstly associated subxact with xact and only then got
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# base snap; now nest one more subxact to get snap first and only then (at
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# commit) associate it with toplevel.
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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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