diff options
| -rw-r--r-- | src/backend/commands/vacuum.c | 17 |
1 files changed, 12 insertions, 5 deletions
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c index 15c36eca94f..6820b70229f 100644 --- a/src/backend/commands/vacuum.c +++ b/src/backend/commands/vacuum.c @@ -797,6 +797,12 @@ vac_truncate_clog(TransactionId frozenXID) /* * Scan pg_database to compute the minimum datfrozenxid * + * Since vac_update_datfrozenxid updates datfrozenxid in-place, + * the values could change while we look at them. Fetch each one just + * once to ensure sane behavior of the comparison logic. (Here, as in + * many other places, we assume that fetching or updating an XID in shared + * storage is atomic.) + * * Note: we need not worry about a race condition with new entries being * inserted by CREATE DATABASE. Any such entry will have a copy of some * existing DB's datfrozenxid, and that source DB cannot be ours because @@ -812,15 +818,16 @@ vac_truncate_clog(TransactionId frozenXID) while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL) { - Form_pg_database dbform = (Form_pg_database) GETSTRUCT(tuple); + volatile FormData_pg_database *dbform = (Form_pg_database) GETSTRUCT(tuple); + TransactionId datfrozenxid = dbform->datfrozenxid; - Assert(TransactionIdIsNormal(dbform->datfrozenxid)); + Assert(TransactionIdIsNormal(datfrozenxid)); - if (TransactionIdPrecedes(nextXID, dbform->datfrozenxid)) + if (TransactionIdPrecedes(nextXID, datfrozenxid)) frozenAlreadyWrapped = true; - else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID)) + else if (TransactionIdPrecedes(datfrozenxid, frozenXID)) { - frozenXID = dbform->datfrozenxid; + frozenXID = datfrozenxid; oldest_datoid = HeapTupleGetOid(tuple); } } |