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WAL-log inplace update before revealing it to other sessions.

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A buffer lock won't stop a reader having already checked tuple
visibility.  If a vac_update_datfrozenid() and then a crash happened
during inplace update of a relfrozenxid value, datfrozenxid could
overtake relfrozenxid.  That could lead to "could not access status of
transaction" errors.  Back-patch to v12 (all supported versions).  In
v14 and earlier, this also back-patches the assertion removal from
commit 7fcf2faf9c.

Discussion: https://postgr.es/m/20240620012908.92.nmisch@google.com
This commit is contained in:
Noah Misch
2024-10-25 06:51:03 -07:00
parent 0ea9d40a66
commit 1e74e31f3c
3 changed files with 46 additions and 18 deletions
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4
src/backend/access/heap/README.tuplock
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@@ -203,6 +203,4 @@ Inplace updates create an exception to the rule that tuple data won't change
under a reader holding a pin. A reader of a heap_fetch() result tuple may
witness a torn read. Current inplace-updated fields are aligned and are no
wider than four bytes, and current readers don't need consistency across
fields. Hence, they get by with just fetching each field once. XXX such a
caller may also read a value that has not reached WAL; see
systable_inplace_update_finish().
fields. Hence, they get by with just fetching each field once.