Re: Adding REPACK [concurrently]

Hello!

Antonin Houska <ah(at)cybertec(dot)at>:
> I'll apply it to the next version of the "Add CONCURRENTLY option to REPACK
> command" patch.
I have added it to the v21 patchset.

Also, I’ve updated the MVCC-safe patch:
* it uses the "XactLockTableWait before replay + SnapshotSelf" approach from [0]
* it includes a TAP test to ensure MVCC safety - not intended to be
committed in its current form (too heavy)
* documentation has been updated.

It's now much simpler and does not negatively impact performance. It
is less aggressive in tuple freezing, but can be updated to match the
non-MVCC-safe version if needed.

While testing MVCC-safe version with stress-tests
007_repack_concurrently_mvcc.pl I encountered some random crashes with
such logs:

25-09-02 12:24:40.039 CEST client backend[261907]
007_repack_concurrently_mvcc.pl ERROR: relcache reference
0x7715b9f394a8 is not owned by resource owner TopTransaction
2025-09-02 12:24:40.039 CEST client backend[261907]
007_repack_concurrently_mvcc.pl STATEMENT: REPACK (CONCURRENTLY) tbl1
USING INDEX tbl1_pkey;
TRAP: failed Assert("rel->rd_refcnt > 0"), File:
"../src/backend/utils/cache/relcache.c", Line: 6992, PID: 261907
postgres: CIC_test: nkey postgres [local]
REPACK(ExceptionalCondition+0xbe)[0x5b7ac41d79f9]
postgres: CIC_test: nkey postgres [local] REPACK(+0x852d2e)[0x5b7ac41cbd2e]
postgres: CIC_test: nkey postgres [local] REPACK(+0x8aa4a6)[0x5b7ac42234a6]
postgres: CIC_test: nkey postgres [local] REPACK(+0x8aad3b)[0x5b7ac4223d3b]
postgres: CIC_test: nkey postgres [local] REPACK(+0x8aac69)[0x5b7ac4223c69]
postgres: CIC_test: nkey postgres [local]
REPACK(ResourceOwnerRelease+0x32)[0x5b7ac4223c26]
postgres: CIC_test: nkey postgres [local] REPACK(+0x1f43bf)[0x5b7ac3b6d3bf]
postgres: CIC_test: nkey postgres [local] REPACK(+0x1f4dfa)[0x5b7ac3b6ddfa]
postgres: CIC_test: nkey postgres [local]
REPACK(AbortCurrentTransaction+0xe)[0x5b7ac3b6dd6b]
postgres: CIC_test: nkey postgres [local]
REPACK(PostgresMain+0x57d)[0x5b7ac3fd7238]
postgres: CIC_test: nkey postgres [local] REPACK(+0x654102)[0x5b7ac3fcd102]
postgres: CIC_test: nkey postgres [local]
REPACK(postmaster_child_launch+0x191)[0x5b7ac3eceb7a]
postgres: CIC_test: nkey postgres [local] REPACK(+0x55c8c1)[0x5b7ac3ed58c1]
postgres: CIC_test: nkey postgres [local] REPACK(+0x559d1e)[0x5b7ac3ed2d1e]
postgres: CIC_test: nkey postgres [local]
REPACK(PostmasterMain+0x168a)[0x5b7ac3ed25f8]
postgres: CIC_test: nkey postgres [local] REPACK(main+0x3a1)[0x5b7ac3da2bd6]
/lib/x86_64-linux-gnu/libc.so.6(+0x2a1ca)[0x7715b962a1ca]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x8b)[0x7715b962a28b]

This time I was clever and tried to attempt to reproduce the issue on
a non-MVCC safe version at first - and it is reproducible.

Just comment \if :p_t1 != :p_t2 (and its internals, because they
catching non-mvcc behaviour which is expected without 0006 patch); and
set
'--no-vacuum --client=30 --jobs=4 --exit-on-abort --transactions=25000'

It takes about a minute on my PC to get the crash.

[0]: https://www.postgresql.org/message-id/flat/CADzfLwXCTXNdxK-XGTKmObvT%3D_QnaCviwgrcGtG9chsj5sYzrg%40mail.gmail.com#6d6ccf34e6debcd386d735db593379d8

Best regards,
Mikhail.