Re: nbtree backwards scan test coverage

Hello Peter,

20.07.2026 02:23, Peter Geoghegan wrote:
> Attached patches add an isolation test to the nbtree test module,
> which uses injection points. This provides thorough test coverage for
> nbtree backwards scans, including cases where we must recover from
> concurrent page splits, and page deletions by VACUUM.

The new backwards-scan-concurrent-splits test has failed a couple of times
so far: [1], [2]:
# --- /mnt/build/HEAD/pgsql/src/test/modules/nbtree/expected/backwards-scan-concurrent-splits.out
# +++ /mnt/build/HEAD/pgsql.build/src/test/modules/nbtree/output_iso/results/backwards-scan-concurrent-splits.out
# @@ -208,7 +208,6 @@
#
#  (1 row)
#
# -scan_session: NOTICE:  notice triggered for injection point nbtree-walk-left-step-right
#  step b_scan: <... completed>
#  col
#  ---
not ok 1     - backwards-scan-concurrent-splits          310 ms
ok 2         - predicate-empty-index                     214 ms
1..2
# 1 of 2 tests failed.

Looks like it is not immune to autovacuum. I've reproduced this locally with:
--- a/src/test/modules/nbtree/Makefile
+++ b/src/test/modules/nbtree/Makefile
@@ -8 +8 @@ REGRESS = nbtree_half_dead_pages \
-ISOLATION = backwards-scan-concurrent-splits \
+ISOLATION = $(shell printf 'backwards-scan-concurrent-splits %.0s' `seq 100`) \

and
echo "autovacuum_naptime=1s" >/tmp/extra.config
TEMP_CONFIG=/tmp/extra.config make -s check -C src/test/modules/nbtree
...
ok 17        - backwards-scan-concurrent-splits          160 ms
# diff -U3 .../src/test/modules/nbtree/expected/backwards-scan-concurrent-splits.out
.../src/test/modules/nbtree/output_iso/results/backwards-scan-concurrent-splits.out
# --- .../src/test/modules/nbtree/expected/backwards-scan-concurrent-splits.out 2026-07-28 22:26:18.306287159 +0300
# +++ .../src/test/modules/nbtree/output_iso/results/backwards-scan-concurrent-splits.out 2026-07-29 07:26:51.016349487
+0300
# @@ -277,8 +277,6 @@
#
#  (1 row)
#
# -scan_session: NOTICE:  notice triggered for injection point nbtree-walk-left-deleted
# -scan_session: NOTICE:  notice triggered for injection point nbtree-walk-left-restart
#  step b_scan_999: <... completed>
#  col
#  ---
not ok 18    - backwards-scan-concurrent-splits          162 ms
ok 19        - backwards-scan-concurrent-splits          165 ms
...
ok 99        - backwards-scan-concurrent-splits          168 ms
ok 100       - backwards-scan-concurrent-splits          160 ms
ok 101       - predicate-empty-index                     104 ms
1..101
# 8 of 101 tests failed.

Could you please have a look?

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=jay&dt=2026-07-28%2021%3A00%3A31
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=basilisk&dt=2026-07-26%2006%3A58%3A54

Best regards,
Alexander