Re: pgsql: test_aio: Add basic tests for StartReadBuffers()

One of the AIO commits around this one (2026-03-28 00:00Z) broke the
postgresql-19 builds on apt.pg.o. Only the Debian unstable (sid,
assertions enabled) builds are working, everything else including the
nearly identical testing (forky) is broken (where assertions are off):

05:01:04 # +++ tap check in src/test/modules/test_aio +++
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, doesn't combine hits, block 0-1: expected stdout'
05:01:04 # at t/001_aio.pl line 1474.
05:01:04 # '0|0|f|0
05:01:04 # 0|0|f|0'
05:01:04 # doesn't match '(?^:^0\|0\|f\|1\n1\|1\|f\|1$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, doesn't combine hits, block 0-1: expected stderr'
05:01:04 # at t/001_aio.pl line 1474.
05:01:04 # 'psql:<stdin>:19: WARNING: resource was not closed: [4728] (rel=base/5/16413, blockNum=0, flags=0x83000000, refcount=1 1)
05:01:04 # psql:<stdin>:19: WARNING: resource was not closed: [4729] (rel=base/5/16413, blockNum=1, flags=0x83000000, refcount=1 1)'
05:01:04 # doesn't match '(?^:^$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, interrupted by hit on 3, block 2-5: expected stdout'
05:01:04 # at t/001_aio.pl line 1490.
05:01:04 # '0|2|t|1
05:01:04 # 1|3|f|0
05:01:04 # 1|3|t|2'
05:01:04 # doesn't match '(?^:^0\|2\|t\|1\n1\|3\|f\|1\n2\|4\|t\|2$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, interrupted by hit on 3, block 2-5: expected stderr'
05:01:04 # at t/001_aio.pl line 1490.
05:01:04 # 'psql:<stdin>:27: WARNING: resource was not closed: [4730] (rel=base/5/16413, blockNum=3, flags=0x83000000, refcount=1 1)'
05:01:04 # doesn't match '(?^:^$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, hit, block 0: expected stdout'
05:01:04 # at t/001_aio.pl line 1508.
05:01:04 # '0|0|f|0'
05:01:04 # doesn't match '(?^:^0\|0\|f\|1$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, hit, block 0: expected stderr'
05:01:04 # at t/001_aio.pl line 1508.
05:01:04 # 'psql:<stdin>:39: WARNING: resource was not closed: [4734] (rel=base/5/16413, blockNum=0, flags=0x83000000, refcount=1 1)'
05:01:04 # doesn't match '(?^:^$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, hit, block 1: expected stdout'
05:01:04 # at t/001_aio.pl line 1522.
05:01:04 # '0|1|f|0'
05:01:04 # doesn't match '(?^:^0\|1\|f\|1$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, hit, block 1: expected stderr'
05:01:04 # at t/001_aio.pl line 1522.
05:01:04 # 'psql:<stdin>:47: WARNING: resource was not closed: [4735] (rel=base/5/16413, blockNum=1, flags=0x83000000, refcount=1 1)'
05:01:04 # doesn't match '(?^:^$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, hit, block 0-1: expected stdout'
05:01:04 # at t/001_aio.pl line 1529.
05:01:04 # '0|0|f|0
05:01:04 # 0|0|f|0'
05:01:04 # doesn't match '(?^:^0\|0\|f\|1\n1\|1\|f\|1$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, hit, block 0-1: expected stderr'
05:01:04 # at t/001_aio.pl line 1529.
05:01:04 # 'psql:<stdin>:51: WARNING: resource was not closed: [4734] (rel=base/5/16413, blockNum=0, flags=0x83000000, refcount=1 1)
05:01:04 # psql:<stdin>:51: WARNING: resource was not closed: [4735] (rel=base/5/16413, blockNum=1, flags=0x83000000, refcount=1 1)'
05:01:04 # doesn't match '(?^:^$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, hit 0-1, miss 2: expected stdout'
05:01:04 # at t/001_aio.pl line 1536.
05:01:04 # '0|0|f|0
05:01:04 # 0|0|f|0
05:01:04 # 0|0|t|1'
05:01:04 # doesn't match '(?^:^0\|0\|f\|1\n1\|1\|f\|1\n2\|2\|t\|1$)'
05:01:04
05:01:04 # Failed test 'worker: normal: read buffers, hit 0-1, miss 2: expected stderr'
05:01:04 # at t/001_aio.pl line 1536.
05:01:04 # 'psql:<stdin>:55: WARNING: resource was not closed: [4734] (rel=base/5/16413, blockNum=0, flags=0x83000000, refcount=1 1)
05:01:04 # psql:<stdin>:55: WARNING: resource was not closed: [4735] (rel=base/5/16413, blockNum=1, flags=0x83000000, refcount=1 1)'
05:01:04 # doesn't match '(?^:^$)'
...

https://jengus.postgresql.org/view/Snapshot/job/postgresql-19-binaries-snapshot/
https://jengus.postgresql.org/view/Snapshot/job/postgresql-19-binaries-snapshot/781/architecture=amd64,distribution=forky/consoleFull

Christoph