AIX and EAGAIN on open()

Hello,

a customer running PG on AIX [1] is occasionally seeing "Resource
temporarily unavailable" (EAGAIN) returned by open() calls:

[1] We have PostgreSQL 11.13 on powerpc-ibm-aix7.2.5.0, compiled by /opt/IBM/xlc/13.1.0/bin/xlc, 64-bit

2022-05-19 03:28:13 CEST:127.0.0.1(63265):x(at)x:[64029168]: ERROR: could not open file "base/16401/935915821_fsm": Resource temporarily unavailable
2022-05-19 03:28:13 CEST:127.0.0.1(63265):x(at)x:[64029168]: CONTEXT: SQL statement "INSERT INTO s[...]"
PL/pgSQL function s...() line 12 at SQL statement
2022-05-19 03:28:13 CEST:127.0.0.1(63265):x(at)x:[64029168]: STATEMENT: PREPARE ... AS insert into ...

2022-04-16 01:45:31 CEST:127.0.0.1(58946):x(at)x:[20906970]: ERROR: could not access status of transaction 0
2022-04-16 01:45:31 CEST:127.0.0.1(58946):x(at)x:[20906970]: DETAIL: Could not open file "pg_subtrans/6158": Resource temporarily unavailable.
2022-04-16 01:45:31 CEST:127.0.0.1(58946):x(at)x:[20906970]: STATEMENT: PREPARE ... AS update ...

2020-12-01 09:24:30 CET:127.0.0.1(59898):x(at)x:[6227520]: ERROR: could not access status of transaction 0
2020-12-01 09:24:30 CET:127.0.0.1(59898):x(at)x:[6227520]: DETAIL: Could not open file "pg_subtrans/AC9E": Resource temporarily unavailable.
2020-12-01 09:24:30 CET:127.0.0.1(59898):x(at)x:[6227520]: STATEMENT: PREPARE ... AS DELETE FROM ....

open() should not return EAGAIN as per POSIX [2],

[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html#tag_16_357_05

and the AIX documentation says it would only return EAGAIN if O_TRUNC
is used [3], but as far as I can tell, PG does not use that flag.

[3] https://www.ibm.com/docs/en/aix/7.2?topic=o-open-openat-openx-openxat-open64-open64at-open64x-open64xat-creat-creat64-subroutine

IBM's reply to the issue back in December 2020 was this:

The man page / infocenter document is not intended as an exhaustive
list of all possible error codes returned and their circumstances.
"Resource temporarily unavailable" may also be returned for
O_NSHARE, O_RSHARE with O_NONBLOCK.

Afaict, PG does not use these flags either.

We also ruled out that the system is using any anti-virus or similar
tooling that would intercept IO traffic.

Does anything of that ring a bell for someone? Is that an AIX bug, a
PG bug, or something else?

Christoph
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