Re: pg_ls_waldir does not work on standy

Nikhil Shetty <nikhil(dot)dba04(at)gmail(dot)com> writes:
> Postgres version is 10.12

That's a couple years out of date. 10.13 and up have a probably-relevant
bug fix:

Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Branch: master Release: REL_13_BR [085b6b667] 2020-03-11 15:27:59 -0400
Branch: REL_12_STABLE Release: REL_12_3 [3c8864fc1] 2020-03-11 15:27:59 -0400
Branch: REL_11_STABLE Release: REL_11_8 [63b51dfec] 2020-03-11 15:27:59 -0400
Branch: REL_10_STABLE Release: REL_10_13 [05283dd5b] 2020-03-11 15:28:00 -0400

Avoid holding a directory FD open across pg_ls_dir_files() calls.

This coding technique is undesirable because (a) it leaks the FD for
the rest of the transaction if the SRF is not run to completion, and
(b) allocated FDs are a scarce resource, but multiple interleaved
uses of the relevant functions could eat many such FDs.

In v11 and later, a query such as "SELECT pg_ls_waldir() LIMIT 1"
yields a warning about the leaked FD, and the only reason there's
no warning in earlier branches is that fd.c didn't whine about such
leaks before commit 9cb7db3f0. Even disregarding the warning, it
wouldn't be too hard to run a backend out of FDs with careless use
of these SQL functions.

Hence, rewrite the function so that it reads the directory within
a single call, returning the results as a tuplestore rather than
via value-per-call mode.

There are half a dozen other built-in SRFs with similar problems,
but let's fix this one to start with, just to see if the buildfarm
finds anything wrong with the code.

In passing, fix bogus error report for stat() failure: it was
whining about the directory when it should be fingering the
individual file. Doubtless a copy-and-paste error.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Back-patch to v10 where this function was added.

Justin Pryzby, with cosmetic tweaks and test cases by me

Discussion: https://postgr.es/m/20200308173103.GC1357@telsasoft.com

regards, tom lane