pgsql: Handle concurrent sequence drops during synchronization

Handle concurrent sequence drops during synchronization

Commit d4a657b0a4d added a call to has_sequence_privilege() while
fetching sequence information from the publisher, so that
publisher-side permission failures could be distinguished from missing
sequences. It also assumed that has_sequence_privilege() could never
return NULL, and asserted accordingly.

However, that assumption was incorrect. If a sequence is dropped after
the synchronization worker collects its metadata but while fetching the
sequence information, has_sequence_privilege() can return NULL.
This can trigger the assertion failure. This was also reported in
a buildfarm failure on member culicidae.

Fix this by treating a NULL result from has_sequence_privilege() as
indicating that the sequence was dropped concurrently, and report it as
a missing sequence instead of asserting that the result is never NULL.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Author: Vignesh C <vignesh21(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/20260710045217.f0.noahmisch@microsoft.com
Discussion: https://postgr.es/m/CALDaNm2fHGLeiQKj0r6OG7N9QeayxSmpLrWYJRyt4dL_m3VRWw@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/24a2b541bd450a2db9d8f3ddefc92ace58ef9a42

Modified Files
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src/backend/replication/logical/sequencesync.c | 15 +++++--
src/test/subscription/t/036_sequences.pl | 56 +++++++++++++++++++++++++-
2 files changed, 67 insertions(+), 4 deletions(-)