pgsql: Tolerate partial pgstats entries in pgstat_gc_entry_refs()

Tolerate partial pgstats entries in pgstat_gc_entry_refs()

pgstat_get_entry_ref_cached() inserts a local entry_ref with
shmem-related fields set to NULL, expecting pgstat_get_entry_ref() (its
sole caller) to fill them up before returning. If an ERROR happens
while pgstat_get_entry_ref() runs, it could be possible to finish with a
local pgstats entry partially filled.

This could lead to a crash of pgstat_gc_entry_refs(), which tolerates a
NULL shared_stats in an assertion but unconditionally dereferenced its
"dropped" and "generation" fields.

This extends 4069df21beb8, being a cheap insurance against NULL pointer
dereference, if some code paths of pgstat_get_entry_ref() are not able
to perform any cleanup actions (for example after a dsm_create()
throwing an ERROR).

Reviewed-by: Grigorev Jurij <ju(dot)grigorev(at)ftdata(dot)ru>
Discussion: https://postgr.es/m/aqtiKTvl519bu8-V@paquier.xyz
Backpatch-through: 15

Branch
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REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e99e00b2751d3d4b5516d3c6243070da53ecad12

Modified Files
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src/backend/utils/activity/pgstat_shmem.c | 9 +++++++++
1 file changed, 9 insertions(+)