> On Mon, Aug 25, 2025 at 12:58:08PM -0500, Sami Imseih wrote: > >> If this fails, why doesn't the call in pgss_shmem_startup() also fail? Was > >> pg_stat_statements not loaded via shared_preload_libraries? > > > > because the array is valid in postmaster, but it's not for a normal backend, > > since it's not getting copied. > > > > Yes, pg_stat_statements was loaded via shared_preload_libraries. Nothing > > was done out of the ordinary. > > > >> And is the > >> failure a segfault or a "requested tranche is not registered" error? > > > > It's a segfault. > > Oh, I see what's happening. We are never calling GetNamedLWLockTranche() > in backends because ShmemInitStruct() always sets *found. > > I'd argue that calling GetNamedLWLockTranche() outside of an "if (!found)" > block in a shmem_startup_hook doesn't follow convention, but the docs have > this note: > > shmem_startup_hook provides a convenient place for the initialization > code, but it is not strictly required that all such code be placed in > this hook. > > Like the inaccurate sentence immediately following this one [0], it was > added by commit 964152c. IMHO we should adjust the documentation to match > reality. > > [0] https://postgr.es/m/aJ9QIF3g2QQko9wq%40nathan
Why not ERROR out completely if we are calling this function outside
of postmaster
startup? I don't think we should be satisfied with just a
documentation change, since this
leads to a segfault. We can do something like below at the top of
GetNamedLWLockTranche
```
if (IsUnderPostmaster)
elog(ERROR, "GetNamedLWLockTranche can only be called during
postmaster startup");
```
Another approach is to just change GetNamedLWLockTranche to use
NamedLWLockTrancheArray since that is already copied in EXEC_BACKEND, and allow
GetNamedLWLockTranche to continue to be used outside of startup. In
this case, we
will need to add num_lwlocks field to NamedLWLockTrancheArray. This
might be better
to backpatch, since we will not be changing user facing behavior.
Attached is a repro patch. You will need to set
shared_preload_libraries = 'pg_stat_statements'
as well.
--
Sami
v1-0001-repro.patch
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