On Fri, Aug 15, 2025 at 09:17:22AM -0500, Sami Imseih wrote: > While working on a related area, I noticed that shmem_startup_hook > is called twice under EXEC_BACKEND. > > The first call occurs in CreateSharedMemoryAndSemaphores() during > postmaster startup (!IsUnderPostmaster), which is expected. > > The second call happens in AttachSharedMemoryStructs() (when > EXEC_BACKEND is defined), and this occurs in normal backends > (IsUnderPostmaster). > > The second call does not seem correct. The startup hook that should > only run during > postmaster initialization, AFAIK. > > Blame shows that this change was introduced in commit 69d903367c, > but I could not determine the rationale from the discussion, > so it may have been an oversight.
I think the startup hook must run in each backend for EXEC_BACKEND, else we won't properly initialize pointers to shared memory in that case, right? I added the following wording in commit 964152c: + <literal>shmem_startup_hook</literal> provides a convenient place for the + initialization code, but it is not strictly required that all such code + be placed in this hook. Each backend will execute the registered + <literal>shmem_startup_hook</literal> shortly after it attaches to shared + memory. Note that add-ins should still acquire + <function>AddinShmemInitLock</function> within this hook, as shown in the + example above. IIUC commit 69d9033 actually makes that inaccurate for the non-EXEC_BACKEND case. Presumably this is okay because we don't need to re-initialize pointers to shmem when forking. I must've missed this change when updating the documentation. -- nathan
