Re: Clean up NamedLWLockTranche stuff

On Fri, Mar 27, 2026 at 04:50:12PM -0500, Nathan Bossart wrote:
> On Fri, Mar 27, 2026 at 05:22:33PM -0400, Andres Freund wrote:
>> TRAP: failed Assert("MemoryContextIsValid(context)"), File: "mcxt.c", Line: 1270, PID: 230491
>> [...](ExceptionalCondition+0x54)[0xaaaae186c204]
>> [...](MemoryContextAllocExtended+0x0)[0xaaaae18a2a24]
>> [...](RequestNamedLWLockTranche+0x6c)[0xaaaae16e7310]
>> [...](process_shmem_requests+0x28)[0xaaaae1881628]
>> [...](PostgresSingleUserMain+0xc4)[0xaaaae1701a34]
>> [...](main+0x6ac)[0xaaaae12a2adc]
>> /lib/aarch64-linux-gnu/libc.so.6(__libc_start_main+0xe8)[0xffff99713dd8]
>> [...](+0xf2b98)[0xaaaae12a2b98]
>> Aborted
>> pg_rewind: error: postgres single-user mode in target cluster failed
>
> Hm. AFAICT PostmasterContext isn't created in single-user mode, and the
> commit in question has RequestNamedLWLockTranche() allocate requests there.
> I guess the idea is to allow backends to free that memory after forking
> from postmaster, but we don't do that for the NamedLWLockTrancheRequests
> list. Maybe we should surround the last part of that function with
> MemoryContextSwitchTo(...) to either TopMemoryContext or PostmasterContext
> depending on whether we're in single-user mode.

Concretely, like the attached.

--
nathan