[Nathan Bossart <nathandbossart@gmail.com>]

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

Attachments:

0001-fix-RequestNamedLWLockTranche.patch (text/plain, 1.5K) download
From 714c765399656c9743e3ad76fe52d1cfc8994952 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Fri, 27 Mar 2026 17:00:49 -0500
Subject: [PATCH 1/1] fix RequestNamedLWLockTranche

---
 src/backend/storage/lmgr/lwlock.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index 7a68071302a..f5b2a6d479d 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -630,6 +630,7 @@ void
 RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
 {
 	NamedLWLockTrancheRequest *request;
+	MemoryContext oldcontext;
 
 	if (!process_shmem_requests_in_progress)
 		elog(FATAL, "cannot request additional LWLocks outside shmem_request_hook");
@@ -652,10 +653,17 @@ RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
 				 errdetail("No more than %d tranches may be registered.",
 						   MAX_USER_DEFINED_TRANCHES)));
 
-	request = MemoryContextAllocZero(PostmasterContext, sizeof(NamedLWLockTrancheRequest));
+	if (IsPostmasterEnvironment)
+		oldcontext = MemoryContextSwitchTo(PostmasterContext);
+	else
+		oldcontext = MemoryContextSwitchTo(TopMemoryContext);
+
+	request = palloc0(sizeof(NamedLWLockTrancheRequest));
 	strlcpy(request->tranche_name, tranche_name, NAMEDATALEN);
 	request->num_lwlocks = num_lwlocks;
 	NamedLWLockTrancheRequests = lappend(NamedLWLockTrancheRequests, request);
+
+	MemoryContextSwitchTo(oldcontext);
 }
 
 /*
-- 
2.50.1 (Apple Git-155)