Re: Avoid memory leaks during base backups

On Mon, Oct 17, 2022 at 1:09 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Fri, Oct 14, 2022 at 09:56:31PM +0000, Cary Huang wrote:
> > I applied your v5 patch on the current master and run valgrind on it
> > while doing a basebackup with simulated error. No memory leak
> > related to backup is observed. Regression is also passing.
>
> Echoing with what I mentioned upthread in [1], I don't quite
> understand why this patch needs to touch basebackup.c, walsender.c
> and postgres.c. In the case of a replication command processed by a
> WAL sender, memory allocations happen in the memory context created
> for replication commands, which is itself, as far as I understand, the
> message memory context when we get a 'Q' message for a simple query.
> Why do we need more code for a cleanup that should be already
> happening? Am I missing something obvious?
>
> [1]: https://www.postgresql.org/message-id/YzPKpKEk/JMjhWEz(at)paquier(dot)xyz

My bad, I missed that. You are right. We have "Replication command
context" as a child of "MessageContext" memory context for base backup
that gets cleaned upon error in PostgresMain() [1].

> xlogfuncs.c, by storing stuff in the TopMemoryContext of the process
> running the SQL commands pg_backup_start/stop() is different, of
> course. Perhaps the point of centralizing the base backup context in
> xlogbackup.c makes sense, but my guess that it makes more sense to
> keep that with the SQL functions as these are the only ones in need of
> a cleanup, coming down to the fact that the start and stop functions
> happen in different queries, aka these are not bind to a message
> context.

Yes, they're not related to "MessageContext" memory context.

Please see the attached v6 patch that deals with memory leaks for
backup SQL-callable functions.

[1]
(gdb) bt
#0 MemoryContextDelete (context=0x558b7cd0de50) at mcxt.c:378
#1 0x0000558b7c655733 in MemoryContextDeleteChildren
(context=0x558b7ccda8c0) at mcxt.c:430
#2 0x0000558b7c65546d in MemoryContextReset (context=0x558b7ccda8c0)
at mcxt.c:309
#3 0x0000558b7c43b5cd in PostgresMain (dbname=0x558b7cd11fb8 "",
username=0x558b7ccd6298 "ubuntu")
at postgres.c:4358
#4 0x0000558b7c364a88 in BackendRun (port=0x558b7cd09620) at postmaster.c:4482
#5 0x0000558b7c36431b in BackendStartup (port=0x558b7cd09620) at
postmaster.c:4210
#6 0x0000558b7c3603be in ServerLoop () at postmaster.c:1804
#7 0x0000558b7c35fb1b in PostmasterMain (argc=3, argv=0x558b7ccd4200)
at postmaster.c:1476
#8 0x0000558b7c229a0e in main (argc=3, argv=0x558b7ccd4200) at main.c:197

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com