Re: Avoid memory leaks during base backups

On Wed, Oct 19, 2022 at 9:23 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Wed, Oct 19, 2022 at 8:10 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> > One option is to just have do_pg_start_backup() blow
> > away any old memory context before it allocates any new memory, and
> > forget about releasing anything in PostgresMain(). That means memory
> > could remain allocated after a failure until you next retry the
> > operation, but I don't think that really matters. It's not a lot of
> > memory; we just don't want it to accumulate across many repetitions.
>
> This seems reasonable to me.

I tried implementing this, please see the attached v7 patch.
Currently, memory allocated in the new memory context is around 4KB
[1]. In the extreme and rarest of the rare cases where somebody
executes select pg_backup_start(repeat('foo', 1024)); or a failure
occurs before reaching pg_backup_stop() on all of the sessions
(max_connections) at once, the maximum/peak memory bloat/leak is
around max_connections*4KB, which will still be way less than the
total amount of RAM. Hence, I think this approach seems very
reasonable and non-invasive yet can solve the memory leak problem.
Thoughts?

[1]
(gdb) p *backupcontext
$4 = {type = T_AllocSetContext, isReset = false, allowInCritSection =
false, mem_allocated = 4232,
  methods = 0x55c925b81f90 <mcxt_methods+240>, parent =
0x55c92766d2a0, firstchild = 0x0, prevchild = 0x0,
  nextchild = 0x55c92773f1f0, name = 0x55c9258be05c "on-line backup
context", ident = 0x0, reset_cbs = 0x0}

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