Re: Memory leak in WAL sender with pgoutput (v10~)

On Tue, Dec 17, 2024 at 2:48 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Dec 13, 2024 at 1:22 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Wed, Dec 11, 2024 at 9:43 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Dec 11, 2024 at 11:09 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > >
> > > > On Tue, Dec 10, 2024 at 6:13 PM Zhijie Hou (Fujitsu)
> > > > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > > > >
> > > > > On Wednesday, December 11, 2024 2:14 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > > > > >
> > > > > > On Tue, Dec 10, 2024 at 1:16 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> > > > > > wrote:
> > > > > > >
> > > > > > > On Tue, Dec 10, 2024 at 11:24 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> > > > > > wrote:
> > > > > > > >
> > > > > > > > On Tue, Dec 10, 2024 at 8:54 AM vignesh C <vignesh21(at)gmail(dot)com>
> > > > > > wrote:
> > > > > > > > >
> > > > > > > > > On Tue, 10 Dec 2024 at 04:56, Michael Paquier <michael(at)paquier(dot)xyz>
> > > > > > wrote:
> > > > > > > > > >
> > > > > > > > > > On Mon, Dec 09, 2024 at 03:36:15PM +0530, Amit Kapila wrote:
> > > > > > > > > > > It couldn't solve the problem completely even in back-branches. The
> > > > > > > > > > > SQL API case I mentioned and tested by Hou-San in the email [1]
> > > > > > won't
> > > > > > > > > > > be solved.
> > > > > > > > > > >
> > > > > > > > > > > [1] -
> > > > > > https://www.postgresql.org/message-id/OS0PR01MB57166A4DA0ABBB94F
> > > > > > 2FBB28694362%40OS0PR01MB5716.jpnprd01.prod.outlook.com
> > > > > > > > > >
> > > > > > > > > > Yeah, exactly (wanted to reply exactly that yesterday but lacked time,
> > > > > > > > > > thanks!).
> > > > > > > > >
> > > > > > > > > Yes, that makes sense. How about something like the attached patch.
> > > > > > > > >
> > > > > > > >
> > > > > > > > - oldctx = MemoryContextSwitchTo(CacheMemoryContext);
> > > > > > > > - if (data->publications)
> > > > > > > > - {
> > > > > > > > - list_free_deep(data->publications);
> > > > > > > > - data->publications = NIL;
> > > > > > > > - }
> > > > > > > > + static MemoryContext pubctx = NULL;
> > > > > > > > +
> > > > > > > > + if (pubctx == NULL)
> > > > > > > > + pubctx = AllocSetContextCreate(CacheMemoryContext,
> > > > > > > > + "logical replication publication list context",
> > > > > > > > + ALLOCSET_SMALL_SIZES);
> > > > > > > > + else
> > > > > > > > + MemoryContextReset(pubctx);
> > > > > > > > +
> > > > > > > > + oldctx = MemoryContextSwitchTo(pubctx);
> > > > > > > >
> > > > > > > > Considering the SQL API case, why is it okay to allocate this context
> > > > > > > > under CacheMemoryContext?
> > > > > > > >
> > > > > > >
> > > > > > > On further thinking, we can't allocate it under
> > > > > > > LogicalDecodingContext->context because once that is freed at the end
> > > > > > > of SQL API pg_logical_slot_get_changes(), pubctx will be pointing to a
> > > > > > > dangling memory. One idea is that we use
> > > > > > > MemoryContextRegisterResetCallback() to invoke a reset callback
> > > > > > > function where we can reset pubctx but not sure if we want to go there
> > > > > > > in back branches. OTOH, the currently proposed fix won't leak memory
> > > > > > > on repeated calls to pg_logical_slot_get_changes(), so that might be
> > > > > > > okay as well.
> > > > > > >
> > > > > > > Thoughts?
> > > > > >
> > > > > > Alternative idea is to declare pubctx as a file static variable. And
> > > > > > we create the memory context under LogicalDecodingContext->context in
> > > > > > the startup callback and free it in the shutdown callback.
> > > > >
> > > > > I think when an ERROR occurs during the execution of the pg_logical_slot_xx()
> > > > > API, the shutdown callback function is not invoked. This would result in the
> > > > > static variable not being reset, which, I think, is why Amit mentioned the use
> > > > > of MemoryContextRegisterResetCallback().
> > > >
> > > > My idea is that since that new context is cleaned up together with its
> > > > parent context (LogicalDecodingContext->context), we unconditionally
> > > > set that new context to the static variable at the startup callback.
> > > > That being said, Amit's idea would be cleaner.
> > > >
> > >
> > > Your preference is not completely clear. Are you okay with the idea of
> > > Vignesh's currently proposed patch for back-branches, or do you prefer
> > > to use a memory context reset callback, or do you have a different
> > > idea that should be adopted for back-branches?
> >
> > IIUC the current Vignesh's patch[1] doesn't solve the memory leak in
> > case of using logical decoding APIs, as you mentioned.
> >
>
> Right, but note that it wouldn't leak memory on repeated calls to the
> API. Only if the backend ever makes a single call for get_changes will
> it leak memory once, which is not ideal. Still, we can live with it if
> the other approaches are complex for back branches.

True. I missed that point.

>
> > I've tried the
> > idea of using memory context reset callback to reset pubctx. We need
> > to register the callback to LogicalContextDecodingContext->context,
> > meaning that we need to pass it to get_rel_sync_entry() (see
> > fix_memory_leak_v1.patch). I don't prefer this approach as it could
> > make backpatching complex in the future. Alternatively, we can declare
> > pubctx as a file static variable, create the memory context at the
> > startup callback, reset the pubctx at the shutdown callback, and use
> > the memory context reset callback to ensure the pubctx is reset (see
> > fix_memory_leak_v2.patch).Or I think we might not necessarily need to
> > use the memory context reset callback (see fix_memory_leak_v3.patch).
> > I prefer the latter two approaches.
> >
>
> Will fix_memory_leak_v3.patch avoid the leak in case of an ERROR in
> SQL API? If so, how?

The pubctx is created as a child of LogicalDecodingContext->context.
On an error, the pubctx is cleaned up altogether when cleaning up
LogicalDecodingContext->context.

Regards,

--
Masahiko Sawada
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