Re: Parallel Apply

On Tue, 6 Oct 2026 at 12:07, Hayato Kuroda (Fujitsu)
<kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
>
> Thanks for the contact. Here is an updated version.

Thanks for the updated version Kuroda-san.
I found an issue while reviewing the patch where an error relayed from
a parallel apply worker can get an unrelated CONTEXT line from the
leader process. ProcessParallelApplyMessage() in applyparallelworker.c
sets error_context_stack to the leader's apply_error_context_stack
before calling ereport(ERROR). As a result, when errfinish() processes
the new error, the leader's error context callback runs again and adds
the leader's current replication context.

This can be incorrect because the leader may be processing a
completely different transaction when it receives the error from the
parallel worker. For example, in my test:
- tab_a is processed by a parallel worker and fails due to a trigger.
- The leader is processing an unrelated insert into tab_b.
- The parallel worker's error is received while the leader is processing tab_b.
- The final error incorrectly contains context for both tab_a and tab_b.

The resulting error looks like:
ERROR: logical replication parallel apply worker exited due to error
CONTEXT: PL/pgSQL function public.tab_a_boom_fn() line 3 at RAISE
processing remote data ... relation "public.tab_a" ...
logical replication parallel apply worker
processing remote data ... relation "public.tab_b" ...

The tab_b context is unrelated to the actual failure and comes from
the leader's current activity. I have attached a regression test that
reproduces the issue. The test verifies that the relayed error
contains the context for tab_a but must not include the leader's
unrelated tab_b context. It currently fails on the second assertion.

I think the error handling in ProcessParallelApplyMessage() needs to
avoid running the leader's current error-context callbacks when
re-throwing the worker's error.
Thoughts?

Regards,
Vignesh