Re: Re: Parallel scan with SubTransGetTopmostTransaction assert coredump - Mailing list pgsql-hackers

On Sat, May 15, 2021 at 12:37 PM 刘鹏程 <pengchengliu@tju.edu.cn> wrote:
>
>
>   BTW, I test it in a high performance server. It is verly easily be reproduced. My colleague and me use different
environmentboth can reproduce it. 
>

Hi Pengcheng,

Although the issue won't reproduce easily in my system, I can
certainly see how, for the snapshots used in the parallel worker case,
the Active snapshot used is potentially an earlier snapshot that the
Transaction snapshot. I don't know why it is getting a newer
Transaction snapshot in InitializeParallelDSM(), when it has
previously pushed the return value of GetTransactionSnapshot() as the
Active snapshot.

So I too hope Tom or Robert can explain what is going on here and how
to resolve it (as you requested them to, in your other post).

I actually think that the Assert in SubTransGetTopmostTransaction() is
correct, but in the parallel-worker case, the snapshots are not being
setup correctly.

Can you try the trivial change below and see if it prevents the coredump?

Regards,
Greg Nancarrow
Fujitsu Australia


diff --git a/src/backend/access/transam/parallel.c
b/src/backend/access/transam/parallel.c
index 14a8690019..870889053f 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -216,7 +216,7 @@ InitializeParallelDSM(ParallelContext *pcxt)
  int i;
  FixedParallelState *fps;
  dsm_handle session_dsm_handle = DSM_HANDLE_INVALID;
- Snapshot transaction_snapshot = GetTransactionSnapshot();
+ Snapshot transaction_snapshot = GetActiveSnapshot();
  Snapshot active_snapshot = GetActiveSnapshot();

  /* We might be running in a very short-lived memory context. */