Re: After promote, DB can not UPDATE with SAVEPOINT (could not access status of transaction 1409172)

Hi,

On 2013-08-22 22:17:40 +0900, ダンミンフーン wrote:
> I've get found out that issue in my project.
> Update in PRIMARY while pg_basebackup is performing,
> Can cause the STANDBY could not UPDATE after promote.
>
> In Standby Node, when first XID of a page come, pg_subtrans
> must be extended by ExtendSUBTRANS function.
>
> But if that XID created while pg_basebackup (with "-x" option)
> was performing, it did not extend.
>
> Due to that, after basebackup complete, start that DB and do
> promote complete. It failed in SAVEPOINT UPDATE query like follows.
>
> ---
> postgres=# BEGIN;
> postgres=# SAVEPOINT testsavepoint;
> postgres=# UPDATE test_tbl SET name = 'test';
> ERROR: could not access status of transaction 1409172
> DETAIL: Could not read from file "pg_subtrans/0015" at offset 131072: Success.
> ---
>
> I've also confirm source and realize that,
>
> When StartupXLOG call RecordKnownAssignedTransactionIds
> (in the "main redo apply loop"), the "standbyState" still
> was STANDBY_INITIALIZED, so it is returned without goto
> ExtendSUBTRANS (to check and extend space for pg_subtrans).
>
> Therefor, after STANDBY promote, when UPDATE query made by
> SAVEPOINT is executed, the process as follow is performed
> and get the above ERROR in SimpleLruReadPage function.
>
> AssignTransactionId => SubTransSetParent => SubTransSetParent => SimpleLruReadPage
>
> I think that ExtendSUBTRANS must be called even if "standbyState"
> in STANDBY_INITIALIZED in order to avoid the case above.
>
> I also attach a patch. Could anyone confirm for me.

Nice diagnosis and thanks for the patch!

I think the the issue has already been fixed while investigating a
similar issue, could you verify that the current git version works for
you?

c.f. commit 99ee15b315c187045a95db7b27fd9d866aea93e0

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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