Re: [sqlsmith] FailedAssertion("!(XLogCtl->Insert.exclusiveBackup)", File: "xlog.c", Line: 10200)

On Thu, Aug 4, 2016 at 2:19 AM, Andreas Seltenreich <seltenreich@gmx.de> wrote:
> testing with sqlsmith shows that the following assertion doesn't hold:
>
>     FailedAssertion("!(XLogCtl->Insert.exclusiveBackup)", File: "xlog.c", Line: 10200)
>
> The triggering statements always contain a call to pg_start_backup with
> the third argument 'true', i.e. it's trying to start an exlusive backup.
>
> I didn't manage to put together a stand-alone testcase yet.

While I have not been able to trigger this assertion directly, I have
bumped into the fact that pg_stop_backup can reset unconditionally
XLogCtl->Insert.exclusiveBackup *before* pg_start_backup finishes or
even creates the backup_label file if it is set. So the in-memory
state of the backup is like there is no backups running at all
(including exclusive and non-exclusive), but there could be a
backup_label file present. In this state, it is not possible to
trigger pg_start_backup or pg_stop_backup again except if the
backup_label file is manually removed.

In do_pg_stop_backup, both steps would be better reversed, like in the
patch attached. So what we should actually do in pg_stop_backup is
first look at if the backup_label file exists, and then we reset the
in-memory flag as the last thing that do_pg_start_backup does is
writing the backup_label file. This does not close completely the
window though. After the backup_label file is created, it could still
be possible to trigger  the assertion if there is an error on the
tablespace map file.

This window exists as well on back-branches btw, this is not new to
9.6. Magnus, what do you think?
--
Michael