Re: Remove Deprecated Exclusive Backup Mode

On 7/1/20 4:39 PM, Magnus Hagander wrote:
> On Wed, Jul 1, 2020 at 10:28 PM David Steele <david@pgmasters.net
>     Here's a thought. What if we just stored the oldest starting LSN and a
>     count of how many backups have been requested. When the backup ends it
>     checks that backup count is > 0 and starting LSN is <= its starting
>     LSN.
>     If not, it throws an error. When backups go to 0 FPWs are turned off if
>     they were off before the first backup.
>
> I guess the weak spot of that one is if some script does stop without
> doing start first, it will break somebody else's backup. (And yes, I've
> seen scripts make this mistake many times -- it equally breaks the
> exclusive backups in the current system...)

Well, they'd have to pass in a backup_label with a start LSN >= the min
LSN or they would just get an error and not decrement the backup count.

The real issue would be if they called pg_stop_backup twice. We might be
able to stop that with a rolling max stop lsn to keep anyone from
calling pg_stop_backup() twice. 

But yeah, it would be possible to kill somebody else's session with some
finagling. Still, worse case would be an error'd backup rather than a
corrupt one.

But really, that's only if FPWs are turned off. We can also do some
extra validation if the session is left open, which for most software is
the norm now.

> And don't we need the combination of the start/stop location for the
> history file?

You mean the .backup file for the WAL? All that needs is the
backup_label and the stop LSN that's determined in pg_stop_backup(). Am
I missing something?