Subject: Re: PATCH: Make pg_stop_backup() archive wait optional
To: Robert Haas <robertmhaas@gmail.com>
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Cc: Michael Paquier <michael.paquier@gmail.com>,
 Pg Hackers <pgsql-hackers@postgresql.org>
From: David Steele <david@pgmasters.net>
Message-ID: <aa775a6f-059c-47dc-ef87-1f017073014b@pgmasters.net>
Date: Tue, 28 Feb 2017 22:37:14 -0500
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On 2/28/17 10:22 PM, Robert Haas wrote:
> On Tue, Feb 28, 2017 at 6:22 AM, David Steele <david@pgmasters.net> wrote:
>>>> I'm not sure that's the case.  It seems like it should lock just as
>>>> multiple backends would now.  One process would succeed and the others
>>>> would error.  Maybe I'm missing something?
>>>
>>> Hm, any errors happening in the workers would be reported to the
>>> leader, meaning that even if one worker succeeded to run
>>> pg_start_backup() it would be reported as an error at the end to the
>>> client, no? By marking the exclusive function restricted we get sure
>>> that it is just the leader that fails or succeeds.
>>
>> Good point, and it strengthens the argument beyond, "it just seems right."
> 
> I think the argument should be based on whether or not the function
> depends on backend-private state that will not be synchronized.
> That's the definition of what makes something parallel-restricted or
> not.

Absolutely.  Yesterday was a long day so I may have (perhaps) become a
bit flippant.

> It looks like pg_start_backup() and pg_stop_backup() depend on the
> backend-private global variable nonexclusive_backup_running, so they
> should be parallel-restricted.

Agreed.

-- 
-David
david@pgmasters.net


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