Re: pg_basebackup behavior on non-existent slot

On Tue, Sep 12, 2017 at 7:35 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:

> On Wed, Sep 6, 2017 at 2:50 AM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
> wrote:
>
>> Magnus Hagander wrote:
>> > On Mon, Sep 4, 2017 at 3:21 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
>> wrote:
>>
>> > > Should the parent process of pg_basebackup be made to respond to
>> SIGCHLD?
>> > > Or call waitpid(bgchild, &status, WNOHANG) in some strategic loop?
>> >
>> > I think it's ok to just call waitpid() -- we don't need to react super
>> > quickly, but we should react.
>>
>> Hmm, not sure about that ... in the normal case (slotname is correct)
>> you'd be doing thousands of useless waitpid() system calls during the
>> whole operation, no? I think it'd be better to have a SIGCHLD handler
>> that sets a flag (just once), which can be quickly checked without
>> accessing kernel space.
>>
>
> If we don't want polling by waitpid, then my next thought would be to move
> the data copy into another process, then have the main process do nothing
> but wait for the first child to exit. If the first to exit is the WAL
> receiver, then we must have an error and the data receiver can be killed.
> I don't know how to translate that to Windows, however.
>
>
Well, we could do something similar -- run the main process and the
streamer in separate threads on windows and have a main thread wait on
both. The main thread would have to be in charge of cleanup as well of
course. But I think that's likely going to be more complicated than using
non blocking libpq APIs.

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>