Re: Allow some recovery parameters to be changed with reload

On 10.08.2020 23:20, Robert Haas wrote:
> On Sun, Aug 9, 2020 at 1:21 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>> Sorry for the late reply. I have been looking at that stuff again,
>> and restore_command can be called in the context of a WAL sender
>> process within the page_read callback of logical decoding via
>> XLogReadDetermineTimeline(), as readTimeLineHistory() could look for a
>> timeline history file. So restore_command is not used only in the
>> startup process.
> Hmm, interesting. But, does that make this change wrong, apart from
> the comments? Like, in the case of primary_conninfo, maybe some
> confusion could result if the startup process decided whether to ask
> for a WAL receiver based on thinking primary_conninfo being set, while
> that process thought that it wasn't actually set after all, as
> previously discussed in
> http://postgr.es/m/CA+TgmoZVmJX1+QTWw2tSnPHrnkwKZxC3ZsRynFB-Fpzm1Oxuhw@mail.gmail.com
> ... but what's the corresponding hazard here, exactly? It doesn't seem
> that there's any way in which the decision one process makes affects
> the decision the other process makes. There's still a race condition:
> it's possible for a walsender
Did you mean walreceiver here?
> to use the old restore_command after the
> startup process had already used the new one, or the other way around.
> However, it doesn't seem like that should confuse anything inside the
> server, and therefore I'm not sure we need to code around it.
I came up with following scenario. Let's say we have xlog files 1,2,3 in
dir1 and files 4,5 in dir2. If startup process had only handled files 1
and 2, before we switched restore_command from reading dir1 to reading
dir2, it will fail to find next file. IIUC, it will assume that recovery
is done, start server and walreceiver. The walreceiver will fail as
well. I don't know, how realistic is this case, though.

In general,. this feature looks useful and consistent with previous
changes, so I am interested in pushing it forward.
Sergey, could you please attach this thread to the upcoming CF, if
you're going to continue working on it.

 A few more questions:
- RestoreArchivedFile() is also used by pg_rewind. I don't see any
particular problem with it, just want to remind that we should test it too.
- How will it interact with possible future optimizations of archive
restore? For example, WAL prefetch [1].

 [1]
https://www.postgresql.org/message-id/flat/601EE1F5-0B78-47E1-9AAE-C15F74A1C21D(at)postgrespro(dot)ru

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Anastasia Lubennikova
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The Russian Postgres Company