Re: Nicer error when connecting to standby with hot_standby=off

From: Andres Freund <andres(at)anarazel(dot)de>
To: James Coleman <jtc331(at)gmail(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Nicer error when connecting to standby with hot_standby=off
Date: 2020-03-09 22:28:34
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On 2020-03-08 20:12:21 -0400, James Coleman wrote:
> I recently noticed while setting up a test environment that attempting to
> connect to a standby running without hot_standby=on results in a fairly
> generic error (I believe "the database system is starting up"). I don't
> have my test setup running right now, so can't confirm with a repro case at
> the moment, but with a little bit of spelunking I noticed that error text
> only shows up in src/backend/postmaster/postmaster.c when
> port->canAcceptConnections has the value CAC_STARTUP.
> Ideally the error message would include something along the lines of "The
> server is running as a standby but cannot accept connections with
> hot_standby=off".

Yea, something roughly like that would be good.

> I wanted to get some initial feedback on the idea before writing a patch:
> does that seem like a reasonable change? Is it actually plausible to
> distinguish between this state and "still recovering" (i.e., when starting
> up a hot standby but initial recovery hasn't completed so it legitimately
> can't accept connections yet)? If so, should we include the possibility if
> hot_standby isn't on, just in case?

Yes, it is feasible to distinguish those cases. And we should, if we're
going to change things around.

> The enum value CAC_STARTUP is defined in src/include/libpq/libpq-be.h,
> which makes me wonder if changing this value would result in a wire
> protocol change/something the client wants to know about? If so, I assume
> it's not reasonable to change the value, but would it still be reasonable
> to change the error text?

The value shouldn't be visible to clients in any way. While not obvious
from the name, there's this comment at the top of the header:

* Note that this is backend-internal and is NOT exported to clients.
* Structs that need to be client-visible are in pqcomm.h.


Andres Freund

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