Re: Add client connection check during the execution of the query

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: s(dot)cherkashin(at)postgrespro(dot)ru
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Add client connection check during the execution of the query
Date: 2019-01-13 23:05:39
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s(dot)cherkashin(at)postgrespro(dot)ru writes:
> This patch adds verification of the connection with the client during
> the execution of the SQL query. The feature enables using the GUC
> variable ‘client_connection_check_interval’. The default check interval
> is 1 second. If you set the value of ‘client_connection_check_interval’
> to 0, then the check will not be performed.

I took a quick look through this.

* It won't even compile on non-Linux platforms, because MSG_DONTWAIT
is a Linux-ism. Perhaps that can be replaced by putting the client
socket into nonblock mode, but I'm not very sure that that'll work
(especially when using OpenSSL or another TLS implementation).

* I'm not convinced that this will reliably detect client connection loss.
AFAICS, if there is any unread data pending, it'd report that all is well
even if the client dropped off the net after sending that data. It's hard
to evaluate how likely such a situation is, but one really obvious case
is that the client might've sent an 'X' message to try to close the
connection gracefully. Also, use of TLS would again make things much
harder to reason about, because the TLS layer may send or receive data
that we don't know about.

* The management of the pending timeout interrupt seems like a mess.
Why did you involve ProcessInterrupts in that? It seems likely to queue
extra timeouts at random times due to unrelated interrupts causing that
bit of code to run, and/or cause weird gaps in the timeout intervals due
to not being run promptly. I'd be inclined to set this up so that the
timeout handler itself re-queues the timeout (I think that will work, or
if not, we should probably fix timeout.c so that it can).

* BTW, I am not on board with making this enabled-by-default.

This does seem like possibly a useful option if we can make it
work portably/reliably, but I don't have very high hopes for that.

regards, tom lane

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