Re: psql not responding to SIGINT upon db reconnection

On 22/11/2023 23:29, Tristan Partin wrote:
> Ha, you're right. I had this working yesterday, but convinced myself it
> didn't. I had a do while loop wrapping the blocking call. Here is a v4,
> which seems to pass the tests that were pointed out to be failing
> earlier.

Thanks! This suffers from a classic race condition:

> +			if (cancel_pressed)
> +				break;
> +
> +			sock = PQsocket(n_conn);
> +			if (sock == -1)
> +				break;
> +
> +			rc = pqSocketPoll(sock, for_read, !for_read, (time_t)-1);
> +			Assert(rc != 0); /* Timeout is impossible. */
> +			if (rc == -1)
> +			{
> +				success = false;
> +				break;
> +			}

If a signal arrives between the "if (cancel_pressed)" check and 
pqSocketPoll(), pgSocketPoll will "miss" the signal and block 
indefinitely. There are three solutions to this:

1. Use a timeout, so that you wake up periodically to check for any 
missed signals. Easy solution, the downsides are that you will not react 
as quickly if the signal is missed, and you waste some cycles by waking 
up unnecessarily.

2. The Self-pipe trick: https://cr.yp.to/docs/selfpipe.html. We also use 
that in src/backend/storage/ipc/latch.c. It's portable, but somewhat 
complicated.

3. Use pselect() or ppoll(). They were created specifically to address 
this problem. Not fully portable, I think it's missing on Windows at least.

Maybe use pselect() if it's available, and select() with a timeout if 
it's not.

-- 
Heikki Linnakangas
Neon (https://neon.tech)