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)