Re: [Libguestfs] [nbdkit PATCH 3/3] server: Add and use nbdkit_nanosleep

There are a couple of problems with filters trying to sleep. First, when it is time to shut down nbdkit, we wait until all pending transactions have had a chance to wind down. But consider what happens if one or more of those pending transactions are blocked in a sleep. POSIX says nanosleep is interrupted with EINTR if that thread handles a signal, but wiring up signal masks just to ensure a specific thread will get the signal is not pretty, and that means the thread processing SIGINT is NOT the thread blocked in nanosleep. Couple that with the fact that if multiple threads are sleeping, only one thread needs to process the signal, so the rest continue to sleep. Thus, even though we know the user wants nbdkit to quit, we are blocked waiting for a potentially long sleep to complete. This problem can be solved by realizing we already have a pipe-to-self to learn about a quit request or the end of a particular connection, and check for activities on those pipes in parallel with a timeout through pselect or ppoll to break our wait as soon as we know there is no reason to continue on with the transaction.

+++ b/server/public.c

+#else + struct timespec ts; + struct connection *conn = threadlocal_get_conn (); + struct pollfd fds[2] = { + [0].fd = quit_fd, + [0].events = POLLIN, + [1].fd = conn ? conn->status_pipe[0] : -1, + [1].events = POLLIN,