Re: [Libguestfs] [nbdkit PATCH 1/2] nbd: Don't reference stale errno in reader loop

When switching to libnbd in commit ab7760fcfd, I mistakenly assumed that after a POLLIN event fires on the pipe-to-self, then read() will return either 1 or -1. But this is not true; read() can also return 0 (if the pipe hits EOF), in which case POSIX says errno has an unspecified value, and we should not be deciding whether to log an error based on a random value. I did not manage to fix the bug even when later refactoring the pipe-to-self to be non-blocking, although it appears to be latent as long as the pipe is non-blocking and the write end is not closed. Fixes: e0d32468 Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- plugins/nbd/nbd.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c index d020beec..e5b8f338 100644 --- a/plugins/nbd/nbd.c +++ b/plugins/nbd/nbd.c @@ -332,9 +332,11 @@ nbdplug_reader (void *handle) /* Check if we were kicked because a command was started */ if (fds[1].revents & POLLIN) { - while (read (h->fds[0], &c, 1) == 1) + int r; + + while ((r = read (h->fds[0], &c, 1)) == 1) /* Drain any backlog */; - if (errno != EAGAIN) { + if (r == -1 && errno != EAGAIN) { nbdkit_error ("failed to read pipe: %m"); break; }