[PATCH v6 03/15] aio-posix: fix spurious return from ->wait() due to signals

[Stefan Hajnoczi <stefanha@redhat.com>]

io_uring_enter(2) only returns -EINTR in some cases when interrupted by
a signal. Therefore the while loop in fdmon_io_uring_wait() is
incomplete and can lead to a spurious early return.

Handle the case when a signal interrupts io_uring_enter(2) but the
syscall returns the number of SQEs submitted (that takes priority over
-EINTR).

This patch probably makes little difference for QEMU, but the test suite
relies on the exact pattern of aio_poll() return values, so it's best to
hide this io_uring syscall interface quirk.

Here is the strace of test-aio receiving 3 SIGCONT signals after this
fix has been applied. Notice how the io_uring_enter(2) return value is 1
the first time because an SQE was submitted, but -EINTR the other times:

  eventfd2(0, EFD_CLOEXEC|EFD_NONBLOCK) = 9
  io_uring_enter(7, 1, 0, 0, NULL, 8) = 1
  clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7ffe38a46240) = 0
  io_uring_enter(7, 1, 1, IORING_ENTER_GETEVENTS, NULL, 8) = 1
  --- SIGCONT {si_signo=SIGCONT, si_code=SI_USER, si_pid=596096, si_uid=1000} ---
  io_uring_enter(7, 0, 1, IORING_ENTER_GETEVENTS, NULL, 8) = -1 EINTR (Interrupted system call)
  --- SIGCONT {si_signo=SIGCONT, si_code=SI_USER, si_pid=596096, si_uid=1000} ---
  io_uring_enter(7, 0, 1, IORING_ENTER_GETEVENTS, NULL, 8 <unfinished ...>
  <... io_uring_enter resumed>) = -1 EINTR (Interrupted system call)
  --- SIGCONT {si_signo=SIGCONT, si_code=SI_USER, si_pid=596096, si_uid=1000} ---
  io_uring_enter(7, 0, 1, IORING_ENTER_GETEVENTS, NULL, 8 <unfinished ...>
  <... io_uring_enter resumed>) = 0

Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 util/fdmon-io_uring.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c
index b64ce42513..3d8638b0e5 100644
--- a/util/fdmon-io_uring.c
+++ b/util/fdmon-io_uring.c
@@ -299,9 +299,16 @@ static int fdmon_io_uring_wait(AioContext *ctx, AioHandlerList *ready_list,
 
     fill_sq_ring(ctx);
 
+    /*
+     * Loop to handle signals in both cases:
+     * 1. If no SQEs were submitted, then -EINTR is returned.
+     * 2. If SQEs were submitted then the number of SQEs submitted is returned
+     *    rather than -EINTR.
+     */
     do {
         ret = io_uring_submit_and_wait(&ctx->fdmon_io_uring, wait_nr);
-    } while (ret == -EINTR);
+    } while (ret == -EINTR ||
+             (ret >= 0 && wait_nr > io_uring_cq_ready(&ctx->fdmon_io_uring)));
 
     assert(ret >= 0);
 
-- 
2.51.1