Fix atimer setting and overdue expiration (bug#55628)

* src/atimer.c (set_alarm): If the atimer has already expired, signal
it right away instead of postponing it further.  Previously this could
occur repeatedly, blocking atimers indefinitely.
Also only use `alarm` as fallback if `setitimer` is unavailable, not
both at the same time (which makes no sense, and they both typically
use the same mechanism behind the curtains).

* test/src/eval-tests.el (eval-tests/funcall-with-delayed-message):
New test, verifying proper functioning of funcall-with-delayed-message
which also serves as test for this bug (which also caused
debug-timer-check to fail, but that test is only run when Emacs is
built with enable-checking).

1 files changed, 16 insertions, 17 deletions

diff --git a/src/atimer.c b/src/atimer.c
index 1c6c881fc02..c26904e1f01 100644
--- a/src/atimer.c
+++ b/src/atimer.c
@@ -297,11 +297,6 @@ set_alarm (void)
 {
   if (atimers)
     {
-#ifdef HAVE_SETITIMER
-      struct itimerval it;
-#endif
-      struct timespec now, interval;
-
 #ifdef HAVE_ITIMERSPEC
       if (0 <= timerfd || alarm_timer_ok)
         {
@@ -337,20 +332,24 @@ set_alarm (void)
         }
 #endif
 
-      /* Determine interval till the next timer is ripe.
-         Don't set the interval to 0; this disables the timer.  */
-      now = current_timespec ();
-      interval = (timespec_cmp (atimers->expiration, now) <= 0
-                  ? make_timespec (0, 1000 * 1000)
-                  : timespec_sub (atimers->expiration, now));
+      /* Determine interval till the next timer is ripe.  */
+      struct timespec now = current_timespec ();
+      if (timespec_cmp (atimers->expiration, now) <= 0)
+        {
+          /* Timer is (over)due -- just trigger the signal right way.  */
+          raise (SIGALRM);
+        }
+      else
+        {
+          struct timespec interval = timespec_sub (atimers->expiration, now);
 
 #ifdef HAVE_SETITIMER
-
-      memset (&it, 0, sizeof it);
-      it.it_value = make_timeval (interval);
-      setitimer (ITIMER_REAL, &it, 0);
-#endif /* not HAVE_SETITIMER */
-      alarm (max (interval.tv_sec, 1));
+          struct itimerval it = {.it_value = make_timeval (interval)};
+          setitimer (ITIMER_REAL, &it, 0);
+#else
+          alarm (max (interval.tv_sec, 1));
+#endif
+        }
     }
 }