i#7748: Fix signal stack overflow during thread synch (#7749)

I have noticed an intermittent failure of ap.detach_state which appears
to be caused by signal stack overflow while the two threads in the test
are synching.

The intended order of events goes something like this:

- Thread 1 is trying to synch with thread 2 and sends it a suspend
signal.
- Thread 2 handles the suspend and waits to be woken up.
- Thread 1 finds that thread 2 was not suspended at a safe spot, wakes
up thread 2 and waits to be notified that thread 2 has resumed.
- Thread 2 wakes up, notifies 1 that it has resumed and returns from the
signal handler.
- Thread 1 goes back around the synch loop and sends another suspend
signal. The process loops until thread 2 is suspended in a good state.

The problem comes if thread 2 is interrupted during step 4 after it has
notified thread 1 but before the signal handler returns. In that case
thread 1 can send another suspend signal before the first signal handler
call exits.

SIGILL (the suspend signal) is unblocked during the signal handler (see
i#184) so on a busy system these signals stack up until we run out of
signal stack and crash.

The solution I have found is for thread 2 to block the suspend signal
before notifying thread 1 to make sure it cannot be interrupted with a
second suspend signal.

Fixes #7748

Author: jackgallagher-arm

core/unix/signal.c

@@ -8837,7 +8837,17 @@ handle_suspend_signal(dcontext_t *dcontext, kernel_siginfo_t *siginfo,
 
     if (unblocked_sigs) {
         /* re-block so our exit from main_signal_handler is not interrupted */
+        kernel_sigaddset(&prevmask, SUSPEND_SIGNAL);
         sigprocmask_syscall(SIG_SETMASK, &prevmask, NULL, sizeof(prevmask));
+    } else {
+        /* Prevent another SUSPEND_SIGNAL interrupting main_signal_handler before we
+         * exit.
+         */
+        kernel_sigset_t mask;
+        kernel_sigemptyset(&mask);
+        kernel_sigaddset(&mask, SUSPEND_SIGNAL);
+
+        sigprocmask_syscall(SIG_BLOCK, &mask, NULL, sizeof(mask));
     }
     ostd->suspended_sigcxt = NULL;