Handle unexpected exits in desched processing (fixes #3000)

There are two issues here:
1. We shouldn't attempt to do a `waitpid` if we know the process is
   dying, because our `waitpid` calling is pretty carefully choreographed
   after receiving PTRACE_EVENT_EXIT and an extra waitpid messes that up.
2. `advance_to_disarm_desched_syscall` wasn't robust to the possibility
   that the task could die and would infinite loop.

Fix both by adding appropriate checks. Fixes #3000.

src/RecordSession.cc

@@ -874,6 +874,9 @@ static void advance_to_disarm_desched_syscall(RecordTask* t) {
   /* TODO: mask off signals and avoid this loop. */
   do {
     t->resume_execution(RESUME_SYSCALL, RESUME_WAIT, RESUME_UNLIMITED_TICKS);
+    if (t->is_dying()) {
+      return;
+    }
     /* We can safely ignore TIME_SLICE_SIGNAL while trying to
      * reach the disarm-desched ioctl: once we reach it,
      * the desched'd syscall will be "done" and the tracee

src/Task.cc

@@ -1413,7 +1413,7 @@ void Task::resume_execution(ResumeRequest how, WaitRequest wait_how,
   flush_regs();
 
   pid_t wait_ret = 0;
-  if (session().is_recording()) {
+  if (session().is_recording() && !is_dying()) {
     /* There's a nasty race where a stopped task gets woken up by a SIGKILL
      * and advances to the PTRACE_EXIT_EVENT ptrace-stop just before we
      * send a PTRACE_CONT. Our PTRACE_CONT will cause it to continue and exit,
@@ -1446,7 +1446,7 @@ void Task::resume_execution(ResumeRequest how, WaitRequest wait_how,
           << "waitpid(" << tid << ", NOHANG) failed with " << wait_ret;
     }
   }
-  if (wait_ret > 0 || handled_ptrace_exit_event) {
+  if (wait_ret > 0 || is_dying()) {
     LOG(debug) << "Task " << tid << " exited unexpectedly";
     // wait() will see this and report the ptrace-exit event.
     detected_unexpected_exit = true;

src/test/desched_sigkill.c

@@ -40,7 +40,7 @@ int main(void) {
     pids[i] = fork();
     if (pids[i] == 0) {
         test_assert(1 == write(child_to_parent[1], "x", 1));
-        test_assert(1 == read(parent_to_child[0], &ch, 1) && ch == 'y');
+        test_assert(1 == read(parent_to_child[0], &ch, 1) && ch == 'x');
         pause();
         return 77;
     }