Handle receiving a signal while we're trying to deliver a signal.

Should fix test failure in https://buildkite.com/julialang/rr/builds/1542#018c344a-bc09-477b-bdd6-4726071ce99b

src/RecordSession.cc

@@ -1569,23 +1569,45 @@ static bool inject_handled_signal(RecordTask* t) {
   t->stashed_signal_processed();
 
   int sig = t->ev().Signal().siginfo.si_signo;
-  do {
+  int stop_sig;
+  while (true) {
     // We are ready to inject our signal.
-    // XXX we assume the kernel won't respond by notifying us of a different
-    // signal. We don't want to do this with signals blocked because that will
-    // save a bogus signal mask in the signal frame.
+    // Ideally we would block signals here so only the signal we care about can be raised.
+    // But that breaks all kinds of things because signal delivery itself can update
+    // the sigmaks (e.g. via sa_mask). Emulating all that would be painful, so we
+    // don't change the sigmask.
     if (!t->resume_execution(RESUME_SINGLESTEP, RESUME_WAIT_NO_EXIT, RESUME_NO_TICKS, sig)) {
       return false;
     }
-    // Signal injection can change the sigmask due to sa_mask effects, lack of
-    // SA_NODEFER, and signal frame construction triggering a synchronous
-    // SIGSEGV.
-    t->invalidate_sigmask();
-    // Repeat injection if we got a desched signal. We observe in Linux 4.14.12
-    // that we get SYSCALLBUF_DESCHED_SIGNAL here once in a while.
-  } while (t->stop_sig() == t->session().syscallbuf_desched_sig());
+    // We're expecting a kernel SIGTRAP from the singlestep, but we can also be notified
+    // of other pending signals unfortunately :-(.
+    stop_sig = t->stop_sig();
+    ASSERT(t, stop_sig) << "Only signal-stops should occur here";
+    if (t->get_siginfo().si_code > 0) {
+      // Signal was generated by the kernel (i.e. not via kill() etc).
+      // Signal injection can change the sigmask due to sa_mask effects, lack of
+      // SA_NODEFER, and signal frame construction triggering a synchronous
+      // SIGSEGV.
+      t->invalidate_sigmask();
+      // We should either have a SIGTRAP or possibly a SIGSEGV due to
+      // sigframe construction overflowing the stack.
+      break;
+    }
+    if (stop_sig == t->session().syscallbuf_desched_sig()) {
+      // Repeat injection if we got a desched signal. We observe in Linux 4.14.12
+      // that we get SYSCALLBUF_DESCHED_SIGNAL here once in a while.
+       continue;
+    }
+    if (stop_sig == sig && sig < SIGRTMIN) {
+      // Repeat injection if we got the same non-RT signal. We can coalesce
+      // this with the current undelivered signal so there's no need to stash it.
+      continue;
+    }
+    // Stash it for later processing and retry injecting our signal.
+    t->stash_sig();
+  }
 
-  if (t->stop_sig() == SIGSEGV) {
+  if (stop_sig == SIGSEGV) {
     // Constructing the signal handler frame must have failed. Stash the signal
     // to deliver it later.
     t->stash_sig();
@@ -1600,7 +1622,7 @@ static bool inject_handled_signal(RecordTask* t) {
   }
 
   // We stepped into a user signal handler.
-  ASSERT(t, t->stop_sig() == SIGTRAP)
+  ASSERT(t, stop_sig == SIGTRAP)
       << "Got unexpected status " << t->status() << " trying to deliver "
       << signal_name(sig) << " siginfo is " << t->get_siginfo();
   ASSERT(t, t->get_signal_user_handler(sig) == t->ip())