[NativeProcessLinux] Avoid race in SIGSTOP processing

[labath]

Summary

Processing of SIGSTOP could result in a bogus stop notification from NPL if there was a pending
notification waiting for a thread to stop. The notification would fire even though the thread
would be resumed immediately. I fix this by using a lower-level resume operation which does not
check for stop notifications. The same approach was used for other occurences of this problem
(e.g. D9696).

[labath]

It seems that SIGSTOP processing was broken to begin with https://llvm.org/bugs/show_bug.cgi?id=23264 so this will not help. I will take a step back and see what is the underlying problem.

source/Plugins/Process/Linux/NativeProcessLinux.cpp

if (is_from_llgs && (info->si_code == SI_TKILL) && (signo == SIGSTOP))
 
         // Done handling.
         return;
     }
 
     if (log)
         log->Printf ("NativeProcessLinux::%s() received signal %s", __FUNCTION__, GetUnixSignals ().GetSignalAsCString (signo));
 
-    // This thread is stopped.
-    ThreadDidStop (pid, false);
-
-    switch (signo)
-    {
-    case SIGSTOP:
-        {
-            std::static_pointer_cast<NativeThreadLinux> (thread_sp)->SetStoppedBySignal (signo);
-            if (log)
-            {
-                if (is_from_llgs)
-                    log->Printf ("NativeProcessLinux::%s pid = %" PRIu64 " tid %" PRIu64 " received SIGSTOP from llgs, most likely an interrupt", __FUNCTION__, GetID (), pid);
-                else
-                    log->Printf ("NativeProcessLinux::%s pid = %" PRIu64 " tid %" PRIu64 " received SIGSTOP from outside of debugger", __FUNCTION__, GetID (), pid);
-            }
+    if (signo == SIGSTOP)
 
-            // Resume this thread to get the group-stop mechanism to fire off the true group stops.
-            // This thread will get stopped again as part of the group-stop completion.
-            ResumeThread(pid,
-                    [=](lldb::tid_t tid_to_resume, bool supress_signal)
-                    {
-                        std::static_pointer_cast<NativeThreadLinux> (thread_sp)->SetRunning ();
-                        // Pass this signal number on to the inferior to handle.
-                        return Resume (tid_to_resume, (supress_signal) ? LLDB_INVALID_SIGNAL_NUMBER : signo);
-                    },
-                    true);
-        }
-        break;
+        Resume (pid, signo);
+        return;
+
+    // This thread is stopped.
+    ThreadDidStop (pid, false);
+
+    switch (signo)
+    {
     case SIGSEGV:
     case SIGILL:
     case SIGFPE:
     case SIGBUS:
         if (thread_sp)
             std::static_pointer_cast<NativeThreadLinux> (thread_sp)->SetCrashedWithException (*info);
         break;
     default: