[Lldb-commits] [lldb] Refactor `ThreadList::WillResume()` to prepare to support reverse execution (PR #120817)

[Robert O'Callahan via lldb-commits]

================
@@ -582,31 +531,92 @@ bool ThreadList::WillResume() {
   // There are two special kinds of thread that have priority for "StopOthers":
   // a "ShouldRunBeforePublicStop thread, or the currently selected thread.  If
   // we find one satisfying that critereon, put it here.
-  ThreadSP stop_others_thread_sp;
-
+  ThreadSP thread_to_run;
   for (pos = m_threads.begin(); pos != end; ++pos) {
     ThreadSP thread_sp(*pos);
     if (thread_sp->GetResumeState() != eStateSuspended &&
         thread_sp->GetCurrentPlan()->StopOthers()) {
-      if ((*pos)->IsOperatingSystemPluginThread() &&
-          !(*pos)->GetBackingThread())
+      if (thread_sp->IsOperatingSystemPluginThread() &&
+          !thread_sp->GetBackingThread())
         continue;
 
       // You can't say "stop others" and also want yourself to be suspended.
       assert(thread_sp->GetCurrentPlan()->RunState() != eStateSuspended);
       run_me_only_list.AddThread(thread_sp);
 
       if (thread_sp == GetSelectedThread())
-        stop_others_thread_sp = thread_sp;
-        
+        thread_to_run = thread_sp;
+
       if (thread_sp->ShouldRunBeforePublicStop()) {
         // This takes precedence, so if we find one of these, service it:
-        stop_others_thread_sp = thread_sp;
+        thread_to_run = thread_sp;
         break;
       }
     }
   }
 
+  if (run_me_only_list.GetSize(false) > 0 && !thread_to_run) {
+    if (run_me_only_list.GetSize(false) == 1) {
+      thread_to_run = run_me_only_list.GetThreadAtIndex(0);
+    } else {
+      int random_thread =
+          (int)((run_me_only_list.GetSize(false) * (double)rand()) /
+                (RAND_MAX + 1.0));
+      thread_to_run = run_me_only_list.GetThreadAtIndex(random_thread);
+    }
+  }
+
+  // Give all the threads that are likely to run a last chance to set up their
+  // state before we negotiate who is actually going to get a chance to run...
+  // Don't set to resume suspended threads, and if any thread wanted to stop
+  // others, only call setup on the threads that request StopOthers...
+  bool wants_solo_run = run_me_only_list.GetSize(false) > 0;
+  for (pos = m_threads.begin(); pos != end; ++pos) {
----------------
rocallahan wrote:

FYI the new code does change behavior --- when some threads return true for `StopOthers()`, the old code calls `SetupForResume()` on all threads that return true for `StopOthers()`, but now we only call `SetupForResume()` on the one thread we have chosen to run.

Also, when no threads return true for `StopOthers()` but multiple threads have breakpoints that need to be skipped, the old code would choose randomly between those threads, but the new code picks the last one we encounter looping over `m_threads` (if none return true for `ShouldRunBeforePublicStop()`).

https://github.com/llvm/llvm-project/pull/120817