[lldb-vscode] Synchronize calls to SendTerminatedEvent

[asahay]

Summary

If an inferior exits prior to the processing of a disconnect request,
then the threads executing EventThreadFunction and request_discontinue
respectively may call SendTerminatedEvent simultaneously, in turn,
testing and/or setting g_vsc.sent_terminated_event without any
synchronization. In case the thread executing EventThreadFunction sets
it before the thread executing request_discontinue has had a chance to
test it, the latter would move ahead to issue a response to the
disconnect request. Said response may be dispatched ahead of the
terminated event compelling the client to terminate the debug session
without consuming any console output that might've been generated by
the execution of terminateCommands.

[wallace]

thanks

lldb/tools/lldb-vscode/lldb-vscode.cpp

void SendThreadExitedEvent(lldb::tid_t tid) {
   body.try_emplace("threadId", (int64_t)tid);
   event.try_emplace("body", std::move(body));
   g_vsc.SendJSON(llvm::json::Value(std::move(event)));
 }
 
 // Send a "terminated" event to indicate the process is done being
 // debugged.
 void SendTerminatedEvent() {
+  // If an inferior exits prior to the processing of a disconnect request, then
+  // the threads executing EventThreadFunction and request_discontinue
+  // respectively may call SendTerminatedEvent simultaneously. Without any
+  // synchronization, the thread executing EventThreadFunction may set
+  // g_vsc.sent_terminated_event before the thread executing
+  // request_discontinue has had a chance to test it, in which case the latter
+  // would move ahead to issue a response to the disconnect request. Said
+  // response may get dispatched ahead of the terminated event compelling the
+  // client to terminate the debug session without consuming any console output
+  // that might've been generated by the execution of terminateCommands. So,
+  // synchronize simultaneous calls to SendTerminatedEvent.
+  static std::mutex mutex;
+  std::lock_guard<std::mutex> locker(mutex);
   if (!g_vsc.sent_terminated_event) {
     g_vsc.sent_terminated_event = true;
     g_vsc.RunTerminateCommands();
     // Send a "terminated" event
     llvm::json::Object event(CreateEventObject("terminated"));
     g_vsc.SendJSON(llvm::json::Value(std::move(event)));
   }
 }