[lldb-dap] Send terminated event only once (#93172)

This is a regression from #91591. Turns out std::mutex does not prevent
code from running twice. 🤦

Author: labath

lldb/tools/lldb-dap/DAP.h

@@ -26,6 +26,7 @@
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/JSON.h"
+#include "llvm/Support/Threading.h"
 #include "llvm/Support/raw_ostream.h"
 
 #include "lldb/API/SBAttachInfo.h"
@@ -169,6 +170,7 @@ struct DAP {
   std::optional<llvm::json::Object> last_launch_or_attach_request;
   lldb::tid_t focus_tid;
   bool disconnecting = false;
+  llvm::once_flag terminated_event_flag;
   bool stop_at_entry;
   bool is_attach;
   bool enable_auto_variable_summaries;

lldb/tools/lldb-dap/lldb-dap.cpp

@@ -227,13 +227,12 @@ void SendContinuedEvent() {
 // debugged.
 void SendTerminatedEvent() {
   // Prevent races if the process exits while we're being asked to disconnect.
-  static std::mutex mutex;
-  std::lock_guard<std::mutex> locker(mutex);
-
-  g_dap.RunTerminateCommands();
-  // Send a "terminated" event
-  llvm::json::Object event(CreateTerminatedEventObject());
-  g_dap.SendJSON(llvm::json::Value(std::move(event)));
+  llvm::call_once(g_dap.terminated_event_flag, [&] {
+    g_dap.RunTerminateCommands();
+    // Send a "terminated" event
+    llvm::json::Object event(CreateTerminatedEventObject());
+    g_dap.SendJSON(llvm::json::Value(std::move(event)));
+  });
 }
 
 // Send a thread stopped event for all threads as long as the process