Merge pull request llvm#11914 from swiftlang/cherry-pick-#163821

[lldb-dap] Fix mutex acquisition order for modules event. (llvm#163821)

Author: adrian-prantl

lldb/tools/lldb-dap/DAP.cpp

@@ -27,6 +27,7 @@
 #include "lldb/API/SBEvent.h"
 #include "lldb/API/SBLanguageRuntime.h"
 #include "lldb/API/SBListener.h"
+#include "lldb/API/SBMutex.h"
 #include "lldb/API/SBProcess.h"
 #include "lldb/API/SBStream.h"
 #include "lldb/Utility/IOObject.h"
@@ -1357,7 +1358,11 @@ void DAP::EventThread() {
           const bool remove_module =
               event_mask & lldb::SBTarget::eBroadcastBitModulesUnloaded;
 
-          std::lock_guard<std::mutex> guard(modules_mutex);
+          // NOTE: Both mutexes must be acquired to prevent deadlock when
+          // handling `modules_request`, which also requires both locks.
+          lldb::SBMutex api_mutex = GetAPIMutex();
+          const std::scoped_lock<lldb::SBMutex, std::mutex> guard(
+              api_mutex, modules_mutex);
           for (uint32_t i = 0; i < num_modules; ++i) {
             lldb::SBModule module =
                 lldb::SBTarget::GetModuleAtIndexFromEvent(i, event);