[lldb] Protect the selected frame idx in StackFrameList

Protected m_selected_frame_idx using a mutex.

Author: JDevlieghere

lldb/include/lldb/Target/StackFrameList.h

@@ -174,6 +174,11 @@ class StackFrameList {
   /// change the frame if this is the first time GetSelectedFrame is called.
   std::optional<uint32_t> m_selected_frame_idx;
 
+  /// Protect access to m_selected_frame_idx. Always acquire after m_list_mutex
+  /// to avoid lock inversion. A recursive mutex because GetSelectedFrameIndex
+  /// may indirectly call SetSelectedFrame.
+  std::recursive_mutex m_selected_frame_mutex;
+
   /// The number of concrete frames fetched while filling the frame list. This
   /// is only used when synthetic frames are enabled.
   uint32_t m_concrete_frames_fetched;

lldb/source/Target/StackFrameList.cpp

@@ -783,6 +783,8 @@ void StackFrameList::SelectMostRelevantFrame() {
 
 uint32_t
 StackFrameList::GetSelectedFrameIndex(SelectMostRelevant select_most_relevant) {
+  std::lock_guard<std::recursive_mutex> guard(m_selected_frame_mutex);
+
   if (!m_selected_frame_idx && select_most_relevant)
     SelectMostRelevantFrame();
   if (!m_selected_frame_idx) {
@@ -798,6 +800,8 @@ StackFrameList::GetSelectedFrameIndex(SelectMostRelevant select_most_relevant) {
 
 uint32_t StackFrameList::SetSelectedFrame(lldb_private::StackFrame *frame) {
   std::shared_lock<std::shared_mutex> guard(m_list_mutex);
+  std::lock_guard<std::recursive_mutex> selected_frame_guard(
+      m_selected_frame_mutex);
 
   const_iterator pos;
   const_iterator begin = m_frames.begin();
@@ -851,6 +855,8 @@ void StackFrameList::Clear() {
   std::unique_lock<std::shared_mutex> guard(m_list_mutex);
   m_frames.clear();
   m_concrete_frames_fetched = 0;
+  std::lock_guard<std::recursive_mutex> selected_frame_guard(
+      m_selected_frame_mutex);
   m_selected_frame_idx.reset();
 }