[lldb] Flush the global thread pool in Debugger::Terminate

Use the Initialize/Terminate pattern for the global thread pool to make
sure it gets flushed during teardown.

Differential revision: https://reviews.llvm.org/D131407

Author: JDevlieghere

lldb/source/Core/Debugger.cpp

@@ -104,6 +104,7 @@ static std::recursive_mutex *g_debugger_list_mutex_ptr =
     nullptr; // NOTE: intentional leak to avoid issues with C++ destructor chain
 static DebuggerList *g_debugger_list_ptr =
     nullptr; // NOTE: intentional leak to avoid issues with C++ destructor chain
+static llvm::ThreadPool *g_thread_pool = nullptr;
 
 static constexpr OptionEnumValueElement g_show_disassembly_enum_values[] = {
     {
@@ -538,13 +539,19 @@ void Debugger::Initialize(LoadPluginCallbackType load_plugin_callback) {
          "Debugger::Initialize called more than once!");
   g_debugger_list_mutex_ptr = new std::recursive_mutex();
   g_debugger_list_ptr = new DebuggerList();
+  g_thread_pool = new llvm::ThreadPool(llvm::optimal_concurrency());
   g_load_plugin_callback = load_plugin_callback;
 }
 
 void Debugger::Terminate() {
   assert(g_debugger_list_ptr &&
          "Debugger::Terminate called without a matching Debugger::Initialize!");
 
+  if (g_thread_pool) {
+    // The destructor will wait for all the threads to complete.
+    delete g_thread_pool;
+  }
+
   if (g_debugger_list_ptr && g_debugger_list_mutex_ptr) {
     // Clear our global list of debugger objects
     {
@@ -2005,11 +2012,7 @@ Status Debugger::RunREPL(LanguageType language, const char *repl_options) {
 }
 
 llvm::ThreadPool &Debugger::GetThreadPool() {
-  // NOTE: intentional leak to avoid issues with C++ destructor chain
-  static llvm::ThreadPool *g_thread_pool = nullptr;
-  static llvm::once_flag g_once_flag;
-  llvm::call_once(g_once_flag, []() {
-    g_thread_pool = new llvm::ThreadPool(llvm::optimal_concurrency());
-  });
+  assert(g_thread_pool &&
+         "Debugger::GetThreadPool called before Debugger::Initialize");
   return *g_thread_pool;
 }