MT#62181 fix join race condition

In some cases it can happen that ::join() is called from multiple
threads at the same time. Checking the ::joinable() flag is not
race-free and so is unreliable to make sure that only one other thread
attempts to join at any given time. Add a mutex and a state variable to
make sure only one thread attempts to join, and make any other threads
wait.

Change-Id: I02fd236a416b035c98642535f1521be4a3a63fd9

core/AmThread.cpp

@@ -82,9 +82,18 @@ void AmThread::stop()
 
 void AmThread::join()
 {
-  // only when neither stopped nor joined
-  if (_td.joinable())
-    _td.join();
+  // don't attempt to join thread that doesn't exist
+  if (_state == state::idle)
+    return;
+
+  // make sure only one other thread joins this one. all others
+  // are made to wait through the mutex
+  std::lock_guard<std::mutex> _l(_join_mt);
+  if (!_joined) {
+    if (_td.joinable())
+      _td.join();
+    _joined = true;
+  }
 }
 
 

core/AmThread.h

@@ -122,6 +122,9 @@ class AmThread
 
   std::atomic<state> _state;
 
+  std::mutex _join_mt;
+  bool _joined;
+
   void _start();
 
 protected:
@@ -135,7 +138,8 @@ public:
   unsigned long _pid;
 
   AmThread()
-    : _state(state::idle)
+    : _state(state::idle),
+      _joined(false)
   {}
 
   virtual ~AmThread();