rtos: Return an error when a Thread is re-used

Calling Thread::start multiple times leads to undefined behavior since
the Thread class was not designed to handle being restarted.  Return an
error code if Thread::start is called a second time to prevent this
behavior.

rtos/Thread.cpp

@@ -44,6 +44,7 @@ namespace rtos {
 void Thread::constructor(osPriority priority,
         uint32_t stack_size, unsigned char *stack_pointer) {
     _tid = 0;
+    _finished = false;
     _dynamic_stack = (stack_pointer == NULL);
 
 #if defined(__MBED_CMSIS_RTOS_CA9) || defined(__MBED_CMSIS_RTOS_CM)
@@ -74,7 +75,7 @@ void Thread::constructor(Callback<void()> task,
 osStatus Thread::start(Callback<void()> task) {
     _mutex.lock();
 
-    if (_tid != 0) {
+    if ((_tid != 0) || _finished) {
         _mutex.unlock();
         return osErrorParameter;
     }
@@ -117,6 +118,7 @@ osStatus Thread::terminate() {
     osThreadId local_id = _tid;
     _join_sem.release();
     _tid = (osThreadId)NULL;
+    _finished = true;
 
     ret = osThreadTerminate(local_id);
 
@@ -367,6 +369,7 @@ void Thread::_thunk(const void * thread_ptr)
     t->_task();
     t->_mutex.lock();
     t->_tid = (osThreadId)NULL;
+    t->_finished = true;
     t->_join_sem.release();
     // rtos will release the mutex automatically
 }

rtos/Thread.h

@@ -197,6 +197,7 @@ public:
     /** Starts a thread executing the specified function.
       @param   task           function to be executed by this thread.
       @return  status code that indicates the execution status of the function.
+      @note a thread can only be started once
     */
     osStatus start(mbed::Callback<void()> task);
 
@@ -344,9 +345,10 @@ private:
     mbed::Callback<void()> _task;
     osThreadId _tid;
     osThreadDef_t _thread_def;
-    bool _dynamic_stack;
     Semaphore _join_sem;
     Mutex _mutex;
+    bool _dynamic_stack;
+    bool _finished;
 };
 
 }