Wait for main thread to finish before to exit

Currently after a 'quit' command UI thread raises stop
signal, exits from uci_loop() and calls Threads.exit()
while the search threads are still active.

In Threads.exit() main thread is asked to terminate, but
if it is parked in idle_loop() it will exit and free its
resources (in particular the shared Movepicker object) while
sibling slaves are still active and this leads to a crash.

The fix is to let the UI thread always wait for main thread
to finish the search before to return from uci_loop().

Found by Valgrind when running with 8 threads.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/thread.cpp

@@ -197,7 +197,7 @@ void ThreadsManager::exit() {
 
   for (int i = 0; i <= MAX_THREADS; i++)
   {
-      threads[i].do_terminate = true;
+      threads[i].do_terminate = true; // Search must be already finished
       threads[i].wake_up();
 
       // Wait for thread termination
@@ -458,6 +458,27 @@ void ThreadsManager::start_thinking(const Position& pos, const LimitsType& limit
 }
 
 
+// ThreadsManager::stop_thinking() is used by UI thread to raise a stop request
+// and to wait for the main thread finishing the search. Needed to wait exiting
+// and terminate the threads after a 'quit' command.
+
+void ThreadsManager::stop_thinking() {
+
+  Thread& main = threads[0];
+
+  Search::Signals.stop = true;
+
+  lock_grab(&main.sleepLock);
+
+  cond_signal(&main.sleepCond); // In case is waiting for stop or ponderhit
+
+  while (!main.do_sleep)
+      cond_wait(&sleepCond, &main.sleepLock);
+
+  lock_release(&main.sleepLock);
+}
+
+
 // ThreadsManager::wait_for_stop_or_ponderhit() is called when the maximum depth
 // is reached while the program is pondering. The point is to work around a wrinkle
 // in the UCI protocol: When pondering, the engine is not allowed to give a