Fix locking in waitForEvent() and sample playing

libs/core/linux.cpp

@@ -269,11 +269,13 @@ void waitForEvent(int source, int value) {
             pthread_mutex_lock(&eventMutex);
             t->waitSource = source;
             t->waitValue = value;
+            stopUser();
             // spourious wake ups may occur they say
             while (t->waitSource) {
                 pthread_cond_wait(&t->waitCond, &eventMutex);
             }
             pthread_mutex_unlock(&eventMutex);
+            startUser();
             return;
         }
     }

libs/core/pxt.h

@@ -7,6 +7,8 @@ namespace pxt {
 void raiseEvent(int id, int event);
 int allocateNotifyEvent();
 void sleep_core_us(uint64_t us);
+void startUser();
+void stopUser();
 
 class Button;
 typedef Button *Button_;

libs/music/music.cpp

@@ -119,6 +119,7 @@ void playSample(Buffer buf) {
         pthread_detach(pid);
     }
 
+    stopUser();
     pthread_mutex_lock(&pumpMutex);
     *lmsSoundMMap = 1; // BUSY
     uint8_t cmd[] = {SOUND_CMD_PLAY, (uint8_t)((currVolume / 33) + 1)};
@@ -130,6 +131,7 @@ void playSample(Buffer buf) {
     pumpMusic();
     pthread_cond_wait(&sampleDone, &pumpMutex);
     pthread_mutex_unlock(&pumpMutex);
+    startUser();
 }
 
 /**