opensl: Attempt to avoid a race with active callbacks during shutdown.

Based on examining crash dumps and investigating OpenSL workarounds in
liboboe, there is a potential race between active buffer callbacks and
stream shutdown, resulting in a buffer callback attempting to use a new
buffer after stream destroy has freed the backing allocation.

Ideally there would be a concrete event to watch for to ensure the
last callbacks have completed before destroying the stream, but I'm not
aware of one and the existing workaround in liboboe relies on an
arbitrary ("long enough") sleep between stopping and destroying streams.

src/cubeb_opensl.c

@@ -1655,6 +1655,16 @@ opensl_stream_destroy(cubeb_stream * stm)
 {
   assert(stm->draining || stm->shutdown);
 
+  // If we're still draining at stream destroy time, pause the streams now so we
+  // can destroy them safely.
+  if (stm->draining) {
+    opensl_stream_stop(stm);
+  }
+  // Sleep for 10ms to give active streams time to pause so that no further
+  // buffer callbacks occur.  Inspired by the same workaround (sleepBeforeClose)
+  // in liboboe.
+  usleep(10 * 1000);
+
   if (stm->playerObj) {
     (*stm->playerObj)->Destroy(stm->playerObj);
     stm->playerObj = NULL;