Avoid forming deadlock when steam drains upon device switching

This solves the deadlock mentioned in BMO 1572273 comment 6. It's very
likely to have a deadlock when the output callback tries to stop the
input AudioUnit when the input device is unplugged during the duplex
stream is working.

When the input device is unplugged, the stream will be reinitialized and
the AudioUnits will be stopped then. Hence we could only feed silence to
the output buffer and let stream reinit stop the AudioUnits.

Author: ChunMinChang

src/cubeb_audiounit.cpp

@@ -612,13 +612,20 @@ audiounit_output_callback(void * user_ptr,
   }
 
   if (stm->draining) {
-    OSStatus r = AudioOutputUnitStop(stm->output_unit);
-    assert(r == 0);
-    if (stm->input_unit) {
-      r = AudioOutputUnitStop(stm->input_unit);
+    if (stm->reinit_pending) {
+      ALOG("(%p) stream is reinitializing.", stm);
+      // The stream reinit will stop the AudioUnits anyway.
+    } else {
+      ALOG("(%p) stop the AudioUnits.", stm);
+      OSStatus r = AudioOutputUnitStop(stm->output_unit);
       assert(r == 0);
+      if (stm->input_unit) {
+        r = AudioOutputUnitStop(stm->input_unit);
+        assert(r == 0);
+      }
+      stm->state_callback(stm, stm->user_ptr, CUBEB_STATE_DRAINED);
     }
-    stm->state_callback(stm, stm->user_ptr, CUBEB_STATE_DRAINED);
+    ALOG("(%p) stream is draining. feed silence.", stm);
     audiounit_make_silent(&outBufferList->mBuffers[0]);
     return noErr;
   }