audio/alsa: Exit write loop if underlying device is already paused.

If the alsasink thread starts the write loop but another thread pauses
the underlying alsa device, the sink thread will endlessly loop.

snd_pcm_writei() will return 0 if the state is SND_PCM_STATE_PAUSED
and the loop will never make any progress.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1097>

ext/alsa/gstalsasink.c

@@ -1089,6 +1089,10 @@ gst_alsasink_write (GstAudioSink * asink, gpointer data, guint length)
         goto write_error;
       }
       continue;
+    } else if (err == 0 && alsa->hw_support_pause) {
+      /* We might be already paused, if so, just bail */
+      if (snd_pcm_state (alsa->handle) == SND_PCM_STATE_PAUSED)
+        break;
     }
 
     ptr += snd_pcm_frames_to_bytes (alsa->handle, err);

gst-libs/gst/audio/gstaudiosink.c

@@ -254,6 +254,9 @@ audioringbuffer_thread_func (GstAudioRingBuffer * buf)
               GST_DEBUG_FUNCPTR_NAME (writefunc),
               (errno > 1 ? g_strerror (errno) : "unknown"), left, written);
           break;
+        } else if (written == 0 && G_UNLIKELY (g_atomic_int_get (&buf->state) !=
+                GST_AUDIO_RING_BUFFER_STATE_STARTED)) {
+          break;
         }
         left -= written;
         readptr += written;