[droidcamsrc] call droid_media_camera_take_picture from another thread

take_picture call in HAL sometimes need to read information from preview
frames for e.g. auto exposure when using flash. If the calling thread
also handles pulling preview frames, this will create a deadlock.

To prevent this, I've moved the actual droid_media_camera_take_picture()
call to a function that will be called in a background thread. The
GstTaskPool is used to execute the function in the glib's thread pool,
so that the new thread doesn't have to be created every time.

This fixed taking a picture with the flash on on Fairphone 2 with
Halium 7.1-based port and Ubuntu Touch.

gst/droidcamsrc/gstdroidcamsrcdev.c

@@ -48,6 +48,7 @@ GST_DEBUG_CATEGORY_EXTERN (gst_droid_camsrc_debug);
 
 struct _GstDroidCamSrcImageCaptureState
 {
+  gboolean running;
   gboolean image_preview_sent;
   gboolean image_start_sent;
 };
@@ -542,6 +543,8 @@ gst_droidcamsrc_dev_new (GstDroidCamSrcPad * vfsrc,
 
   dev->viewfinder_format = GST_VIDEO_FORMAT_UNKNOWN;
 
+  dev->task_pool = gst_task_pool_new ();
+
   return dev;
 }
 
@@ -646,6 +649,9 @@ gst_droidcamsrc_dev_destroy (GstDroidCamSrcDev * dev)
 
   gst_droidcamsrc_recorder_destroy (dev->recorder);
 
+  gst_task_pool_cleanup (dev->task_pool);       // Just in case
+  gst_object_unref (dev->task_pool);
+
   g_slice_free (GstDroidCamSrcImageCaptureState, dev->img);
   g_slice_free (GstDroidCamSrcVideoCaptureState, dev->vid);
   g_slice_free (GstDroidCamSrcDev, dev);
@@ -655,6 +661,8 @@ gst_droidcamsrc_dev_destroy (GstDroidCamSrcDev * dev)
 gboolean
 gst_droidcamsrc_dev_init (GstDroidCamSrcDev * dev)
 {
+  GError *err = NULL;
+
   GST_DEBUG ("dev init");
 
   g_rec_mutex_lock (dev->lock);
@@ -690,6 +698,13 @@ gst_droidcamsrc_dev_init (GstDroidCamSrcDev * dev)
 
   g_rec_mutex_unlock (dev->lock);
 
+  gst_task_pool_prepare (dev->task_pool, &err);
+  if (err != NULL) {
+    GST_ERROR ("Failed to prepare thread pool: %s", err->message);
+    g_error_free (err);
+    return FALSE;
+  }
+
   return TRUE;
 }
 
@@ -706,6 +721,8 @@ gst_droidcamsrc_dev_deinit (GstDroidCamSrcDev * dev)
   }
 
   g_rec_mutex_unlock (dev->lock);
+
+  gst_task_pool_cleanup (dev->task_pool);
 }
 
 gboolean
@@ -832,22 +849,58 @@ gst_droidcamsrc_dev_set_params (GstDroidCamSrcDev * dev)
   return ret;
 }
 
+static void
+_take_picture (gpointer user_data)
+{
+  GstDroidCamSrcDev *dev = (GstDroidCamSrcDev *) user_data;
+  GstDroidCamSrc *src = GST_DROIDCAMSRC (GST_PAD_PARENT (dev->imgsrc->pad));
+
+  const int msg_type = dev->c.CAMERA_MSG_SHUTTER | dev->c.CAMERA_MSG_RAW_IMAGE
+      | dev->c.CAMERA_MSG_POSTVIEW_FRAME | dev->c.CAMERA_MSG_COMPRESSED_IMAGE;
+
+  GST_DEBUG ("really calling droid_media_camera_take_picture");
+
+  g_rec_mutex_lock (dev->lock);
+
+  if (!droid_media_camera_take_picture (dev->cam, msg_type)) {
+    GST_ERROR ("error capturing image");
+    GST_ELEMENT_ERROR (src, LIBRARY, FAILED, (NULL), ("take_picture fails"));
+  }
+
+  dev->img->running = FALSE;
+
+  g_rec_mutex_unlock (dev->lock);
+}
+
 gboolean
 gst_droidcamsrc_dev_capture_image (GstDroidCamSrcDev * dev)
 {
   gboolean ret = FALSE;
-  int msg_type = dev->c.CAMERA_MSG_SHUTTER | dev->c.CAMERA_MSG_RAW_IMAGE
-      | dev->c.CAMERA_MSG_POSTVIEW_FRAME | dev->c.CAMERA_MSG_COMPRESSED_IMAGE;
+  GError *err = NULL;
 
   GST_DEBUG ("dev capture image");
 
   g_rec_mutex_lock (dev->lock);
 
+  if (dev->img->running) {
+    GST_ERROR ("another capture is already in progress (?)");
+    goto out;
+  }
+
+  dev->img->running = TRUE;
   dev->img->image_preview_sent = FALSE;
   dev->img->image_start_sent = FALSE;
 
-  if (!droid_media_camera_take_picture (dev->cam, msg_type)) {
-    GST_ERROR ("error capturing image");
+  /*
+   * Call droid_media_camera_take_picture from another thread. take_picture
+   * call in HAL sometimes need to read information from preview frames for
+   * e.g. auto exposure when using flash. If the calling thread also handles
+   * pulling preview frames, this will create a deadlock.
+   */
+  gst_task_pool_push (dev->task_pool, _take_picture, dev, &err);
+  if (err) {
+    GST_ERROR ("Failed to start take_picture task: %s", err->message);
+    g_error_free (err);
     goto out;
   }
 

gst/droidcamsrc/gstdroidcamsrcdev.h

@@ -58,6 +58,8 @@ struct _GstDroidCamSrcDev
 
   gboolean use_recorder;
   GstDroidCamSrcRecorder *recorder;
+
+  GstTaskPool *task_pool;
 };
 
 GstDroidCamSrcDev *gst_droidcamsrc_dev_new (GstDroidCamSrcPad *vfsrc,