Review of attachment 235403 [details] [review]:

::: panels/background/bg-pictures-source.c
@@ +127,3 @@
+    {
+      g_object_unref (bg_source->priv->picture_dir_monitor);
+      bg_source->priv->picture_dir_monitor = NULL;

Use g_clear_object to clear the reference.

@@ +133,3 @@
+    {
+      g_object_unref (bg_source->priv->cache_dir_monitor);
+      bg_source->priv->cache_dir_monitor = NULL;

Same as above.

@@ +382,3 @@
 gboolean
 bg_pictures_source_remove (BgPicturesSource *bg_source,
+                          const char       *uri)

Alignment is slightly wrong here.

@@ +568,3 @@
+            BgPicturesSource *self)
+{
+  if (!bg_pictures_source_is_known (self, g_file_get_uri (file)))

The return value of g_file_get_uri needs to be freed.

@@ +571,3 @@
+    {
+      /* sync up the ref count so we can re-use the add_single_item code path */
+      g_object_ref (file);

Probably best to move this and the explanation up to to where add_single_item is called.

@@ +636,3 @@
+                                                        G_FILE_MONITOR_NONE,
+                                                        NULL,
+                                                        NULL);

g_file_monitor_directory can return NULL on error, so this should be checked before attempting to connect the signal.

@@ +656,3 @@
+                                                      G_FILE_MONITOR_NONE,
+                                                      NULL,
+                                                      NULL);

Same as above.

Created attachment 235695 [details] [review]
background: Monitor the Pictures and cache directory for new files

Updated patch

Review of attachment 235695 [details] [review]:

::: panels/background/bg-pictures-source.c
@@ +595,3 @@
+  switch (event_type)
+    {
+      case G_FILE_MONITOR_EVENT_CREATED:

You don't want to use created here. New files might get opened before they're done writing, and you don't check for changed.
You likely want G_FILE_MONITOR_EVENT_CHANGES_DONE_HINT

G_FILE_MONITOR_EVENT_CHANGES_DONE_HINT fires three times if a file is overwritten.

I am checking before add_single_file to see if the file exists in the "known_items" hash table, but it seems there isn't anything ever being added to that in the first place.. I could fix the inserting as well but not sure what the side effects are.

Created attachment 235810 [details] [review]
background: Monitor the Pictures and cache directory for new files

Change to using G_FILE_MONITOR_EVENT_CHANGES_DONE_HINT. This required the known_items table to be re-instated. 

There is a bit of clean up to do in bg-pictures-source which I'll open a separate bug/patch about, would be good to do this on top of this patch and #691800

Review of attachment 235810 [details] [review]:

Looks good other than that.

::: panels/background/bg-pictures-source.c
@@ +553,3 @@
+                               G_FILE_QUERY_INFO_NONE,
+                               G_PRIORITY_LOW,
+                               NULL,

Needs to use a GCancellable here, so we can cancel any queries when exiting the panel.

Created attachment 236771 [details] [review]
background: Monitor the Pictures and cache directory for new files

Added cancellable to file monitors

Review of attachment 236771 [details] [review]:

Feel free to commit after making those changes.

::: panels/background/bg-pictures-source.c
@@ +125,3 @@
     }
 
+  if (bg_source->priv->picture_dir_monitor)

You don't need to do "if..." with g_clear_object.

@@ +128,3 @@
+      g_clear_object (&bg_source->priv->picture_dir_monitor);
+
+  if (bg_source->priv->cache_dir_monitor)

Ditto.

Created attachment 237445 [details] [review]
background: Monitor the Pictures and cache directory for new files

fix up patch

Attachment 237445 [details] pushed as ce409d4 - background: Monitor the Pictures and cache directory for new files