libweston: add weston_head destroy signal

Add support for subscribing to weston_head destruction.

The primary use case for heads being destroyed arbitrarily is the
DRM-backend with MST connectors, which may disappear on unplug. It is
not just the connector becoming disconnected, it is the connector
actually disappearing.

The compositor needs to know about disappearing heads so that it has a
chance to clean up "orphaned" outputs which do get disabled but still
need destroying at run time. Shutdown would destroy them as well.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>

libweston/compositor.c

@@ -4443,6 +4443,7 @@ weston_head_init(struct weston_head *head, const char *name)
 	memset(head, 0, sizeof *head);
 
 	wl_list_init(&head->compositor_link);
+	wl_signal_init(&head->destroy_signal);
 	wl_list_init(&head->output_link);
 	wl_list_init(&head->resource_list);
 	head->name = strdup(name);
@@ -4708,6 +4709,8 @@ weston_head_detach(struct weston_head *head)
 WL_EXPORT void
 weston_head_release(struct weston_head *head)
 {
+	wl_signal_emit(&head->destroy_signal, head);
+
 	weston_head_detach(head);
 
 	free(head->make);
@@ -4900,6 +4903,48 @@ weston_head_get_output(struct weston_head *head)
 	return head->output;
 }
 
+/** Add destroy callback for a head
+ *
+ * \param head The head to watch for.
+ * \param listener The listener to add. The \c notify member must be set.
+ *
+ * Heads may get destroyed for various reasons by the backends. If a head is
+ * attached to an output, the compositor should listen for head destruction
+ * and reconfigure or destroy the output if necessary.
+ *
+ * The destroy callbacks will be called on weston_head destruction before any
+ * automatic detaching from an associated weston_output and before any
+ * weston_head information is lost.
+ *
+ * The \c data argument to the notify callback is the weston_head being
+ * destroyed.
+ */
+WL_EXPORT void
+weston_head_add_destroy_listener(struct weston_head *head,
+				 struct wl_listener *listener)
+{
+	wl_signal_add(&head->destroy_signal, listener);
+}
+
+/** Look up destroy listener for a head
+ *
+ * \param head The head to query.
+ * \param notify The notify function used used for the added destroy listener.
+ * \return The listener, or NULL if not found.
+ *
+ * This looks up the previously added destroy listener struct based on the
+ * notify function it has. The listener can be used to access user data
+ * through \c container_of().
+ *
+ * \sa wl_signal_get()
+ */
+WL_EXPORT struct wl_listener *
+weston_head_get_destroy_listener(struct weston_head *head,
+				 wl_notify_func_t notify)
+{
+	return wl_signal_get(&head->destroy_signal, notify);
+}
+
 /* Move other outputs when one is resized so the space remains contiguous. */
 static void
 weston_compositor_reflow_outputs(struct weston_compositor *compositor,

libweston/compositor.h

@@ -155,6 +155,7 @@ enum dpms_enum {
 struct weston_head {
 	struct weston_compositor *compositor;	/**< owning compositor */
 	struct wl_list compositor_link;	/**< in weston_compositor::head_list */
+	struct wl_signal destroy_signal;	/**< destroy callbacks */
 
 	struct weston_output *output;	/**< the output driving this head */
 	struct wl_list output_link;	/**< in weston_output::head_list */
@@ -2052,6 +2053,14 @@ weston_head_get_output(struct weston_head *head);
 void
 weston_head_detach(struct weston_head *head);
 
+void
+weston_head_add_destroy_listener(struct weston_head *head,
+				 struct wl_listener *listener);
+
+struct wl_listener *
+weston_head_get_destroy_listener(struct weston_head *head,
+				 wl_notify_func_t notify);
+
 struct weston_head *
 weston_compositor_iterate_heads(struct weston_compositor *compositor,
 				struct weston_head *iter);