libei: implement ei_peek_event()

At least for the test suite, I'll need something that allows me to look at an
event without removing it from the event queue. Just the event type isn't
enough, I need things like "what are the caps on the device". Simplest way to
allow this is a peek function that doesn't remove the event from the queue.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

src/libei.c

@@ -99,6 +99,8 @@ ei_event_destroy(struct ei_event *event)
 
 static
 OBJECT_IMPLEMENT_INIT(ei_event);
+static
+OBJECT_IMPLEMENT_REF(ei_event);
 _public_
 OBJECT_IMPLEMENT_UNREF(ei_event);
 _public_
@@ -468,6 +470,16 @@ ei_get_event(struct ei *ei)
 	return e;
 }
 
+_public_ struct ei_event*
+ei_peek_event(struct ei *ei)
+{
+	if (list_empty(&ei->event_queue))
+		return NULL;
+
+	struct ei_event *e = list_first_entry(&ei->event_queue, e, link);
+	return ei_event_ref(e);
+}
+
 _public_ enum ei_event_type
 ei_next_event_type(struct ei *ei)
 {

src/libei.h

@@ -175,6 +175,25 @@ ei_dispatch(struct ei *ei);
 struct ei_event *
 ei_get_event(struct ei *ei);
 
+/**
+ * Returns the next event in the internal event queue (or NULL) without
+ * removing that event from the queue, i.e. the next call to ei_get_event()
+ * will return that same event.
+ *
+ * This call is useful for checking whether there is an event and/or what
+ * type of event it is.
+ *
+ * Repeated calls to ei_peek_event() return the same event.
+ *
+ * The returned event is refcounted, use ei_event_unref() to drop the
+ * reference.
+ *
+ * A caller must not call ei_get_event() while holding a ref to an event
+ * returned by ei_peek_event().
+ */
+struct ei_event *
+ei_peek_event(struct ei *ei);
+
 /**
  * Return the type of the next event in the event queue or @ref
  * EI_EVENT_NONE if none are available. This function does not remove the