parse: do delayed set only if the target child was not found and fail otherwise

When using the child proxy notation (child::property=value) it may
happen that the target child does not exist at the time of parsing
(i.e: decodebin creates the encoder according to the contents of the
stream). On this cases, we want to delay the setting of the property
to later, when new elements are added. Previous logic performed a
delayed set even if the target child was found but the property
was not found in it. This should be treated as a failure because,
unlike missing elements, properties should not appear dynamically.
By not failing, typos in property names may go unnoticed to the end
user.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2908>

subprojects/gstreamer/gst/parse/grammar.y.in

@@ -327,6 +327,33 @@ static void gst_parse_add_delayed_set (GstElement *element, gchar *name, gchar *
   }
 }
 
+static gboolean
+gst_parse_separate_prop_from_children (const gchar *name, gchar **children, gchar **property)
+{
+  static const gchar *separator = "::";
+  const gchar *prop = NULL;
+
+  g_return_val_if_fail (name, FALSE);
+  g_return_val_if_fail (children, FALSE);
+  g_return_val_if_fail (property, FALSE);
+
+  /* Given "child1::child2::prop" isolate "prop" */
+  prop = g_strrstr (name, separator);
+  if (!prop) {
+    GST_WARNING ("%s is not a valid childproxy path", name);
+    return FALSE;
+  }
+
+  /* Make a copy of prop skipping "::" */
+  *property = g_strdup (prop + 2);
+
+  /* Extract "child1::child2" from "child1::child2::prop" */
+  *children =
+    g_strndup (name, strlen (name) - strlen (prop));
+
+  return TRUE;
+}
+
 static void gst_parse_new_child(GstChildProxy *child_proxy, GObject *object,
     const gchar * name, gpointer data)
 {
@@ -367,10 +394,54 @@ static void gst_parse_new_child(GstChildProxy *child_proxy, GObject *object,
     const gchar *obj_name = GST_OBJECT_NAME(object);
     gint len = strlen (obj_name);
 
-    /* do a delayed set */
-    if ((strlen (set->name) > (len + 2)) && !strncmp (set->name, obj_name, len) && !strncmp (&set->name[len], "::", 2)) {
-      gst_parse_add_delayed_set (GST_ELEMENT(child_proxy), set->name, set->value_str);
+    /*
+     * We've been notified that a new child has beed added, but the
+     * property was still not found. Three things could be happening:
+     *
+     * 1. The target property is of the form obj_name::child_name::property
+     * and is for a child that does not (yet) exist:
+     *    We need to add the delayed property setting handler on that new child.
+     *
+     * 2. The target property is of the form obj_name::property or
+     * obj_name::child_name::property and the child already exists:
+     *    We warn about a nonexistent property
+     *
+     * 3. The target property is of the form other_obj_name::child_name::property
+     * or other_obj_name::property:
+     *    We ignore this case as another delayed set will catch it.
+     */
+
+    /* Cases 1,2: The child just added corresponds to this delayed set */
+    if ((strlen (set->name) > (len+2)) && !strncmp (set->name, obj_name, len)
+	&& !strncmp (&set->name[len], "::", 2)) {
+      gchar *children = NULL;
+      gchar *prop = NULL;
+      GObject *child = NULL;
+
+      if (!gst_parse_separate_prop_from_children (set->name, &children, &prop)) {
+	/* Malformed property name, ignore */
+	return;
+      }
+
+      child = gst_child_proxy_get_child_by_name_recurse (child_proxy, children);
+      g_free (children);
+      g_free (prop);
+
+      /* Case 1: A child in the hierarchy does not exist yet, add a new delayed set */
+      if (NULL == child) {
+	gst_parse_add_delayed_set (GST_ELEMENT (child_proxy), set->name, set->value_str);
+      }
+      /* Case 2: The target child exists already but there's no such property */
+      else {
+	gst_object_unref (child);
+	GST_ELEMENT_WARNING(GST_ELEMENT (child_proxy), PARSE, NO_SUCH_PROPERTY,
+          (_("No such property.")), (_("no property \"%s\" in element \"%s\""),
+        set->name, GST_ELEMENT_NAME(child_proxy)));
+      }
     }
+    /* Case 3: The child just added does not correspond to this delayed set, just ignore
+     * else { }
+     */
   }
 
 out:
@@ -564,8 +635,30 @@ static GstElement * gst_parse_element_make (graph_t *graph, element_t *data) {
     proxied_property_t *pp = tmp->data;
 
     if (!gst_child_proxy_lookup (GST_CHILD_PROXY (ret), pp->name, &target, &pspec)) {
-      /* do a delayed set */
-      gst_parse_add_delayed_set (ret, pp->name, pp->value);
+      /* the property was not found. if the target child doesn't exist
+         then we do a delayed set waiting for new elements to be added. If
+         the child was found, we fail since the property doesn't exist.
+      */
+      gchar *children = NULL;
+      gchar *property = NULL;
+      if (!gst_parse_separate_prop_from_children (pp->name, &children, &property)) {
+	/* malformed childproxy path, skip */
+	continue;
+      }
+
+      target = gst_child_proxy_get_child_by_name_recurse (GST_CHILD_PROXY (ret), children);
+      g_free (children);
+      g_free (property);
+
+      if (target == NULL) {
+	gst_parse_add_delayed_set (ret, pp->name, pp->value);
+      } else {
+	gst_object_unref (target);
+	SET_ERROR (graph->error, GST_PARSE_ERROR_NO_SUCH_PROPERTY,	\
+          _("no property \"%s\" in element \"%s\""), pp->name, \
+          GST_ELEMENT_NAME (ret));
+	goto done;
+      }
     } else {
       GValue v = { 0, };
 
@@ -598,6 +691,50 @@ done:
   return ret;
 }
 
+static gboolean gst_parse_child_proxy_find_child (
+    GstChildProxy * child_proxy, const gchar *name)
+{
+  gchar **names = NULL, **current = NULL;
+  GObject *obj = NULL;
+  gboolean found = FALSE;
+
+  obj = G_OBJECT (g_object_ref (child_proxy));
+
+  current = names = g_strsplit (name, "::", -1);
+
+  /* find the owner of the property */
+  while (current[1]) {
+    GObject *next = NULL;
+
+    /* Cannot ask for the child of a non-childproxy */
+    if (!GST_IS_CHILD_PROXY (obj)) {
+      break;
+    }
+
+    next = gst_child_proxy_get_child_by_name (GST_CHILD_PROXY (obj),
+        current[0]);
+
+    /* The child doesn't exist yet */
+    if (!next) {
+      break;
+    }
+
+    gst_object_unref (obj);
+    obj = next;
+    current++;
+  }
+
+  gst_object_unref (obj);
+  g_strfreev (names);
+
+  /* The remaining name is the property, we have found the object */
+  if (current[1] == NULL) {
+    found = TRUE;
+  }
+
+  return found;
+}
+
 static void gst_parse_element_set (gchar *value, GstElement *element, graph_t *graph)
 {
   GParamSpec *pspec = NULL;
@@ -613,8 +750,15 @@ static void gst_parse_element_set (gchar *value, GstElement *element, graph_t *g
 
   if (GST_IS_CHILD_PROXY (element) && strstr (value, "::") != NULL) {
     if (!gst_child_proxy_lookup (GST_CHILD_PROXY (element), value, &target, &pspec)) {
-      /* do a delayed set */
-      gst_parse_add_delayed_set (element, value, pos);
+      /* the property was not found. if the target child doesn't exist
+         then we do a delayed set waiting for new elements to be added. If
+         the child was found, we fail since the property doesn't exist.
+      */
+      if (!gst_parse_child_proxy_find_child (GST_CHILD_PROXY (element), value)) {
+	gst_parse_add_delayed_set (element, value, pos);
+      } else {
+	goto error;
+      }
     }
   } else {
     pspec = g_object_class_find_property (G_OBJECT_GET_CLASS (element), value);