taglist: add gst_tag_list_peek_string_index to avoid a copy

Adds a variation of the _get_string_index function that doesn't copy
the string.

API: gst_tag_list_peek_string_index

https://bugzilla.gnome.org/show_bug.cgi?id=621896

docs/gst/gstreamer-sections.txt

@@ -2255,6 +2255,7 @@ gst_tag_list_get_double
 gst_tag_list_get_double_index
 gst_tag_list_get_string
 gst_tag_list_get_string_index
+gst_tag_list_peek_string_index
 gst_tag_list_get_pointer
 gst_tag_list_get_pointer_index
 gst_tag_list_get_date

gst/gsttaglist.c

@@ -1601,6 +1601,44 @@ TAG_MERGE_FUNCS (pointer, gpointer, (*value != NULL))
  */
 TAG_MERGE_FUNCS (string, gchar *, (*value != NULL && **value != '\0'))
 
+/*
+ *FIXME 0.11: Instead of _peek (non-copy) and _get (copy), we could have
+ *            _get (non-copy) and _dup (copy) for strings, seems more
+ *            widely used
+ */
+/**
+ * gst_tag_list_peek_string_index:
+ * @list: a #GstTagList to get the tag from
+ * @tag: tag to read out
+ * @index: number of entry to read out
+ * @value: location for the result
+ *
+ * Peeks at the value that is at the given index for the given tag in the given
+ * list.
+ *
+ * The resulting string in @value will be in UTF-8 encoding and doesn't need
+ * to be freed by the caller. The returned string is also guaranteed to
+ * be non-NULL and non-empty.
+ *
+ * Returns: TRUE, if a value was set, FALSE if the tag didn't exist in the
+ *              given list.
+ */
+gboolean
+gst_tag_list_peek_string_index (const GstTagList * list,
+    const gchar * tag, guint index, const gchar ** value)
+{
+  const GValue *v;
+
+  g_return_val_if_fail (GST_IS_TAG_LIST (list), FALSE);
+  g_return_val_if_fail (tag != NULL, FALSE);
+  g_return_val_if_fail (value != NULL, FALSE);
+
+  if ((v = gst_tag_list_get_value_index (list, tag, index)) == NULL)
+    return FALSE;
+  *value = g_value_get_string (v);
+  return *value != NULL && **value != '\0';
+}
+
 /**
  * gst_tag_list_get_date:
  * @list: a #GstTagList to get the tag from

gst/gsttaglist.h

@@ -345,6 +345,10 @@ gboolean     gst_tag_list_get_string_index  (const GstTagList * list,
                                              const gchar      * tag,
                                              guint              index,
                                              gchar           ** value);
+gboolean     gst_tag_list_peek_string_index (const GstTagList * list,
+                                             const gchar      * tag,
+                                             guint              index,
+                                             const gchar     ** value);
 gboolean     gst_tag_list_get_pointer       (const GstTagList * list,
                                              const gchar      * tag,
                                              gpointer         * value);

win32/common/libgstreamer.def

@@ -991,6 +991,7 @@ EXPORTS
 	gst_tag_list_new
 	gst_tag_list_new_full
 	gst_tag_list_new_full_valist
+	gst_tag_list_peek_string_index
 	gst_tag_list_remove_tag
 	gst_tag_merge_mode_get_type
 	gst_tag_merge_strings_with_comma