lib: add rb_uri_is_descendant

This checks if one canonical URI is a descendant of another, that is,
if removing some number of path components from the first URI would
result in the second.

lib/rb-file-helpers.c

@@ -1483,6 +1483,48 @@ rb_file_find_extant_parent (GFile *file)
 	return file;
 }
 
+/**
+ * rb_uri_is_descendant:
+ * @uri: URI to check
+ * @ancestor: a URI to check against
+ *
+ * Checks if @uri refers to a path beneath @ancestor, such that removing some number
+ * of path segments of @uri would result in @ancestor.
+ * It doesn't do any filesystem operations, it just looks at the URIs as strings.
+ * The URI strings should be built by looking at a filesystem rather than user input,
+ * and must not have path segments that are empty (multiple slashes) or '.' or '..'.
+ *
+ * Given this input, checking if one URI is a descendant of another is pretty simple.
+ * A descendant URI must have the ancestor as a prefix, and if the ancestor ends with
+ * a slash, there must be at least one character after that, otherwise the following
+ * character must be a slash with at least one character after it.
+ *
+ * Return value: %TRUE if @uri is a descendant of @ancestor
+ */
+gboolean
+rb_uri_is_descendant (const char *uri, const char *ancestor)
+{
+	int len;
+
+	if (g_str_has_prefix (uri, ancestor) == FALSE)
+		return FALSE;
+
+	len = strlen(ancestor);
+	if (ancestor[len - 1] == '/') {
+		/*
+		 * following character in uri must be a new path segment, not
+		 * the end of the uri.  not considering multiple slashes here.
+		 */
+		return (uri[len] != '\0');
+	} else {
+		/*
+		 * following character in uri must be a separator, with something after it.
+		 * not considering multiple slashes here.
+		 */
+		return ((uri[len] == '/') && strlen(uri) > (len + 1));
+	}
+}
+
 /**
  * rb_uri_get_filesystem_type:
  * @uri: URI to get filesystem type for

lib/rb-file-helpers.h

@@ -60,6 +60,7 @@ gboolean	rb_uri_is_writable	(const char *uri);
 gboolean	rb_uri_is_local		(const char *uri);
 gboolean	rb_uri_is_hidden	(const char *uri);
 gboolean	rb_uri_could_be_podcast (const char *uri, gboolean *is_opml);
+gboolean	rb_uri_is_descendant	(const char *uri, const char *ancestor);
 char *		rb_uri_make_hidden      (const char *uri);
 char *		rb_uri_get_dir_name	(const char *uri);
 char *		rb_uri_get_short_path_name (const char *uri);

tests/test-file-helpers.c

@@ -100,6 +100,22 @@ START_TEST (test_rb_check_dir_has_space)
 }
 END_TEST
 
+START_TEST (test_rb_uri_is_descendant)
+{
+	ck_assert (rb_uri_is_descendant ("file:///tmp", "file:///"));
+	ck_assert (rb_uri_is_descendant ("file:///tmp/2", "file:///"));
+	ck_assert (rb_uri_is_descendant ("file:///tmp/2", "file:///tmp"));
+	ck_assert (rb_uri_is_descendant ("file:///tmp/2", "file:///tmp/"));
+	ck_assert (rb_uri_is_descendant ("file:///tmp/", "file:///tmp") == FALSE);
+	ck_assert (rb_uri_is_descendant ("file:///tmp", "file:///tmp/") == FALSE);
+	ck_assert (rb_uri_is_descendant ("file:///tmp/", "file:///tmp/") == FALSE);
+	ck_assert (rb_uri_is_descendant ("file:///tmp", "file:///tmp") == FALSE);
+	ck_assert (rb_uri_is_descendant ("file:///tmp2", "file:///tmp") == FALSE);
+	ck_assert (rb_uri_is_descendant ("file:///tmp/2", "file:///tmp2") == FALSE);
+	ck_assert (rb_uri_is_descendant ("file:///tmp/22", "file:///tmp/2") == FALSE);
+}
+END_TEST
+
 static Suite *
 rb_file_helpers_suite ()
 {
@@ -110,6 +126,7 @@ rb_file_helpers_suite ()
 
 	tcase_add_test (tc_chain, test_rb_uri_get_short_path_name);
 	tcase_add_test (tc_chain, test_rb_check_dir_has_space);
+	tcase_add_test (tc_chain, test_rb_uri_is_descendant);
 
 	return s;
 }