bundle-uri: limit recursion depth for bundle lists

The next change will start allowing us to parse bundle lists that are
downloaded from a provided bundle URI. Those lists might point to other
lists, which could proceed to an arbitrary depth (and even create
cycles). Restructure fetch_bundle_uri() to have an internal version that
has a recursion depth. Compare that to a new max_bundle_uri_depth
constant that is twice as high as we expect this depth to be for any
legitimate use of bundle list linking.

We can consider making max_bundle_uri_depth a configurable value if
there is demonstrated value in the future.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bundle-uri.c

@@ -334,11 +334,25 @@ static int unbundle_from_file(struct repository *r, const char *file)
 	return result;
 }
 
-int fetch_bundle_uri(struct repository *r, const char *uri)
+/**
+ * This limits the recursion on fetch_bundle_uri_internal() when following
+ * bundle lists.
+ */
+static int max_bundle_uri_depth = 4;
+
+static int fetch_bundle_uri_internal(struct repository *r,
+				     const char *uri,
+				     int depth)
 {
 	int result = 0;
 	char *filename;
 
+	if (depth >= max_bundle_uri_depth) {
+		warning(_("exceeded bundle URI recursion limit (%d)"),
+			max_bundle_uri_depth);
+		return -1;
+	}
+
 	if (!(filename = find_temp_filename())) {
 		result = -1;
 		goto cleanup;
@@ -366,6 +380,11 @@ cleanup:
 	return result;
 }
 
+int fetch_bundle_uri(struct repository *r, const char *uri)
+{
+	return fetch_bundle_uri_internal(r, uri, 0);
+}
+
 /**
  * General API for {transport,connect}.c etc.
  */