delete_refs(): bail early if the packed-refs file cannot be rewritten

If we fail to delete the doomed references from the packed-refs file,
then it is unsafe to delete their loose references, because doing so
might expose a value from the packed-refs file that is obsolete and
perhaps even points at an object that has been garbage collected.

So if repack_without_refs() fails, emit a more explicit error message
and bail.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -2835,9 +2835,26 @@ int delete_refs(struct string_list *refnames)
 	struct strbuf err = STRBUF_INIT;
 	int i, result = 0;
 
-	if (repack_without_refs(refnames, &err))
-		result |= error("%s", err.buf);
-	strbuf_release(&err);
+	if (!refnames->nr)
+		return 0;
+
+	result = repack_without_refs(refnames, &err);
+	if (result) {
+		/*
+		 * If we failed to rewrite the packed-refs file, then
+		 * it is unsafe to try to remove loose refs, because
+		 * doing so might expose an obsolete packed value for
+		 * a reference that might even point at an object that
+		 * has been garbage collected.
+		 */
+		if (refnames->nr == 1)
+			error(_("could not delete reference %s: %s"),
+			      refnames->items[0].string, err.buf);
+		else
+			error(_("could not delete references: %s"), err.buf);
+
+		goto out;
+	}
 
 	for (i = 0; i < refnames->nr; i++) {
 		const char *refname = refnames->items[i].string;
@@ -2846,6 +2863,8 @@ int delete_refs(struct string_list *refnames)
 			result |= error(_("could not remove reference %s"), refname);
 	}
 
+out:
+	strbuf_release(&err);
 	return result;
 }