--- a/mercurial/cext/manifest.c	Tue Nov 06 11:12:56 2018 -0500
+++ b/mercurial/cext/manifest.c	Mon Nov 12 20:35:22 2018 -0500
@@ -38,6 +38,7 @@
 #define MANIFEST_OOM -1
 #define MANIFEST_NOT_SORTED -2
 #define MANIFEST_MALFORMED -3
+#define MANIFEST_BOGUS_FILENAME -4
 
 /* get the length of the path for a line */
 static size_t pathlen(line *l)
@@ -115,7 +116,13 @@
 	char *prev = NULL;
 	while (len > 0) {
 		line *l;
-		char *next = memchr(data, '\n', len);
+		char *next;
+		if (*data == '\0') {
+			/* It's implausible there's no filename, don't
+			 * even bother looking for the newline. */
+			return MANIFEST_BOGUS_FILENAME;
+		}
+		next = memchr(data, '\n', len);
 		if (!next) {
 			return MANIFEST_MALFORMED;
 		}
@@ -190,6 +197,11 @@
 		PyErr_Format(PyExc_ValueError,
 			     "Manifest did not end in a newline.");
 		break;
+	case MANIFEST_BOGUS_FILENAME:
+		PyErr_Format(
+			PyExc_ValueError,
+			"Manifest had an entry with a zero-length filename.");
+		break;
 	default:
 		PyErr_Format(PyExc_ValueError,
 			     "Unknown problem parsing manifest.");