--- a/mercurial/cext/revlog.c	Sun Dec 02 21:41:24 2018 +0900
+++ b/mercurial/cext/revlog.c	Sun Dec 02 22:10:37 2018 +0900
@@ -194,6 +194,33 @@
 	return 0;
 }
 
+/*
+ * Get parents of the given rev.
+ *
+ * If the specified rev is out of range, IndexError will be raised. If the
+ * revlog entry is corrupted, ValueError may be raised.
+ *
+ * Returns 0 on success or -1 on failure.
+ */
+int HgRevlogIndex_GetParents(PyObject *op, int rev, int *ps)
+{
+	int tiprev;
+	if (!op || !HgRevlogIndex_Check(op) || !ps) {
+		PyErr_BadInternalCall();
+		return -1;
+	}
+	tiprev = (int)index_length((indexObject *)op) - 1;
+	if (rev < -1 || rev > tiprev) {
+		PyErr_Format(PyExc_IndexError, "rev out of range: %d", rev);
+		return -1;
+	} else if (rev == -1) {
+		ps[0] = ps[1] = -1;
+		return 0;
+	} else {
+		return index_get_parents((indexObject *)op, rev, ps, tiprev);
+	}
+}
+
 static inline int64_t index_get_start(indexObject *self, Py_ssize_t rev)
 {
 	uint64_t offset;

--- a/mercurial/cext/revlog.h	Sun Dec 02 21:41:24 2018 +0900
+++ b/mercurial/cext/revlog.h	Sun Dec 02 22:10:37 2018 +0900
@@ -12,4 +12,8 @@
 
 extern PyTypeObject HgRevlogIndex_Type;
 
+#define HgRevlogIndex_Check(op) PyObject_TypeCheck(op, &HgRevlogIndex_Type)
+
+int HgRevlogIndex_GetParents(PyObject *op, int rev, int *ps);
+
 #endif /* _HG_REVLOG_H_ */