--- a/mercurial/cext/revlog.c	Mon Jan 13 19:11:44 2020 -0800
+++ b/mercurial/cext/revlog.c	Tue Jan 14 12:04:12 2020 +0100
@@ -38,6 +38,7 @@
 } nodetreenode;
 
 typedef struct {
+	int abi_version;
 	int (*index_parents)(PyObject *, int, int *);
 } Revlog_CAPI;
 
@@ -3037,6 +3038,9 @@
 #endif /* WITH_RUST */
 
 static Revlog_CAPI CAPI = {
+    /* increment the abi_version field upon each change in the Revlog_CAPI
+       struct or in the ABI of the listed functions */
+    1,
     HgRevlogIndex_GetParents,
 };