diff --git a/mercurial/state.py b/mercurial/state.py
--- a/mercurial/state.py
+++ b/mercurial/state.py
@@ -22,6 +22,7 @@
 from .thirdparty import cbor
 
 from . import (
+    error,
     util,
 )
 
@@ -91,3 +92,23 @@
     def exists(self):
         """check whether the state file exists or not"""
         return self._repo.vfs.exists(self.fname)
+
+oldstatefilefns = {}
+
+def readoldstatefile(path):
+    """decorator for registering function which can read the old format of state
+    files.
+
+    Using the state class defined above, all the state files will be using cbor
+    to serialize data to write to disk. But a user can update hg while in
+    conflict. This decorator registers function for reading old statefiles in
+    such cases."""
+    def dec(func):
+        """func here should accept the file object of the state file and
+        should return a dict of data stored in state file."""
+        if path in oldstatefilefns:
+            raise error.ProgrammingError("duplicate entry for reading old %s"
+                                         " found" % path)
+        oldstatefilefns[path] = func
+        return func
+    return dec