state: write the version number in plain text on top of state files

We will soon be using CBOR format to write the data in state files. But we
should not write the version number of the state files in CBOR format and we
should rather write it in plain text because in future we can change the format
of state files and we should be able to parse the version number of state file
without requiring to understand a certain format.

This will help us in making sure we have a good compatibility story with other
versions of state files.

Differential Revision: https://phab.mercurial-scm.org/D3579

mercurial/state.py

@@ -22,6 +22,7 b' from __future__ import absolute_import'
 from .thirdparty import cbor
 
 from . import (
+    error,
     util,
 )
 
@@ -51,18 +52,28 b' class cmdstate(object):'
         """read the existing state file and return a dict of data stored"""
         return self._read()
 
-    def save(self, data):
+    def save(self, version, data):
         """write all the state data stored to .hg/<filename> file
 
         we use third-party library cbor to serialize data to write in the file.
         """
+        if not isinstance(version, int):
+            raise error.ProgrammingError("version of state file should be"
+                                         " an integer")
+
         with self._repo.vfs(self.fname, 'wb', atomictemp=True) as fp:
+            fp.write('%d\n' % iv)
             cbor.dump(self.opts, fp, canonical=True)
 
     def _read(self):
         """reads the state file and returns a dictionary which contain
         data in the same format as it was before storing"""
         with self._repo.vfs(self.fname, 'rb') as fp:
+            try:
+                version = int(fp.readline())
+            except ValueError:
+                raise error.ProgrammingError("unknown version of state file"
+                                             " found")
             return cbor.load(fp)
 
     def delete(self):