state: import the file to write state files from evolve extension

state: import the file to write state files from evolve extension

The current way of writing state files is very obscure with each state file
having it's own format to store state files. There is no centralized way to
write state files in a good format. Moreover the current state files are not
extensible, you cannot add more data to store in state files in reliable ways.

To solve the problem, I wrote my own serialization and deserialization format,
looked into existing formats like Protobuf, MessagePack, JSON but CBOR looks
very promising and is suggested by people in the community.

The current interface to store state files is to directly write data in files
when things abort. Using the class imported by this commit, we can create
objects which has a dict like interface and can store data on the object and
store it on the file when things abort.

The evolve extension is using the state file for `evolve`, `grab` commands and
using it for resolution of orphaness, phase-divergence and content-divergence.

The file is moved from changeset e4ac2e2c2086f977afa35e23a62f849e9305a225 of the
evolve extension which is also tagged as 7.3.0.
The following changes are made to the file while moving to core:

  * import util from current directory as this file in mercurial/ now
  * make cmdstate class extend object
  * removed mutable default value for opts in cmdstate.__init__
  * some doc changes to replace out of core things with in-core ones

evolve extension can be found at https://bitbucket.org/marmoute/mutable-history

Differential Revision: https://phab.mercurial-scm.org/D2591

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/state.py	Wed Feb 21 17:20:22 2018 +0530
@@ -0,0 +1,93 @@
+# state.py - writing and reading state files in Mercurial
+#
+# Copyright 2018 Pulkit Goyal <[e-mail]>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+"""
+This file contains class to wrap the state for commands and other
+related logic.
+
+All the data related to the command state is stored as dictionary in the object.
+The class has methods using which the data can be stored to disk in a file under
+.hg/ directory.
+
+We store the data on disk in cbor, for which we use the third party cbor library
+to serialize and deserialize data.
+"""
+
+from __future__ import absolute_import
+
+from .thirdparty import cbor
+
+from . import (
+    util,
+)
+
+class cmdstate(object):
+    """a wrapper class to store the state of commands like `rebase`, `graft`,
+    `histedit`, `shelve` etc. Extensions can also use this to write state files.
+
+    All the data for the state is stored in the form of key-value pairs in a
+    dictionary.
+
+    The class object can write all the data to a file in .hg/ directory and
+    can populate the object data reading that file.
+
+    Uses cbor to serialize and deserialize data while writing and reading from
+    disk.
+    """
+
+    def __init__(self, repo, fname, opts=None):
+        """ repo is the repo object
+        fname is the file name in which data should be stored in .hg directory
+        opts is a dictionary of data of the statefile
+        """
+        self._repo = repo
+        self.fname = fname
+        if not opts:
+            self.opts = {}
+        else:
+            self.opts = opts
+
+    def __nonzero__(self):
+        return self.exists()
+
+    def __getitem__(self, key):
+        return self.opts[key]
+
+    def __setitem__(self, key, value):
+        updates = {key: value}
+        self.opts.update(updates)
+
+    def load(self):
+        """load the existing state file into the class object"""
+        op = self._read()
+        self.opts.update(op)
+
+    def addopts(self, opts):
+        """add more key-value pairs to the data stored by the object"""
+        self.opts.update(opts)
+
+    def save(self):
+        """write all the state data stored to .hg/<filename> file
+
+        we use third-party library cbor to serialize data to write in the file.
+        """
+        with self._repo.vfs(self.fname, 'wb', atomictemp=True) as fp:
+            cbor.dump(self.opts, fp)
+
+    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:
+            return cbor.load(fp)
+
+    def delete(self):
+        """drop the state file if exists"""
+        util.unlinkpath(self._repo.vfs.join(self.fname), ignoremissing=True)
+
+    def exists(self):
+        """check whether the state file exists or not"""
+        return self._repo.vfs.exists(self.fname)