add arcconfig accessors

Summary:
This is here to support other arcanist/phabrication integration
modules.

Note: this adds a new package.  I'm not sure what the packaging ramifications
are exactly, but surely there are some as other extensions start to depend on
this.

Test Plan:
integration test is provided:

```
$ ../../hg-crew/tests/run-tests.py -j8
....................................................
# Ran 52 tests, 0 skipped, 0 warned, 0 failed.
```

Reviewers: #sourcecontrol, ttung

Subscribers: mjpieters

Differential Revision: https://phabricator.fb.com/D3215615

phabricator/arcconfig.py

@@ -0,0 +1,58 @@
+# Copyright 2016 Facebook Inc.
+#
+# Locate and load arcanist configuration for a project
+
+import errno
+import json
+import os
+from mercurial import (
+    cmdutil,
+    error
+)
+
+cmdtable = {}
+command = cmdutil.command(cmdtable)
+
+class ArcConfigError(Exception):
+    pass
+
+def _load_file(filename):
+    try:
+        with open(filename, 'r') as f:
+            return json.loads(f.read())
+    except IOError as ex:
+        if ex.errno == errno.ENOENT:
+            return None
+        raise
+
+def load_for_path(path):
+    homedir = os.getenv('HOME')
+    if not homedir:
+        raise ArcConfigError('$HOME environment variable not found')
+
+    # Use their own file as a basis
+    userconfig = _load_file(os.path.join(homedir, '.arcrc')) or {}
+
+    # Walk up the path and augment with an .arcconfig if we find it,
+    # terminating the search at that point.
+    path = os.path.abspath(path)
+    while len(path) > 1:
+        config = _load_file(os.path.join(path, '.arcconfig'))
+        if config is not None:
+            userconfig.update(config)
+            # Return the located path too, as we need this for figuring
+            # out where we are relative to the fbsource root.
+            userconfig['_arcconfig_path'] = path
+            return userconfig
+        path = os.path.dirname(path)
+
+    raise ArcConfigError('no .arcconfig found')
+
+@command('debugarcconfig')
+def debugarcconfig(ui, repo, *args, **opts):
+    """ exists purely for testing and diagnostic purposes """
+    try:
+        config = load_for_path(repo.root)
+        ui.write(json.dumps(config, sort_keys=True), '\n')
+    except ArcConfigError as ex:
+        raise error.Abort(str(ex))

setup.py

@@ -14,6 +14,7 @@ setup(
     long_description="",
     keywords='fb hg mercurial',
     license='',
+    packages=['phabricator'],
     py_modules=[
         'arcdiff',
         'backups',

tests/test-arcconfig.t

@@ -0,0 +1,26 @@
+  $ cat >> $HGRCPATH << EOF
+  > [extensions]
+  > arcconfig=$TESTDIR/../phabricator/arcconfig.py
+  > EOF
+
+Sanity check expectations when there is no arcconfig
+
+  $ hg init repo
+  $ cd repo
+  $ hg debugarcconfig
+  abort: no .arcconfig found
+  [255]
+
+Show that we can locate and reflect the contents of the .arcconfig from
+the repo dir
+
+  $ echo '{"hello": "world"}' > .arcconfig
+  $ hg debugarcconfig
+  {"_arcconfig_path": "$TESTTMP/repo", "hello": "world"}
+
+We expect to see the combination of the user arcrc and the repo rc
+
+  $ echo '{"user": true}' > $HOME/.arcrc
+  $ hg debugarcconfig
+  {"_arcconfig_path": "$TESTTMP/repo", "hello": "world", "user": true}
+