add limited interpolation to eden config parsing

Summary: This allows us to templatize centrally managed config files

Reviewed By: bolinfest

Differential Revision: D4444058

fbshipit-source-id: a44372084d32dcf0b27922490ab40a48478a720d

eden/fs/cli/TARGETS

@@ -14,10 +14,17 @@ python_library(
   name = 'lib',
   srcs = [
     'config.py',
+    'configinterpolator.py',
     'util.py',
   ],
 )
 
+python_unittest(
+  name = 'test',
+  srcs = glob(['test/*.py']),
+  deps = [':lib']
+)
+
 for build_target, suffix in get_daemon_versions():
     # The :all rule is a convenience to ensure that both the CLI and the daemon
     # are built.

eden/fs/cli/config.py

@@ -20,7 +20,7 @@ import subprocess
 import tempfile
 import time
 
-from . import util
+from . import (util, configinterpolator)
 import eden.thrift
 import facebook.eden.ttypes as eden_ttypes
 from fb303.ttypes import fb_status
@@ -61,9 +61,18 @@ class Config:
         if not self._system_config_dir:
             self._system_config_dir = SYSTEM_CONFIG_DIR
         self._user_config_path = os.path.join(home_dir, USER_CONFIG)
+        self._home_dir = home_dir
 
     def _loadConfig(self):
-        parser = configparser.ConfigParser()
+        ''' to facilitate templatizing a centrally deployed config, we
+            allow a limited set of env vars to be expanded.
+            ${HOME} will be replaced by the user's home dir,
+            ${USER} will be replaced by the user's login name.
+        '''
+        defaults = {'USER': os.environ.get('USER'),
+                    'HOME': self._home_dir}
+        parser = configparser.ConfigParser(
+            interpolation=configinterpolator.EdenConfigInterpolator(defaults))
         parser.read(self.get_rc_files())
         return parser
 

eden/fs/cli/configinterpolator.py

@@ -0,0 +1,45 @@
+# Copyright (c) 2017-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree. An additional grant
+# of patent rights can be found in the PATENTS file in the same directory.
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+import configparser
+
+
+class EdenConfigInterpolator(configparser.Interpolation):
+    ''' Python provides a couple of interpolation options but neither
+        of them quite match the simplicity that we want.  This class
+        will interpolate the keys of the provided map and replace
+        those tokens with the values from the map.  There is no
+        recursion or referencing of values from other sections of
+        the config.
+        Limiting the scope interpolation makes it easier to replicate
+        this approach in the C++ implementation of the parser.
+    '''
+
+    def __init__(self, defaults):
+        self._defaults = {}
+        ''' pre-construct the token name that we're going to substitute.
+            eg: {"foo": "bar"} is stored as {"${foo}": "bar"} internally
+        '''
+        for k, v in defaults.items():
+            self._defaults['${' + k + '}'] = v
+
+    def _interpolate(self, value):
+        ''' simple brute force replacement using the defaults that were
+            provided to us during construction '''
+        for k, v in self._defaults.items():
+            value = value.replace(k, v)
+        return value
+
+    def before_get(self, parser, section, option, value, defaults):
+        return self._interpolate(value)
+
+    def before_read(self, parser, section, option, value):
+        return self._interpolate(value)

eden/fs/cli/test/interp_test.py

@@ -0,0 +1,42 @@
+# Copyright (c) 2017-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree. An additional grant
+# of patent rights can be found in the PATENTS file in the same directory.
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+from .. import configinterpolator
+import configparser
+import unittest
+
+
+class InterpolatorTest(unittest.TestCase):
+    def test_basic_subs(self):
+        defaults = {'USER': 'wez', 'RECURSIVE': 'a${RECURSIVE}b'}
+        parser = configparser.ConfigParser(
+            interpolation=configinterpolator.EdenConfigInterpolator(defaults))
+        parser.add_section('section')
+        parser.set('section', 'user', '${USER}')
+        parser.set('section', 'rec', '${RECURSIVE}')
+        parser.set('section', 'simple', 'value')
+
+        self.assertEqual('wez', parser.get('section', 'user'))
+        self.assertEqual('value', parser.get('section', 'simple'))
+        self.assertEqual('a${RECURSIVE}b', parser.get('section', 'rec'))
+
+        actual = {}
+        for section in parser.sections():
+            actual[section] = dict(parser.items(section))
+
+        expect = {
+            'section': {
+                'user': 'wez',
+                'simple': 'value',
+                'rec': 'a${RECURSIVE}b',
+            }
+        }
+        self.assertEqual(expect, actual)