Fix #159. Allow config files to be output from external commands.

* Convention: a filename of $(...) is actually a process specification.
* The process is executed in the current directory of the containing config file.
* The standard output of the process is used as the config file content and parsed.
* The process specification is handed to the platform shell for execution.

user_sync/config.py

@@ -21,6 +21,7 @@
 import logging
 import os
 import re
+import subprocess
 import types
 
 import keyring
@@ -641,6 +642,7 @@ def load_sub_config(cls, filename):
     def load_other_config(cls, filename):
         '''
         same as load_root_config, but does no post-processing.
+        :type filename: str
         '''
         return cls.load_from_yaml(filename, {})
 
@@ -667,25 +669,41 @@ def load_from_yaml(cls, filename, path_keys):
                           does the key have a default value so that must be added to
                           the dictionary if there is not already a value found.
         '''
-        cls.filepath = os.path.abspath(filename)
-        cls.filename = os.path.split(cls.filepath)[1]
-        cls.dirpath = os.path.dirname(cls.filepath)
-        if not os.path.isfile(cls.filepath):
-            raise AssertionException('No such configuration file: %s' % (cls.filepath,))
-
-        # read the dict from the YAML file
-        try:
-            with open(filename, 'r', 1) as input_file:
-                yml = yaml.load(input_file)
-        except IOError as e:
-            # if a file operation error occurred while loading the
-            # configuration file, swallow up the exception and re-raise this
-            # as an configuration loader exception.
-            raise AssertionException('Error reading configuration file: %s' % e)
-        except yaml.error.MarkedYAMLError as e:
-            # same as above, but indicate this problem has to do with
-            # parsing the configuration file.
-            raise AssertionException('Error parsing configuration file: %s' % e)
+        if filename.startswith('$(') and filename.endswith(')'):
+            # it's a command line to execute and read standard output
+            dir_end = filename.index(']')
+            if filename.startswith('$([') and dir_end > 0:
+                dir = filename[3:dir_end]
+                cmd = filename[dir_end+1:-1]
+            else:
+                dir = os.path.abspath(".")
+                cmd = filename[3:-1]
+            try:
+                bytes = subprocess.check_output(cmd, cwd=dir, shell=True)
+                yml = yaml.load(bytes)
+            except subprocess.CalledProcessError as e:
+                raise AssertionException("Error executing process '%s' in dir '%s': %s" % (cmd, dir, e))
+            except yaml.error.MarkedYAMLError as e:
+                raise AssertionException('Error parsing process YAML data: %s' % e)
+        else:
+            # it's a pathname to a configuration file to read
+            cls.filepath = os.path.abspath(filename)
+            if not os.path.isfile(cls.filepath):
+                raise AssertionException('No such configuration file: %s' % (cls.filepath,))
+            cls.filename = os.path.split(cls.filepath)[1]
+            cls.dirpath = os.path.dirname(cls.filepath)
+            try:
+                with open(filename, 'r', 1) as input_file:
+                    yml = yaml.load(input_file)
+            except IOError as e:
+                # if a file operation error occurred while loading the
+                # configuration file, swallow up the exception and re-raise this
+                # as an configuration loader exception.
+                raise AssertionException('Error reading configuration file: %s' % e)
+            except yaml.error.MarkedYAMLError as e:
+                # same as above, but indicate this problem has to do with
+                # parsing the configuration file.
+                raise AssertionException('Error parsing configuration file: %s' % e)
 
         # process the content of the dict
         for path_key, options in path_keys.iteritems():
@@ -773,6 +791,9 @@ def relative_path(cls, val, must_exist):
         if not isinstance(val, types.StringTypes):
             raise AssertionException("Expected pathname for setting %s in config file %s" %
                                      (cls.key_path, cls.filename))
+        if val.startswith('$(') and val.endswith(')'):
+            # this presumes
+            return "$([" + cls.dirpath + "]" + val[2:-1] + ")"
         if cls.dirpath and not os.path.isabs(val):
             val = os.path.abspath(os.path.join(cls.dirpath, val))
         if must_exist and not os.path.isfile(val):