Allow command options to be set from environment variables

Extend the Command base class to allow options to be set from values
in environment variables, if the user has not given the option on the
command line and the environment variable is set.

Derived classes of Command can override the implementation of the method
_GetEnvironmentOptions to configure which of its options may be set from
environment variables.

Change-Id: I7c780bcf9644d6567893d9930984c054bce7351e

command.py

@@ -22,6 +22,7 @@ import sys
 from error import NoSuchProjectError
 from error import InvalidProjectGroupsError
 
+
 class Command(object):
   """Base class for any command line action in repo.
   """
@@ -33,6 +34,27 @@ class Command(object):
   def WantPager(self, opt):
     return False
 
+  def ReadEnvironmentOptions(self, opts):
+    """ Set options from environment variables. """
+
+    env_options = self._RegisteredEnvironmentOptions()
+
+    for env_key, opt_key in env_options.items():
+      # Get the user-set option value if any
+      opt_value = getattr(opts, opt_key)
+
+      # If the value is set, it means the user has passed it as a command
+      # line option, and we should use that.  Otherwise we can try to set it
+      # with the value from the corresponding environment variable.
+      if opt_value is not None:
+        continue
+
+      env_value = os.environ.get(env_key)
+      if env_value is not None:
+        setattr(opts, opt_key, env_value)
+
+    return opts
+
   @property
   def OptionParser(self):
     if self._optparse is None:
@@ -49,6 +71,24 @@ class Command(object):
     """Initialize the option parser.
     """
 
+  def _RegisteredEnvironmentOptions(self):
+    """Get options that can be set from environment variables.
+
+    Return a dictionary mapping environment variable name
+    to option key name that it can override.
+
+    Example: {'REPO_MY_OPTION': 'my_option'}
+
+    Will allow the option with key value 'my_option' to be set
+    from the value in the environment variable named 'REPO_MY_OPTION'.
+
+    Note: This does not work properly for options that are explicitly
+    set to None by the user, or options that are defined with a
+    default value other than None.
+
+    """
+    return {}
+
   def Usage(self):
     """Display usage and terminate.
     """

main.py

@@ -120,6 +120,7 @@ class _Repo(object):
       return 1
 
     copts, cargs = cmd.OptionParser.parse_args(argv)
+    copts = cmd.ReadEnvironmentOptions(copts)
 
     if not gopts.no_pager and not isinstance(cmd, InteractiveCommand):
       config = cmd.manifest.globalConfig