fbcode_builder: add getdeps main entrypoint

Summary:
This is the new getdeps entrypoint.  It lives in `opensource/fbcode_builder` so that
it is synced into the various opensource projects at FB.

In the incarnation in this diff is has a single subcommand that can be used to validate
a manifest file.

Reviewed By: sinancepel

Differential Revision: D14690995

fbshipit-source-id: 53f8ec9bafb7b1930970079e3ce20f496ac1c188

build/fbcode_builder/getdeps/subcmd.py

@@ -0,0 +1,58 @@
+# Copyright (c) 2019-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, division, print_function, unicode_literals
+
+
+class SubCmd(object):
+    NAME = None
+    HELP = None
+
+    def run(self, args):
+        """ perform the command """
+        return 0
+
+    def setup_parser(self, parser):
+        # Subclasses should override setup_parser() if they have any
+        # command line options or arguments.
+        pass
+
+
+CmdTable = []
+
+
+def add_subcommands(parser, cmd_table=CmdTable):
+    """ Register parsers for the defined commands with the provided parser """
+    for cls in cmd_table:
+        command = cls()
+        command_parser = parser.add_parser(command.NAME, help=command.HELP)
+        command.setup_parser(command_parser)
+        command_parser.set_defaults(func=command.run)
+
+
+def cmd(name, help=None, cmd_table=CmdTable):
+    """
+    @cmd() is a decorator that can be used to help define Subcmd instances
+
+    Example usage:
+
+        @subcmd('list', 'Show the result list')
+        class ListCmd(Subcmd):
+            def run(self, args):
+                # Perform the command actions here...
+                pass
+    """
+
+    def wrapper(cls):
+        class SubclassedCmd(cls):
+            NAME = name
+            HELP = help
+
+        cmd_table.append(SubclassedCmd)
+        return SubclassedCmd
+
+    return wrapper