fbcode_builder: implement automatic project detection from the current repo

Summary:
This updates fbcode_builder to try and automatically detect the current
repository's project name by looking for a `.projectid` file in the repository
root.  If the project name can be detected:
- The current repository will automatically be used as the source directory
  for this project (instead of fetching the sources into the scratch
  directory).
- If an explicit project name was not specified on the command line, the
  current project will be built by default.

This also changes the repository detection logic to use the current working
directory, rather than the directory where the fbcode_builder code lives.
This will allow this logic to work even if we move fbcode_builder into its own
repository in the future, and have projects depend on it using git submodules.
This does mean that callers need to invoke fbcode_builder.py from inside the
repository.

Reviewed By: wez

Differential Revision: D17088938

fbshipit-source-id: f14d28fdcfaa330ff837ea52b8bdd4e358b81c61

build/fbcode_builder/getdeps.py

@@ -71,6 +71,23 @@ class ShowHostType(SubCmd):
 class ProjectCmdBase(SubCmd):
     def run(self, args):
         opts = setup_build_options(args)
+
+        if args.current_project is not None:
+            opts.repo_project = args.current_project
+        if args.project is None:
+            if opts.repo_project is None:
+                raise UsageError(
+                    "no project name specified, and no .projectid file found"
+                )
+            if opts.repo_project == "fbsource":
+                # The fbsource repository is a little special.  There is no project
+                # manifest file for it.  A specific project must always be explicitly
+                # specified when building from fbsource.
+                raise UsageError(
+                    "no project name specified (required when building in fbsource)"
+                )
+            args.project = opts.repo_project
+
         ctx_gen = opts.get_context_generator(facebook_internal=args.facebook_internal)
         if args.test_dependencies:
             ctx_gen.set_value_for_all_projects("test", "on")
@@ -101,6 +118,12 @@ class ProjectCmdBase(SubCmd):
 
             return project, os.path.abspath(path)
 
+        # If we are currently running from a project repository,
+        # use the current repository for the project sources.
+        build_opts = loader.build_opts
+        if build_opts.repo_project is not None and build_opts.repo_root is not None:
+            loader.set_project_src_dir(build_opts.repo_project, build_opts.repo_root)
+
         for arg in args.src_dir:
             project, path = parse_project_arg(arg, "--src-dir")
             loader.set_project_src_dir(project, path)
@@ -116,6 +139,7 @@ class ProjectCmdBase(SubCmd):
     def setup_parser(self, parser):
         parser.add_argument(
             "project",
+            nargs="?",
             help=(
                 "name of the project or path to a manifest "
                 "file describing the project"
@@ -133,6 +157,12 @@ class ProjectCmdBase(SubCmd):
             action="store_true",
             help="Enable building tests for dependencies as well.",
         )
+        parser.add_argument(
+            "--current-project",
+            help="Specify the name of the fbcode_builder manifest file for the "
+            "current repository.  If not specified, the code will attempt to find "
+            "this in a .projectid file in the repository root.",
+        )
         parser.add_argument(
             "--src-dir",
             default=[],