mb: Add a top-level comment to GN args when using the Simple Chrome SDK.

This will add a comment to the gn_arg log link on chromium bots:
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8930669115526655984/+/steps/lookup_GN_args/0/logs/gn_args/0

This will hopefully help avoid confusion when a non-CrOS chromium dev
gets a CL blocked by a simplechrome bot. (See the linked bug where
someone wasn't familiar with it and tried copy-pasting the arg dump
outside the SDK.)

I looked into adding the comment further upstream in chromite's
cros_chrome_sdk libs, but it makes heavy use of gn_helpers, and adding
comment support to that is non-trivial.

This can't land until https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1320309
and gets rolled into chromium.

Bug: 901471
Change-Id: I9ad646a9b6ab6fd29d4adb7acfd842b26c3825ae
Reviewed-on: https://chromium-review.googlesource.com/c/1318179Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606257}

tools/mb/mb.py

@@ -1047,6 +1047,17 @@ class MetaBuildWrapper(object):
     # the last instance of each arg is listed.
     gn_args = gn_helpers.ToGNString(gn_helpers.FromGNArgs(gn_args))
 
+    # If we're using the Simple Chrome SDK, add a comment at the top that
+    # points to the doc. This must happen after the gn_helpers.ToGNString()
+    # call above since gn_helpers strips comments.
+    if vals['cros_passthrough']:
+      simplechrome_comment = [
+          '# These args are generated via the Simple Chrome SDK. See the link',
+          '# below for more details:',
+          '# https://chromium.googlesource.com/chromiumos/docs/+/master/simple_chrome_workflow.md',  # pylint: disable=line-too-long
+      ]
+      gn_args = '%s\n%s' % ('\n'.join(simplechrome_comment), gn_args)
+
     args_file = vals.get('args_file', None)
     if args_file:
       gn_args = ('import("%s")\n' % vals['args_file']) + gn_args

tools/mb/mb_unittest.py

@@ -6,6 +6,7 @@
 """Tests for mb.py."""
 
 import json
+import os
 import StringIO
 import sys
 import unittest
@@ -110,6 +111,7 @@ TEST_CONFIG = """\
     'fake_master': {
       'fake_builder': 'rel_bot',
       'fake_debug_builder': 'debug_goma',
+      'fake_simplechrome_builder': 'cros_chrome_sdk',
       'fake_args_bot': '//build/args/bots/fake_master/fake_args_bot.gn',
       'fake_multi_phase': { 'phase_1': 'phase_1', 'phase_2': 'phase_2'},
       'fake_args_file': 'args_file_goma',
@@ -119,12 +121,16 @@ TEST_CONFIG = """\
   'configs': {
     'args_file_goma': ['args_file', 'goma'],
     'args_file_twice': ['args_file', 'args_file'],
+    'cros_chrome_sdk': ['cros_chrome_sdk'],
     'rel_bot': ['rel', 'goma', 'fake_feature1'],
     'debug_goma': ['debug', 'goma'],
     'phase_1': ['phase_1'],
     'phase_2': ['phase_2'],
   },
   'mixins': {
+    'cros_chrome_sdk': {
+      'cros_passthrough': True,
+    },
     'fake_feature1': {
       'gn_args': 'enable_doom_melon=true',
     },
@@ -217,11 +223,17 @@ class UnitTest(unittest.TestCase):
         mbw.files[path] = contents
     return mbw
 
-  def check(self, args, mbw=None, files=None, out=None, err=None, ret=None):
+  def check(self, args, mbw=None, files=None, out=None, err=None, ret=None,
+            env=None):
     if not mbw:
       mbw = self.fake_mbw(files)
 
-    actual_ret = mbw.Main(args)
+    try:
+      prev_env = os.environ.copy()
+      os.environ = env if env else prev_env
+      actual_ret = mbw.Main(args)
+    finally:
+      os.environ = prev_env
 
     self.assertEqual(actual_ret, ret)
     if out is not None:
@@ -583,6 +595,12 @@ class UnitTest(unittest.TestCase):
                     '""" to _path_/args.gn.\n\n'
                     '/fake_src/buildtools/linux64/gn gen _path_\n'))
 
+  def test_lookup_simplechrome(self):
+    simplechrome_env = {
+        'GN_ARGS': 'is_chromeos=1 target_os="chromeos"',
+    }
+    self.check(['lookup', '-c', 'cros_chrome_sdk'], ret=0, env=simplechrome_env)
+
   def test_help(self):
     orig_stdout = sys.stdout
     try: