gn_helpers: Add support for imported args files.

The libassistant build calls gn_helpers to read the out dir's args.gn
file (while it's being invoked by ninja, mind you!) and passes its
contents through gn_helpers, applies a filtering, then creates a sub
build dir (within the original build dir) using its modified set of GN
args.

This is problematic since crbug.com/937821 is trying to transition
CrOS bots to using import() lines in their args file, which breaks
libassistant's build.

This fixes that by adding import support to gn_helpers.

Bug: 937821
Change-Id: I790bc27368611f31b63a0135c0e5919f35b21e6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107709
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#752503}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 00156a211ee71ca14e6dcb1084b88b9ad92944e9

gn_helpers.py

@@ -19,9 +19,14 @@ To use in a random python file in the build:
 Where the sequence of parameters to join is the relative path from your source
 file to the build directory."""
 
+import os
+import re
 import sys
 
 
+IMPORT_RE = re.compile(r'^import\("//(\S+)"\)')
+
+
 class GNException(Exception):
   pass
 
@@ -170,6 +175,31 @@ class GNValueParser(object):
   def IsDone(self):
     return self.cur == len(self.input)
 
+  def ReplaceImports(self):
+    """Replaces import(...) lines with the contents of the imports.
+
+    Recurses on itself until there are no imports remaining, in the case of
+    nested imports.
+    """
+    lines = self.input.splitlines()
+    if not any(line.startswith('import(') for line in lines):
+      return
+    for line in lines:
+      if not line.startswith('import('):
+        continue
+      regex_match = IMPORT_RE.match(line)
+      if not regex_match:
+        raise GNException('Not a valid import string: %s' % line)
+      import_path = os.path.join(
+          os.path.dirname(__file__), os.pardir, regex_match.group(1))
+      with open(import_path) as f:
+        imported_args = f.read()
+      self.input = self.input.replace(line, imported_args)
+    # Call ourselves again if we've just replaced an import() with additional
+    # imports.
+    self.ReplaceImports()
+
+
   def ConsumeWhitespace(self):
     while not self.IsDone() and self.input[self.cur] in ' \t\n':
       self.cur += 1
@@ -218,6 +248,7 @@ class GNValueParser(object):
     """
     d = {}
 
+    self.ReplaceImports()
     self.ConsumeWhitespace()
     self.ConsumeComment()
     while not self.IsDone():

gn_helpers_unittest.py

@@ -2,9 +2,13 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import gn_helpers
+import mock
+import textwrap
 import unittest
 
+import gn_helpers
+
+
 class UnitTest(unittest.TestCase):
   def test_ToGNString(self):
     self.assertEqual(
@@ -122,5 +126,58 @@ class UnitTest(unittest.TestCase):
     self.assertEqual(gn_helpers.FromGNArgs('foo_=true'),
                      {'foo_': True})
 
+  def test_ReplaceImports(self):
+    # Should be a no-op on args inputs without any imports.
+    parser = gn_helpers.GNValueParser(
+        textwrap.dedent("""
+        some_arg1 = "val1"
+        some_arg2 = "val2"
+    """))
+    parser.ReplaceImports()
+    self.assertEquals(
+        parser.input,
+        textwrap.dedent("""
+        some_arg1 = "val1"
+        some_arg2 = "val2"
+    """))
+
+    # A single "import(...)" line should be replaced with the contents of the
+    # file being imported.
+    parser = gn_helpers.GNValueParser(
+        textwrap.dedent("""
+        some_arg1 = "val1"
+        import("//some/args/file.gni")
+        some_arg2 = "val2"
+    """))
+    fake_import = 'some_imported_arg = "imported_val"'
+    with mock.patch('__builtin__.open', mock.mock_open(read_data=fake_import)):
+      parser.ReplaceImports()
+    self.assertEquals(
+        parser.input,
+        textwrap.dedent("""
+        some_arg1 = "val1"
+        some_imported_arg = "imported_val"
+        some_arg2 = "val2"
+    """))
+
+    # No trailing parenthesis should raise an exception.
+    with self.assertRaises(gn_helpers.GNException):
+      parser = gn_helpers.GNValueParser(
+          textwrap.dedent('import("//some/args/file.gni"'))
+      parser.ReplaceImports()
+
+    # No double quotes should raise an exception.
+    with self.assertRaises(gn_helpers.GNException):
+      parser = gn_helpers.GNValueParser(
+          textwrap.dedent('import(//some/args/file.gni)'))
+      parser.ReplaceImports()
+
+    # A path that's not source absolute should raise an exception.
+    with self.assertRaises(gn_helpers.GNException):
+      parser = gn_helpers.GNValueParser(
+          textwrap.dedent('import("some/relative/args/file.gni")'))
+      parser.ReplaceImports()
+
+
 if __name__ == '__main__':
   unittest.main()