Make gn_helpers.py safely ignore comments.

I'd like to add comments to the GN args of some bots. This will let us
do that without mb or simplechrome SDK crashing.

Bug: 901471
Change-Id: I7f49434e02591639a02ef9cfd0f026680f6b9de1
Reviewed-on: https://chromium-review.googlesource.com/c/1318188Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605544}

build/gn_helpers.py

@@ -171,6 +171,19 @@ class GNValueParser(object):
     while not self.IsDone() and self.input[self.cur] in ' \t\n':
       self.cur += 1
 
+  def ConsumeComment(self):
+    if self.IsDone() or self.input[self.cur] != '#':
+      return
+
+    # Consume each comment, line by line.
+    while not self.IsDone() and self.input[self.cur] == '#':
+      # Consume the rest of the comment, up until the end of the line.
+      while not self.IsDone() and self.input[self.cur] != '\n':
+        self.cur += 1
+      # Move the cursor to the next line (if there is one).
+      if not self.IsDone():
+        self.cur += 1
+
   def Parse(self):
     """Converts a string representing a printed GN value to the Python type.
 
@@ -203,6 +216,7 @@ class GNValueParser(object):
     d = {}
 
     self.ConsumeWhitespace()
+    self.ConsumeComment()
     while not self.IsDone():
       ident = self._ParseIdent()
       self.ConsumeWhitespace()
@@ -212,6 +226,7 @@ class GNValueParser(object):
       self.ConsumeWhitespace()
       val = self._ParseAllowTrailing()
       self.ConsumeWhitespace()
+      self.ConsumeComment()
       d[ident] = val
 
     return d

build/gn_helpers_unittest.py

@@ -85,6 +85,15 @@ class UnitTest(unittest.TestCase):
     self.assertEqual(gn_helpers.FromGNArgs('foo="bar baz"'),
                      {'foo': 'bar baz'})
 
+    # Comments should work (and be ignored).
+    gn_args_lines = [
+        '# Top-level comment.',
+        'foo = true',
+        'bar = 1  # In-line comment.',
+    ]
+    self.assertEqual(gn_helpers.FromGNArgs('\n'.join(gn_args_lines)),
+                     {'foo': True, 'bar': 1})
+
     # Lists should work.
     self.assertEqual(gn_helpers.FromGNArgs('foo=[1, 2, 3]'),
                      {'foo': [1, 2, 3]})