Improve PYTHONPATH checker (youtube#3085)

Refactor to make checker work when imported and called from other
module.
Also add to gn.py.

b/299098707

cobalt/build/gn.py

@@ -22,6 +22,7 @@
 from pathlib import Path
 from typing import List
 
+from starboard.build.is_on_path import is_on_same_path
 from starboard.build.platforms import PLATFORMS
 
 _BUILD_TYPES = ['debug', 'devel', 'qa', 'gold']
@@ -51,6 +52,8 @@ def main(out_directory: str, platform: str, build_type: str,
 
 
 if __name__ == '__main__':
+  assert is_on_same_path(__file__), 'Current repo is not first on PYTHONPATH.'
+
   parser = argparse.ArgumentParser()
 
   builds_directory_group = parser.add_mutually_exclusive_group()

starboard/build/is_on_path.py

@@ -16,17 +16,28 @@
 
 import os
 
+SRC_ROOT = src_root_dir = os.path.abspath(
+    os.path.join(os.path.dirname(__file__), os.pardir, os.pardir))
 
-def main():
+
+def is_on_same_path(file_path):
+  """
+  Checks if the passed file path is in the same repo as this file.
+
+  Args:
+    file_path: The path to the file that should be checked.
+
+  Returns:
+    True if file_path is a file in the same repo as this file, otherwise False.
+  """
+  return os.path.abspath(file_path).startswith(SRC_ROOT)
+
+
+if __name__ == '__main__':
   try:
-    # Try to import this file and compare its path to the current file.
     import starboard.build.is_on_path  # pylint: disable=import-outside-toplevel
-    this_file = os.path.realpath(__file__)
-    imported_file = os.path.realpath(starboard.build.is_on_path.__file__)
-    print(str(this_file == imported_file).lower())
+    # Use imported function instead of calling directly to ensure imports are
+    # being made from the same repo as this file is in.
+    print(str(starboard.build.is_on_path.is_on_same_path(__file__)).lower())
   except ImportError:
     print('false')
-
-
-if __name__ == '__main__':
-  main()