Cherry pick PR #1536: Add check for starboard python module (#1674)

Refer to the original PR: youtube/cobalt#1536

b/299098707

---------

Co-authored-by: Oscar Vestlie <[email protected]>
Co-authored-by: Arjun Menon <[email protected]>

starboard/build/config/BUILDCONFIG.gn

@@ -37,6 +37,10 @@ declare_args() {
   build_with_separate_cobalt_toolchain = false
 }
 
+_is_on_pythonpath = exec_script("//starboard/build/is_on_path.py", [], "json")
+assert(!is_starboard || _is_on_pythonpath,
+       "The current repo is not first on the PYTHONPATH.")
+
 assert(!(is_starboard && is_native_target_build),
        "Targets should be built for Starboard or natively, but not both")
 

starboard/build/is_on_path.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+# Copyright 2023 The Cobalt Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Script for checking if the current repo on the path."""
+
+import os
+
+
+def 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())
+  except ImportError:
+    print('false')
+
+
+if __name__ == '__main__':
+  main()