gn_helper: handle absolute path import

Bug: 393209409
Change-Id: Ic26b4502ab1744c7af7c0b06de2e4d1b9f2d3980
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6217699
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Philipp Wollermann <philwo@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

gn_helper.py

@@ -34,7 +34,25 @@ def _gn_lines(output_dir, path):
                     import_path = os.path.normpath(
                         os.path.join(_find_root(output_dir),
                                      raw_import_path[2:]))
+                elif raw_import_path.startswith('/'):
+                    # GN uses "/"-prefix as absolute path,
+                    # https://gn.googlesource.com/gn/+/main/docs/reference.md#labels
+                    # e.g.
+                    #  /usr/local/foo:bar
+                    #  /C:/Program Files/MyLibs:bar
+                    # but Win32's absolute path doesn't have "/"-prefix.
+                    if sys.platform.startswith('win32'):
+                        import_path = raw_import_path[1:]
+                    else:
+                        import_path = raw_import_path
+                    if not os.path.isabs(import_path):
+                        raise Exception('Wrong absolute path for import %s' %
+                                        raw_import_path)
                 else:
+                    if os.path.isabs(raw_import_path):
+                        raise Execption(
+                            'Absolute path "%s" should start with "/" in GN' %
+                            raw_import_path)
                     import_path = os.path.normpath(
                         os.path.join(os.path.dirname(path), raw_import_path))
                 yield from _gn_lines(output_dir, import_path)