mach: fix logic to override paths for legacy layout (servo#34467)

wptrunner internally derives the path to the MANIFEST.json file from the
`metadata_path` passed via `test_paths`. The current logic overrides
only the `metadata_path`, leaving the manifest_path pointing at the
wrong (non-legacy) layout's MANIFEST.json.

In servo#34436 we observed that the recent WPT imports create a transient
`.cache` diretory that wptrunner logic uses for optimization. This has
not been observed until servo#34436 because a [recent bump in MANIFEST.json's
schema version][1] triggered the creation of the cache. Because of the
above issue with incorrect path and the fact that we *first* trigger
`mach update-wpt` for legacy layout during WPT import, the MANIFEST.json
of non-legacy layout gets incorrectly migrated during the invocation of
`update-wpt` for legacy layout but the cache is still created under
legacy-layout's path as it is not based on `manifest_path`. The
subsequent invocation of `mach update-wpt` for non-legacy finds the
MANIFEST.json already migrated so the `.cache` directory is not
constucted.

This change simply replaces the whole object using the wptrunner's
`TestRoot` class constructor so that all derived paths are calculated
correctly. We also add the `.cache` folders to gitignore as it seems
like they are expected to be created during such version migrations.

[1]: web-platform-tests/wpt#49406

Signed-off-by: Mukilan Thiyagarajan <[email protected]>

Author: mukilan

Diff for: .gitignore

@@ -12,6 +12,7 @@
 /python/_venv*
 /python/tidy/servo_tidy.egg-info
 /tests/wpt/sync
+/tests/wpt/**/**/.cache/
 /tests/dromaeo/dromaeo
 *.pkl
 *.pyc

Diff for: python/wpt/__init__.py

@@ -49,15 +49,15 @@ def create_parser():
 
 
 def update_args_for_legacy_layout(kwargs: dict):
-    kwargs["test_paths"]["/"].metadata_path = os.path.join(
-        WPT_PATH, "meta-legacy-layout"
-    )
-    kwargs["test_paths"]["/_mozilla/"].metadata_path = os.path.join(
-        WPT_PATH, "mozilla", "meta-legacy-layout"
-    )
-    kwargs["test_paths"]["/_webgl/"].metadata_path = os.path.join(
-        WPT_PATH, "webgl", "meta-legacy-layout"
-    )
+    def override_metadata_path(url_base, metadata_path):
+        test_root = kwargs["test_paths"][url_base]
+        kwargs["test_paths"][url_base] = wptrunner.wptcommandline.TestRoot(
+            test_root.tests_path,
+            os.path.join(WPT_PATH, *metadata_path)
+        )
+    override_metadata_path("/", ["meta-legacy-layout"])
+    override_metadata_path("/_mozilla/", ["mozilla", "meta-legacy-layout"])
+    override_metadata_path("/_webgl/", ["webgl", "meta-legacy-layout"])
 
 
 def run_tests():

Diff for: servo-tidy.toml

@@ -123,6 +123,13 @@ directories = [
     "./target",
     "./support/crown/target",
     "./third_party",
+    # Cache files generated by wptrunner which fail the EOF newlines check.
+    "./tests/wpt/meta/.cache",
+    "./tests/wpt/meta-legacy-layout/.cache",
+    "./tests/wpt/mozilla/meta/.cache",
+    "./tests/wpt/mozilla/meta-legacy-layout/.cache",
+    "./tests/wpt/webgl/meta/.cache",
+    "./tests/wpt/webgl/meta-legacy-layout/.cache",
 ]
 
 # Directories that are checked for correct file extension