Adds new lint rule to detect test overlap in web feature files

lint.ignore

@@ -847,3 +847,62 @@ PROMISE_REJECTS: wasm/core/js/harness/testharness.js
 
 # Legitimate use of test_driver_internal
 TEST DRIVER INTERNAL: resources/testdriver.js
+
+# Intentional overlapping web feature mappings
+OVERLAPPING-WEB-FEATURES-FILE: css/CSS2/backgrounds/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/CSS2/normal-flow/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/CSS2/page-box/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/CSS2/tables/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/CSS2/text/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-backgrounds/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-break/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-cascade/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-contain/content-visibility/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-display/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-display/parsing/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-flexbox/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-images/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-lists/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-masking/clip-path/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-masking/clip-path/animations/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-multicol/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-overflow/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-page/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-pseudo/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-scrollbars/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-sizing/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-text/text-indent/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-typed-om/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-ui/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-values/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-variables/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-view-transitions/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-writing-modes/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/css-writing-modes/parsing/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/cssom/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/mediaqueries/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/selectors/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/selectors/invalidation/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: css/selectors/parsing/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: custom-elements/reactions/customized-builtins/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: dom/abort/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: dom/nodes/moveBefore/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: fetch/api/basic/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: fetch/api/credentials/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: fetch/http-cache/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: file-system-access/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/editing/the-hidden-attribute/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/rendering/non-replaced-elements/phrasing-content-0/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/rendering/the-details-element/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/semantics/document-metadata/the-link-element/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/semantics/embedded-content/the-img-element/srcset/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/semantics/forms/the-button-element/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/semantics/forms/the-input-element/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/semantics/interactive-elements/the-dialog-element/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: html/semantics/scripting-1/the-script-element/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: shadow-dom/declarative/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: url/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: webmessaging/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: workers/interfaces/WorkerUtils/navigator/WEB_FEATURES.yml
+OVERLAPPING-WEB-FEATURES-FILE: xhr/WEB_FEATURES.yml

tools/lint/lint.py

@@ -777,6 +777,10 @@ def check_web_features_file(repo_root: Text, path: Text, f: IO[bytes]) -> List[r
     base_dir = os.path.join(repo_root, os.path.dirname(path))
     files_in_directory = [
         f for f in os.listdir(base_dir) if os.path.isfile(os.path.join(base_dir, f))]
+    # Track which files are claimed by which feature.
+    file_to_feature: Dict[str, str] = {}
+    # Track overlaps grouped by feature pair: (earlier_feature, later_feature) -> [filenames]
+    overlap_pairs: Dict[Tuple[str, str], List[str]] = {}
     for feature in web_features_file.features:
         if isinstance(feature.files, list):
             # Resolve inclusion patterns to files, then subtract exclusions.
@@ -802,6 +806,26 @@ def check_web_features_file(repo_root: Text, path: Text, f: IO[bytes]) -> List[r
                         errors.append(rules.UnnecessaryExclusionInWebFeaturesFile.error(path, (
                             f"'{file}' in feature '{feature.name}'",)))
                     included -= excluded
+        elif feature.does_feature_apply_recursively():
+            included = set(files_in_directory)
+        else:
+            continue
+        for filename in sorted(included):
+            if filename in file_to_feature:
+                pair = (file_to_feature[filename], feature.name)
+                overlap_pairs.setdefault(pair, []).append(filename)
+            else:
+                file_to_feature[filename] = feature.name
+
+    for (feature_a, feature_b), filenames in overlap_pairs.items():
+        max_files = 5
+        file_list = ", ".join(filenames[:max_files])
+        if len(filenames) > max_files:
+            file_list += f", and {len(filenames) - max_files} more"
+        errors.append(rules.OverlappingWebFeaturesFile.error(path, (
+            f"Features '{feature_a}' and '{feature_b}' share "
+            f"{len(filenames)} overlapping test file{'s' if len(filenames) > 1 else ''}: "
+            f"{file_list}",)))
 
     return errors
 

tools/lint/rules.py

@@ -386,6 +386,13 @@ class UnnecessaryExclusionInWebFeaturesFile(Rule):
     """)
 
 
+class OverlappingWebFeaturesFile(Rule):
+    name = "OVERLAPPING-WEB-FEATURES-FILE"
+    description = collapse("""
+        The WEB_FEATURES.yml file maps the same file to multiple features: %s
+    """)
+
+
 class WebFeaturesFileInNonTestDirectory(Rule):
     name = "WEB-FEATURES-FILE-IN-NON-TEST-DIRECTORY"
     description = collapse("""

tools/lint/tests/test_file_lints.py

@@ -1392,6 +1392,192 @@ def test_duplicate_keys_invalid_web_features_file():
 
     assert errors == []
 
+@pytest.mark.parametrize("files,yml,expected_errors", [
+    # ** before explicit files — 1 overlapping file
+    (
+        ["file-1.html", "file-2.html"],
+        b"""\
+features:
+- name: feature-a
+  files: "**"
+- name: feature-b
+  files:
+  - file-1.html
+""",
+        [
+            ("OVERLAPPING-WEB-FEATURES-FILE",
+             "The WEB_FEATURES.yml file maps the same file to multiple features: "
+             "Features 'feature-a' and 'feature-b' share "
+             "1 overlapping test file: file-1.html",
+             "css/WEB_FEATURES.yml",
+             None),
+        ]
+    ),
+    # Explicit files before ** — 1 overlapping file
+    (
+        ["file-1.html", "file-2.html"],
+        b"""\
+features:
+- name: feature-a
+  files:
+  - file-1.html
+- name: feature-b
+  files: "**"
+""",
+        [
+            ("OVERLAPPING-WEB-FEATURES-FILE",
+             "The WEB_FEATURES.yml file maps the same file to multiple features: "
+             "Features 'feature-a' and 'feature-b' share "
+             "1 overlapping test file: file-1.html",
+             "css/WEB_FEATURES.yml",
+             None),
+        ]
+    ),
+    # ** before wildcard patterns — 2 overlapping files
+    (
+        ["file-1.html", "file-2.html"],
+        b"""\
+features:
+- name: feature-a
+  files: "**"
+- name: feature-b
+  files:
+  - file-*
+""",
+        [
+            ("OVERLAPPING-WEB-FEATURES-FILE",
+             "The WEB_FEATURES.yml file maps the same file to multiple features: "
+             "Features 'feature-a' and 'feature-b' share "
+             "2 overlapping test files: file-1.html, file-2.html",
+             "css/WEB_FEATURES.yml",
+             None),
+        ]
+    ),
+    # Same explicit file in two features
+    (
+        ["file-1.html", "file-2.html"],
+        b"""\
+features:
+- name: feature-a
+  files:
+  - file-1.html
+- name: feature-b
+  files:
+  - file-1.html
+""",
+        [
+            ("OVERLAPPING-WEB-FEATURES-FILE",
+             "The WEB_FEATURES.yml file maps the same file to multiple features: "
+             "Features 'feature-a' and 'feature-b' share "
+             "1 overlapping test file: file-1.html",
+             "css/WEB_FEATURES.yml",
+             None),
+        ]
+    ),
+    # Same wildcard pattern in two features — 2 overlapping files
+    (
+        ["file-1.html", "file-2.html"],
+        b"""\
+features:
+- name: feature-a
+  files:
+  - file-*
+- name: feature-b
+  files:
+  - file-*
+""",
+        [
+            ("OVERLAPPING-WEB-FEATURES-FILE",
+             "The WEB_FEATURES.yml file maps the same file to multiple features: "
+             "Features 'feature-a' and 'feature-b' share "
+             "2 overlapping test files: file-1.html, file-2.html",
+             "css/WEB_FEATURES.yml",
+             None),
+        ]
+    ),
+    # Non-overlapping wildcard patterns — no errors
+    (
+        ["file-1.html", "file-2.html", "bar-1.html"],
+        b"""\
+features:
+- name: feature-a
+  files:
+  - file-*
+- name: feature-b
+  files:
+  - bar-*
+""",
+        []
+    ),
+    # Non-overlapping explicit files — no errors
+    (
+        ["file-1.html", "file-2.html"],
+        b"""\
+features:
+- name: feature-a
+  files:
+  - file-1.html
+- name: feature-b
+  files:
+  - file-2.html
+""",
+        []
+    ),
+    # Overlapping wildcards with exclusions that prevent actual overlap — no errors
+    (
+        ["file-1.html", "file-2.html"],
+        b"""\
+features:
+- name: feature-a
+  files:
+  - file-*
+  - "!file-2.html"
+- name: feature-b
+  files:
+  - file-2.html
+""",
+        []
+    ),
+    # Many overlapping files — capped at 5 in the message
+    (
+        ["f-1.html", "f-2.html", "f-3.html", "f-4.html", "f-5.html", "f-6.html", "f-7.html"],
+        b"""\
+features:
+- name: feature-a
+  files:
+  - f-*
+- name: feature-b
+  files:
+  - f-*
+""",
+        [
+            ("OVERLAPPING-WEB-FEATURES-FILE",
+             "The WEB_FEATURES.yml file maps the same file to multiple features: "
+             "Features 'feature-a' and 'feature-b' share "
+             "7 overlapping test files: f-1.html, f-2.html, f-3.html, f-4.html, f-5.html, and 2 more",
+             "css/WEB_FEATURES.yml",
+             None),
+        ]
+    ),
+])
+def test_overlapping_web_features_file(monkeypatch, files, yml, expected_errors):
+    def listdir(dir):
+        if dir.endswith("css"):
+            return files
+
+    def is_file(file):
+        if os.path.basename(file) in files:
+            return True
+        return False
+
+    monkeypatch.setattr(os.path, "isfile", is_file)
+    monkeypatch.setattr(os, "listdir", listdir)
+    errors = check_file_contents("", "css/WEB_FEATURES.yml", io.BytesIO(yml))
+    check_errors(errors)
+
+    assert errors == expected_errors
+
+
 def test_css_missing_file_manual():
     errors = check_file_contents("", "css/foo/bar-manual.html", io.BytesIO(b""))
     check_errors(errors)