Add a lint that checks paths are unique irrespective of case

Paths that are the same excluding case don't work reliably across
operating systems and can be forbidden by vendor CI. So we should
error on these paths in the lint.

tools/lint/lint.py

@@ -396,6 +396,20 @@ def check_unique_testharness_basenames(repo_root, paths):
     return errors
 
 
+def check_unique_case_insensitive_paths(repo_root, paths):
+    # type: (Text, List[Text]) -> List[rules.Error]
+    seen = {}  # type: Dict[Text, Text]
+    errors = []
+    for path in paths:
+        lower_path = path.lower()
+        if lower_path in seen:
+            context = (seen[lower_path],)
+            errors.append(rules.DuplicatePathCaseInsensitive.error(path, context))
+        else:
+            seen[lower_path] = path
+    return errors
+
+
 def parse_ignorelist(f):
     # type: (IO[Text]) -> Tuple[Ignorelist, Set[Text]]
     """
@@ -1107,7 +1121,8 @@ def process_errors(errors):
 
 path_lints = [check_file_type, check_path_length, check_worker_collision, check_ahem_copy,
               check_mojom_js, check_tentative_directories, check_gitignore_file]
-all_paths_lints = [check_css_globally_unique, check_unique_testharness_basenames]
+all_paths_lints = [check_css_globally_unique, check_unique_testharness_basenames,
+                   check_unique_case_insensitive_paths]
 file_lints = [check_regexp_line, check_parsed, check_python_ast, check_script_metadata,
               check_ahem_system_font]
 

tools/lint/rules.py

@@ -359,6 +359,14 @@ class DuplicateBasenamePath(Rule):
     to_fix = "rename files so they have unique basename paths"
 
 
+class DuplicatePathCaseInsensitive(Rule):
+    name = "DUPLICATE-CASE-INSENSITIVE-PATH"
+    description = collapse("""
+            Path differs from path %s only in case
+    """)
+    to_fix = "rename files so they are unique irrespective of case"
+
+
 class TentativeDirectoryName(Rule):
     name = "TENTATIVE-DIRECTORY-NAME"
     description = "Directories for tentative tests must be named exactly 'tentative'"

tools/lint/tests/test_path_lints.py

@@ -3,7 +3,7 @@
 import mock
 import os
 
-from ..lint import check_path
+from ..lint import check_path, check_unique_case_insensitive_paths
 from .base import check_errors
 import pytest
 
@@ -153,3 +153,15 @@ def test_gitignore_negative(path):
     errors = check_path("/foo/", path)
 
     assert errors == []
+
+
+@pytest.mark.parametrize("paths,errors",
+                         [(["a/b.html", "a/B.html"], ["a/B.html"]),
+                          (["A/b.html", "a/b.html"], ["a/b.html"]),
+                          (["a/b.html", "a/c.html"], [])])
+def test_unique_case_insensitive_paths(paths, errors):
+    got_errors = check_unique_case_insensitive_paths(None, paths)
+    assert len(got_errors) == len(errors)
+    for (name, _, path, _), expected_path in zip(got_errors, errors):
+        assert name == "DUPLICATE-CASE-INSENSITIVE-PATH"
+        assert path == expected_path