decoder: Autodetect encoding for ignore-from-file

Before this change, yamllint would decode files on the ignore-from-file
list using open()’s default encoding [1][2]. This can cause decoding to
fail in some situations (see the previous commit message for details).

This change makes yamllint automatically detect the encoding for files
on the ignore-from-file list. It uses the same algorithm that it uses
for detecting the encoding of YAML files, so the same limitations apply:
files must use UTF-8, UTF-16 or UTF-32 and they must begin with either a
byte order mark or an ASCII character.

[1]: <https://docs.python.org/3.12/library/fileinput.html#fileinput.input>
[2]: <https://docs.python.org/3.12/library/fileinput.html#fileinput.FileInput>

Author: Jayman2000

docs/configuration.rst

@@ -228,6 +228,10 @@ or:
 
 .. note:: However, this is mutually exclusive with the ``ignore`` key.
 
+.. note:: Files on the ``ignore-from-file`` list should use either UTF-8,
+   UTF-16 or UTF-32. See :doc:`Character Encoding <character_encoding>` for
+   details and workarounds.
+
 If you need to know the exact list of files that yamllint would process,
 without really linting them, you can use ``--list-files``:
 

tests/test_config.py

@@ -1,4 +1,5 @@
 # Copyright (C) 2016 Adrien Vergé
+# Copyright (C) 2023–2025 Jason Yundt
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -13,14 +14,20 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import itertools
 import os
 import shutil
 import sys
 import tempfile
 import unittest
 from io import StringIO
 
-from tests.common import build_temp_workspace, RunContext
+from tests.common import (
+    build_temp_workspace,
+    register_test_codecs,
+    RunContext,
+    unregister_test_codecs,
+)
 
 from yamllint import cli, config
 from yamllint.config import YamlLintConfigError
@@ -820,3 +827,44 @@ def test_run_with_ignore_on_ignored_file(self):
             sys.stdout.getvalue().strip(),
             'file-at-root.yaml:4:17: [error] trailing spaces (trailing-spaces)'
         )
+
+    def create_ignore_file(self, text, codec):
+        path = os.path.join(self.wd, f'{codec}.ignore')
+        with open(path, 'wb') as f:
+            f.write(text.encode(codec))
+        self.addCleanup(lambda: os.remove(path))
+        return path
+
+    def test_ignored_from_file_with_multiple_encodings(self):
+        register_test_codecs()
+        self.addCleanup(unregister_test_codecs)
+
+        ignore_files = itertools.starmap(
+            self.create_ignore_file, (
+                ('bin/file.lint-me-anyway.yaml\n', 'utf_32_be'),
+                ('bin/file.yaml\n', 'utf_32_be_sig'),
+                ('file-at-root.yaml\n', 'utf_32_le'),
+                ('file.dont-lint-me.yaml\n', 'utf_32_le_sig'),
+
+                ('ign-dup/file.yaml\n', 'utf_16_be'),
+                ('ign-dup/sub/dir/file.yaml\n', 'utf_16_be_sig'),
+                ('ign-trail/file.yaml\n', 'utf_16_le'),
+                ('include/ign-dup/sub/dir/file.yaml\n', 'utf_16_le_sig'),
+
+                ('s/s/ign-trail/file.yaml\n', 'utf_8'),
+                (
+                    's/s/ign-trail/s/s/file.yaml\n'
+                    's/s/ign-trail/s/s/file2.lint-me-anyway.yaml\n'
+                    '.yamllint\n',
+
+                    'utf_8_sig'
+                ),
+            )
+        )
+        conf = ('---\n'
+                'extends: default\n'
+                f'ignore-from-file: [{", ".join(ignore_files)}]\n')
+
+        with self.assertRaises(SystemExit) as cm:
+            cli.run(('-d', conf, '.'))
+        self.assertEqual(cm.exception.code, 0)

tests/test_decoder.py

@@ -14,6 +14,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import codecs
+import itertools
 import os
 import unittest
 
@@ -22,6 +23,8 @@
     encoding_detectable,
     is_test_codec,
     register_test_codecs,
+    temp_workspace,
+    temp_workspace_with_files_in_many_codecs,
     test_codec_built_in_equivalent,
     unregister_test_codecs,
     uses_bom,
@@ -452,3 +455,30 @@ def test_auto_decode_with_strings_encoded_at_runtime(self):
             msg=("None of the TEST_STRINGS_TO_ENCODE_AT_RUNTIME triggered a "
                  "decoding error.")
         )
+
+    def perform_lines_in_file_test(self, strings):
+        workspace = temp_workspace_with_files_in_many_codecs(
+            '{}',
+            '\n'.join(strings)
+        )
+        with temp_workspace(workspace):
+            iterable = zip(
+                itertools.cycle(strings),
+                decoder.lines_in_files(workspace.keys())
+            )
+            for item in iterable:
+                self.assertEqual(item[0], item[1])
+
+    def test_lines_in_file(self):
+        self.perform_lines_in_file_test((
+            "YAML",
+            "ⓎⒶⓂⓁ",
+            "🅨🅐🅜🅛",
+            "ＹＡＭＬ"
+        ))
+        self.perform_lines_in_file_test((
+            "𝐘𝐀𝐌𝐋",
+            "𝖄𝕬𝕸𝕷",
+            "𝒀𝑨𝑴𝑳",
+            "𝓨𝓐𝓜𝓛"
+        ))

yamllint/config.py

@@ -13,7 +13,6 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import fileinput
 import os.path
 
 import pathspec
@@ -110,8 +109,10 @@ def parse(self, raw_content):
                 raise YamlLintConfigError(
                     'invalid config: ignore-from-file should contain '
                     'filename(s), either as a list or string')
-            with fileinput.input(conf['ignore-from-file']) as f:
-                self.ignore = pathspec.PathSpec.from_lines('gitwildmatch', f)
+            self.ignore = pathspec.PathSpec.from_lines(
+                'gitwildmatch',
+                decoder.lines_in_files(conf['ignore-from-file'])
+            )
         elif 'ignore' in conf:
             if isinstance(conf['ignore'], str):
                 self.ignore = pathspec.PathSpec.from_lines(
@@ -164,9 +165,10 @@ def validate_rule_conf(rule, conf):
                 raise YamlLintConfigError(
                     'invalid config: ignore-from-file should contain '
                     'valid filename(s), either as a list or string')
-            with fileinput.input(conf['ignore-from-file']) as f:
-                conf['ignore'] = pathspec.PathSpec.from_lines(
-                    'gitwildmatch', f)
+            conf['ignore'] = pathspec.PathSpec.from_lines(
+                'gitwildmatch',
+                decoder.lines_in_files(conf['ignore-from-file'])
+            )
         elif ('ignore' in conf and not isinstance(
                 conf['ignore'], pathspec.pathspec.PathSpec)):
             if isinstance(conf['ignore'], str):