Fix "File name too long" error when a title is used as a folder name

src/ytdl_sub/utils/file_path.py

@@ -51,6 +51,13 @@ def _truncate_file_name(cls, file_name: str) -> str:
 
         return f"{file_sub_name}{delimiter}{file_ext}"
 
+    @classmethod
+    def _truncate_directory_name(cls, directory_name: str) -> str:
+        while len(directory_name.encode("utf-8")) > cls._MAX_BASE_FILE_NAME_BYTES:
+            directory_name = directory_name[:-1]
+
+        return directory_name
+
     @classmethod
     def maybe_truncate_file_path(cls, file_path: str) -> str:
         """Turn into a Path, then a string, to get correct directory separators"""
@@ -61,6 +68,30 @@ def maybe_truncate_file_path(cls, file_path: str) -> str:
 
         return str(file_path)
 
+    @classmethod
+    def maybe_truncate_file_name_path(cls, file_name_path: str) -> str:
+        """
+        Truncates each component of a relative output file name path. Both the
+        subdirectories (which can be derived from metadata such as the title) and the
+        final file name are truncated if they exceed the OS limit. The file name's
+        extension is always preserved.
+        """
+        parts = Path(file_name_path).parts
+        if not parts:
+            return file_name_path
+
+        *directory_parts, file_name = parts
+
+        truncated_parts = [
+            cls._truncate_directory_name(part) if cls._is_file_name_too_long(part) else part
+            for part in directory_parts
+        ]
+
+        if cls._is_file_name_too_long(file_name):
+            file_name = cls._truncate_file_name(file_name)
+
+        return str(Path(*truncated_parts, file_name))
+
     @classmethod
     def to_native_filepath(cls, file_path: str) -> str:
         """Ensures file paths use the correct separator"""

src/ytdl_sub/validators/file_path_validators.py

@@ -48,7 +48,7 @@ class StringFormatterFileNameValidator(StringFormatterValidator):
 
     def post_process(self, resolved: str) -> str:
         return FilePathTruncater.to_native_filepath(
-            FilePathTruncater.maybe_truncate_file_path(resolved)
+            FilePathTruncater.maybe_truncate_file_name_path(resolved)
         )
 
 

tests/unit/validators/test_file_path_validators.py

@@ -77,6 +77,46 @@ def test_truncates_file_names_successfully(self, ext: str):
                 assert len(dir_paths) == 1
                 assert Path(truncated_file_path) == dir_paths[0]
 
+    @pytest.mark.parametrize("ext", ["mp4", "-thumb.jpg"])
+    def test_truncates_subdirectory_in_file_name_path(self, ext: str):
+        if "thumb" not in ext:  # do not put . in front of -thumb
+            ext = f".{ext}"
+
+        # Mathematical bold unicode characters are 4 bytes each in UTF-8, so a title used
+        # as a subdirectory can easily exceed the OS file name limit (typically 255 bytes).
+        long_directory = (
+            "[2026] 𝗿𝘁𝗶𝘀𝘁 𝗢𝗻𝗲 - 𝗔 𝗩𝗲𝗿𝘆 𝗟𝗼𝗻𝗴 𝗦𝗼𝗻𝗴 𝗧𝗶𝘁𝗹𝗲 "
+            "𝗧𝗵𝗮𝘁 𝗘𝘅𝗰𝗲𝗲𝗱𝘀 𝗧𝗵𝗲 𝗟𝗶𝗺𝗶𝘁 (𝗘𝘅𝘁𝗲𝗻𝗱𝗲𝗱 𝗠𝗶𝘅 𝘅 "
+            "𝗦𝗲𝗰𝗼𝗻𝗱 𝗔𝗿𝘁𝗶𝘀𝘁 & 𝗧𝗵𝗶𝗿𝗱 𝗔𝗿𝘁𝗶𝘀𝘁)"
+        )
+        # Sanity check that the directory really does exceed the limit
+        assert len(long_directory.encode("utf-8")) > FilePathTruncater._MAX_BASE_FILE_NAME_BYTES
+
+        with tempfile.TemporaryDirectory() as temp_dir:
+            file_path = str(Path(temp_dir) / long_directory / f"video{ext}")
+
+            formatter = StringFormatterFileNameValidator(name="test", value="")
+            truncated_file_path = formatter.post_process(file_path)
+
+            truncated_directory, truncated_file_name = os.path.split(truncated_file_path)
+            _, truncated_directory_name = os.path.split(truncated_directory)
+
+            # Both the subdirectory and the file name must fit within the OS limit
+            assert (
+                len(truncated_directory_name.encode("utf-8"))
+                <= FilePathTruncater._MAX_BASE_FILE_NAME_BYTES
+            )
+            assert (
+                len(truncated_file_name.encode("utf-8"))
+                <= FilePathTruncater._MAX_BASE_FILE_NAME_BYTES
+            )
+            assert truncated_file_name.endswith(ext)
+
+            # The original bug raised "OSError: [Errno 36] Filename too long" here because
+            # the subdirectory component was never truncated.
+            os.makedirs(truncated_directory, exist_ok=True)
+            assert os.path.isdir(truncated_directory)
+
     @pytest.mark.parametrize(
         "file_name_max_bytes, expected_max",
         [