Cast not changes runtype type

[alex-eri]

If name set as string and longname not set there is exception in this part of code TypeError: can't concat str to bytes

asyncssh/asyncssh/sftp.py

Line 7163 in 5972676

name.longname = detail.encode('utf-8') + cast(bytes, name.filename)

In CPython cast do nothing

https://github.com/python/cpython/blob/1af025dd2206eecee3ee6242f2a7cdb67173fb97/Lib/typing.py#L2369-L2377

I suggest remove union types BytesOrStr as bytes always used in input params.

[alex-eri]

Or make SFTPServerHandler to convert paths to string and use only strings in SFTPServer as high-level interface.

[ronf]

Thanks for the report. I'll look into whether I can tighten any of the type names here. It's a bit tricky, though, as SFTPName is used on both the client side and server side, and on the client side the filename and longname parameters can be either bytes or str, depending on the setting of the path_encoding and in some places whether you pass in a str or bytes value in as a path. For instance, you can pass a str to scandir() and it will return SFTPName objects with names of type str. However, if you pass in a bytes value to the same function, it will return names of type bytes.

In the case of format_longname(), it is only called from the _process_readdir() function in the SFTPServerHandler, so in that case the filename will always be of type bytes (making the cast safe, even though it doesn't really do any conversion). Path and file names in the server case are always bytes.

Note that it would not be a good idea here to try and convert all paths to strings. First, we wouldn't necessarily know the encoding used on the server, but more importantly many servers store filenames as raw bytes and so there could be many different encodings within a given filesystem. Since SFTP defines that all file/path names are encoded on the wire as bytes, AsyncSSH uses only bytes everywhere when acting as an SFTP server. However, when acting as an SFTP client, it lets the caller choose whether to use bytes or str, as long as path_encoding is set. By default, this is set to 'uff-8'.

How did you manage to get into format_longname() with a str for the SFTPName's filename?