Checksum command fails if remote doesn't support uname operation

When I try to fs.checksum(path) on a server that does not permit a uname command, I get a generic Channel Open Error: Session request failed. Looking through the debug logs, it seems like the issue is caused by the logic in the _get_system:

https://github.com/fsspec/sshfs/blob/3c10c1bfff44f111926d763a54343726832e2d42/sshfs/spec.py#L295:L300

The server I am working with does support the md5sum and sha1sum commands so the _checksum method as written should work but the actual command never triggers because it errors before that.

Potential Solutions:

• Provide ability to pass checksum commands to the _checksum() method, this would allow me to specify a known command (so the _get_system() check can be bypassed). Ideally, being able to provide both the remote and the local checksum commands would be even better in the case of a Darwin system speaking with a Linux server.
• Include error handling logic for _get_system() to provide a more detailed error message.

If I can get some guidance on the preferred approach, and if contributions are welcome. I'm happy to submit a PR.

Thank you!
Pratheek

Start with sshfs/spec.py at the linked _get_system lines and trace how fs.checksum(path) reaches _checksum. Compare the failure path with the proposed checksum-command and error-handling options; done means the selected approach works when uname is unavailable and provides a useful outcome.