Set cachable to false

[benrutter]

Closes issue #42

At the moment, because SSHFS is considered cachable, creating a new filesystem doesn't create a new connection, and maintains the old one.

That's probably not a good fit for SFTP connections, since they're likely to timeout and be closed off after a while, so almost any long running process using SSHFS is eventually going to hit an issue where the filesystem continues to reference a closed off connection, and the cache needs to be manually cleared.

To fix that, I've just set "cachable" to False here, so that creating a new filesystem would also bring in a new connection.

Honestly, I don't have the knowledge to know the full implications of this, but my main thoughts are:

• Does this mean it's worth adding in something to clean up / close the connection when the SSHFS object goes out of memory? (otherwise there are potentially open but unusable connections floating around)
• Is there a case where you'd want it to be cachable? In which case, the "cachable" setting could theoretically be put behind an __init__ argument? (my guess is that'd cause some kind of side effects nobody is expecting though)

[shcheklein]

Thanks for digging into this @benrutter — the diagnosis (cached instances outliving their connections) is right, but after testing this change we're closing it, for two reasons:

1. As written it's a no-op: fsspec's instance cache lives in the _Cached metaclass, which checks the class attribute cls.cachable both when looking up and when storing instances — an instance attribute set inside __init__ is invisible to it. Verified empirically: with this patch applied, constructing SSHFileSystem twice with the same arguments still returns the same cached object.

2. The working variant (cachable = False as a class attribute) would be the wrong default: every construction would pay a fresh SSH handshake (hundreds of ms), a heavy ecosystem-wide cost — DVC and fsspec.open() construct filesystems repeatedly and rely on the instance cache.

The underlying problem has working knobs today:

• SSHFileSystem(host, keepalive_interval=30) — asyncssh keepalives (forwarded by sshfs) prevent idle connections from being dropped, which is the Initialisation seems to maintain cached filesystem #42 scenario,
• SSHFileSystem(..., skip_instance_cache=True) — per-instance opt-out when you genuinely want a fresh connection,
• SSHFileSystem.clear_instance_cache() — recovery in long-lived processes.

See #42 for the full guidance.