nix-copy-closure to ssh:// store tries to log in with empty SSH username

[the-sun-will-rise-tomorrow]

Describe the bug

I'm not sure if this is supposed to work, but it does seem to almost work, so maybe it should work?

If I run nix-copy-closure --to 'ssh://user@machine?base64-ssh-public-host-key=...&ssh-key=...', Nix seems to try to (eventually) log in to the target machine with an empty SSH username ("").

Local logs:

//ssh://foobar@foobar.example.com: Permission denied (publickey).
error: failed to start SSH connection to '//ssh://foobar@foobar.example.com'

Remote logs:

Jun 12 14:49:09 foobar sshd[1256612]: Accepted publickey for foobar from 192.168.0.2 port 33540 ssh2: ED25519 SHA256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Jun 12 14:49:09 foobar sshd[1256612]: pam_unix(sshd:session): session opened for user foobar(uid=30033) by (uid=0)
Jun 12 14:49:09 foobar systemd-logind[622]: New session 1580 of user foobar.
Jun 12 14:49:09 foobar systemd[1]: Started session-1580.scope - Session 1580 of User foobar.
Jun 12 14:49:09 foobar sshd[1256612]: pam_env(sshd:session): deprecated reading of user environment enabled
Jun 12 14:49:09 foobar sshd[1256619]: Received disconnect from 192.168.0.2 port 33540:11: disconnected by user
Jun 12 14:49:09 foobar sshd[1256619]: Disconnected from user foobar 192.168.0.2 port 33540
Jun 12 14:49:09 foobar sshd[1256612]: pam_unix(sshd:session): session closed for user foobar
Jun 12 14:49:09 foobar systemd[1]: session-1580.scope: Deactivated successfully.
Jun 12 14:49:09 foobar systemd-logind[622]: Session 1580 logged out. Waiting for processes to exit.
Jun 12 14:49:09 foobar systemd-logind[622]: Removed session 1580.
Jun 12 14:49:10 foobar sshd[1256622]: Invalid user  from 192.168.0.2 port 33554
Jun 12 14:49:10 foobar sshd[1256622]: Connection closed by invalid user  192.168.0.2 port 33554 [preauth]

Notice how it seems to log in successfully but the last two lines show that it seems to try logging in with an empty username.

Steps To Reproduce

1. nix-copy-closure --to 'ssh://user@machine?base64-ssh-public-host-key=...&ssh-key=...'

Expected behavior

Work, or fail in a reasonable way.

nix-env --version output

nix-env (Nix) 2.19.1

Additional context

nix --extra-experimental-features nix-command copy instead of nix-copy-closure seems to work OK.

Priorities

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