Describe the bug

A clear and concise description of what the bug is.

c.f. NixOS/nixops#1557

When NixOps sets NIX_SSHOPTS to include -oControlPath=..., nix-copy-closure hangs.

Steps To Reproduce

1. Update to Nix 2.15
2. nixops deploy -d ...
3. Observe: "copying closure..." which never proceeds.

Expected behavior

Files to copy between stores.

nix-env --version output

nix-env (Nix) 2.15.0

Additional context

When stuck in this mode, I noticed that strace attached to the nix-copy-closure process reported

read(7,

and on the remote side, strace attached to the corresponding nix-store process reported

read(0,

The remote side was 2.3 initially, and I was able to use nix-copy-closure without an SSH ControlPath to update to 2.15. Neither worked with 2.15 on the local side; downgrading to 2.14 corrected the issue.