Raito Bezarius
0dd8bf6c1c
The pre-flight `echo started` check over SSH was originally added in
577ebeaefb. As it is usual with these old
commits, understanding why is there a need for something is difficult.
The closest thing would be
> Fix a race starting the SSH master. We now wait synchronously for
> the SSH master to finish starting. This prevents the SSH clients
> from starting their own connections.
But, we removed SSH connection sharing, so this does not apply anymore.
Nonetheless, we believed this check was meant as a way to catch obvious
misconfigurations or SSH failures early, before handing off to
`nix-store`. However, this approach was not fruitful: it assumes the
remote has a `bash`-compatible shell, `echo` behaves in a standard way,
and no `ForceCommand` interferes—all of which are unreliable assumptions
in practice.
While the intent was to provide slightly better diagnostics (e.g. in
case of SSH hanging or returning an interactive shell), in practice it
does not meaningfully catch or improve real failure cases. The
underlying protocol or engine can and should handle those errors more
robustly anyway.
In contrast, this check *does* break several legitimate workflows,
including:
* remote builders using `ForceCommand` wrappers (e.g.
`nix-remote-build`-style setups), see
<https://discourse.nixos.org/t/wrapper-to-restrict-builder-access-through-ssh-worth-upstreaming/25834/15>,
* SSHing into minimal environments lacking `bash` (e.g. initrd,
busybox-based systems),
* configurations that don’t default to POSIX-like shells, e.g., nushell
enthusiasts.
As such, we’re removing this code. Protocol mismatch errors and SSH
failures can be rethought and handled more structurally elsewhere in the
engine.
Change-Id: I187f6881375d42ef83987a13a350c97964bbdb30
Signed-off-by: Raito Bezarius <raito@lix.systems>
952 B
952 B
synopsis, issues, cls, category, credits
| synopsis | issues | cls | category | credits | |||||
|---|---|---|---|---|---|---|---|---|---|
| Remove reliance on Bash for remote stores via SSH |
|
|
Fixes |
|
The pre-flight echo started handshake -- added years ago to catch race conditions -- has been removed.
After removal of connection sharing in Lix 2.93, it required a Bash-compatible shell and a standard echo, so it failed on:
- builders protected by
ForceCommandwrappers (e.g.nix-remote-build), - BusyBox / initrd images with no Bash,
- hosts using non-POSIX shells such as Nushell.
The race the probe once addressed was tied to SSH connection-sharing -- since connection-sharing code has already been removed, the probe is now pointless.
Real connection or protocol errors are now left to SSH/Nix to report directly.
This is technically a breaking change if you had scripts that relied on the literal "started" which needs to be updated to rely on other signals, e.g., exit codes.