ssh-ng: better way to keep SSH errors visible

A better fix than in 104448e75d87d03d2fb0b4ac96d4da72e1dae50d, hence a
revert + the fix.

It turns out that this commit has the side-effect that when having e.g.
`StrictHostKeyChecking=accept-new` for a remote builder, the warnings à la

    Warning: Permanently added 'builder' (ED25519) to the list of known hosts.

actually end up in the derivation's log whereas hostkey verification
errors don't, but only in the stderr of the `nix-build` invocation
(which was the motivation for the patch).

This change writes the stderr from the build-hook to

* the daemon's stderr, so that the SSH errors appear in the journal
  (which was the case before 104448e75d87d03d2fb0b4ac96d4da72e1dae50d)
* the client's stderr, as a log message
* NOT to the drv log (this is handled via `handleJSONLogMessage`)

I tried to fix the issue for legacy-ssh as well, but failed and
ultimately decided to not bother.

I know that we'll sooner or later replace the entire component, however
this is the part of the patch I have working for a while, so I figured I
might still submit it for the time being.

Change-Id: I21ca1aa0d8ae281d2eacddf26e0aa825272707e5

1 files changed, 2 insertions, 1 deletions

diff --git a/tests/nixos/remote-builds-ssh-ng.nix b/tests/nixos/remote-builds-ssh-ng.nix
index 8deb9a504..ec12f9066 100644
--- a/tests/nixos/remote-builds-ssh-ng.nix
+++ b/tests/nixos/remote-builds-ssh-ng.nix
@@ -97,7 +97,8 @@ in
       builder.wait_for_unit("sshd.service")
 
       out = client.fail("nix-build ${expr nodes.client 1} 2>&1")
-      assert "error: failed to start SSH connection to 'root@builder': Host key verification failed" in out, f"No host verification error in {out}"
+      assert "Host key verification failed." in out, f"No host verification error:\n{out}"
+      assert "warning: SSH to 'root@builder' failed, stdout first line: '''" in out, f"No details about which host:\n{out}"
 
       client.succeed(f"ssh -o StrictHostKeyChecking=no {builder.name} 'echo hello world' >&2")