ssh-ng: Set log-fd for ssh to `4` by default

That's expected by `build-remote` and makes sure that errors are
correctly forwarded to the user. For instance, let's say that the
host-key of `example.org` is unknown and

    nix-build ../nixpkgs -A hello -j0 --builders 'ssh-ng://example.org'

is issued, then you get the following output:

    cannot build on 'ssh-ng://example.org?&': error: failed to start SSH connection to 'example.org'
    Failed to find a machine for remote build!
    derivation: yh46gakxq3kchrbihwxvpn5bmadcw90b-hello-2.12.1.drv
    required (system, features): (x86_64-linux, [])
    2 available machines:
    [...]

The relevant information (`Host key verification failed`) ends up in the
daemon's log, but that's not very obvious considering that the daemon
isn't very chatty normally.

This can be fixed - the same way as its done for legacy-ssh - by passing
fd 4 to the SSH wrapper. Now you'd get the following error:

    cannot build on 'ssh-ng://example.org': error: failed to start SSH connection to 'example.org': Host key verification failed.
    Failed to find a machine for remote build!
    [...]

...and now it's clear what's wrong.

Please note that this is won't end up in the derivation's log.

For previous discussion about this change see
https://github.com/NixOS/nix/pull/7659.

Change-Id: I5790856dbf58e53ea3e63238b015ea06c347cf92

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstore/machines.cc b/src/libstore/machines.cc
index ecae3054e..700c9b3dd 100644
--- a/src/libstore/machines.cc
+++ b/src/libstore/machines.cc
@@ -69,10 +69,10 @@ ref<Store> Machine::openStore() const
     Store::Params storeParams;
     if (storeUri.starts_with("ssh://")) {
         storeParams["max-connections"] = "1";
-        storeParams["log-fd"] = "4";
     }
 
     if (storeUri.starts_with("ssh://") || storeUri.starts_with("ssh-ng://")) {
+        storeParams["log-fd"] = "4";
         if (sshKey != "")
             storeParams["ssh-key"] = sshKey;
         if (sshPublicHostKey != "")