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

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 != "")

src/libstore/ssh-store.cc

@@ -32,6 +32,10 @@ struct SSHStoreConfig : virtual RemoteStoreConfig, virtual CommonSSHStoreConfig
 class SSHStore : public virtual SSHStoreConfig, public virtual RemoteStore
 {
 public:
+    // Hack for getting remote build log output.
+    // Intentionally not in `SSHStoreConfig` so that it doesn't appear in
+    // the documentation
+    const Setting<int> logFD{(StoreConfig*) this, -1, "log-fd", "file descriptor to which SSH's stderr is connected"};
 
     SSHStore(const std::string & scheme, const std::string & host, const Params & params)
         : StoreConfig(params)
@@ -47,7 +51,8 @@ public:
             sshPublicHostKey,
             // Use SSH master only if using more than 1 connection.
             connections->capacity() > 1,
-            compress)
+            compress,
+            logFD)
     {
     }
 

tests/nixos/remote-builds-ssh-ng.nix

@@ -95,6 +95,10 @@ in
       builder.succeed("mkdir -p -m 700 /root/.ssh")
       builder.copy_from_host("key.pub", "/root/.ssh/authorized_keys")
       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}"
+
       client.succeed(f"ssh -o StrictHostKeyChecking=no {builder.name} 'echo hello world' >&2")
 
       # Perform a build