feat: Use identity_file as deployment key

Will now use the provided `-i` (identity_file) as a stable deployment
key - if specified - rather than always generating a random ephemeral
SSH key.

This is primarily useful for the following reasons:
- error recovery
- controlled phases

*Error Recovery*

If `nixos-anywhere` stops with an error, we might want to connect
remotely to the deployed machine. With the ephemeral key this might not
be possible, because we don't necessarily have access to it anymore.

*Controlled Phases*

It is already possible to control which phases should be run.
With an ephemeral key outside our control we are not able to resume
the phases with a second `nixos-anywhere` invocation, because a
different deployment key will now be generated.

docs/quickstart.md

@@ -113,7 +113,9 @@ example uses a local directory on the source machine.
 If your SSH key is not found, you will be asked for your password. If you are
 using a non-root user, you must have access to sudo without a password. To avoid
 SSH password prompts, set the `SSHPASS` environment variable to your password
-and add `--env-password` to the `nixos-anywhere` command.
+and add `--env-password` to the `nixos-anywhere` command. If providing a
+specific SSH key through `-i` (identity_file), this key will then be used for
+the installation and no temporary SSH key will be created.
 
 ### 7. (Optional) Test your NixOS and Disko configuration
 

src/nixos-anywhere.sh

@@ -367,10 +367,15 @@ runVmTest() {
 }
 
 uploadSshKey() {
-  # we generate a temporary ssh keypair that we can use during nixos-anywhere
   # ssh-copy-id requires this directory
   mkdir -p "$HOME/.ssh/"
-  ssh-keygen -t ed25519 -f "$sshKeyDir"/nixos-anywhere -P "" -C "nixos-anywhere" >/dev/null
+  if [[ -n ${sshPrivateKeyFile} ]]; then
+    cp "$sshPrivateKeyFile" "$sshKeyDir/nixos-anywhere"
+    ssh-keygen -y -f "$sshKeyDir/nixos-anywhere" >"$sshKeyDir/nixos-anywhere.pub"
+  else
+    # we generate a temporary ssh keypair that we can use during nixos-anywhere
+    ssh-keygen -t ed25519 -f "$sshKeyDir"/nixos-anywhere -P "" -C "nixos-anywhere" >/dev/null
+  fi
 
   declare -a sshCopyIdArgs
   if [[ -n ${sshPrivateKeyFile} ]]; then

tests/flake-module.nix

@@ -20,5 +20,6 @@
       from-nixos-with-sudo-stable = import ./from-nixos-with-sudo.nix testInputsStable;
       from-nixos-with-generated-config = import ./from-nixos-generate-config.nix testInputsUnstable;
       from-nixos-build-on-remote = import ./from-nixos-build-on-remote.nix testInputsUnstable;
+      from-nixos-separated-phases = import ./from-nixos-separated-phases.nix testInputsUnstable;
     });
 }

tests/from-nixos-separated-phases.nix

@@ -0,0 +1,52 @@
+(import ./lib/test-base.nix) {
+  name = "from-nixos-separated-phases";
+  nodes = {
+    installer = ./modules/installer.nix;
+    installed = {
+      services.openssh.enable = true;
+      virtualisation.memorySize = 1024;
+
+      users.users.nixos = {
+        isNormalUser = true;
+        openssh.authorizedKeys.keyFiles = [ ./modules/ssh-keys/ssh.pub ];
+        extraGroups = [ "wheel" ];
+      };
+      security.sudo.enable = true;
+      security.sudo.wheelNeedsPassword = false;
+    };
+  };
+  testScript = ''
+    start_all()
+
+    with subtest("Kexec Phase"):
+      installer.succeed("""
+        nixos-anywhere \
+          -i /root/.ssh/install_key \
+          --debug \
+          --kexec /etc/nixos-anywhere/kexec-installer \
+          --phases kexec \
+          --store-paths /etc/nixos-anywhere/disko /etc/nixos-anywhere/system-to-install \
+          nixos@installed >&2
+      """)
+
+    with subtest("Disko Phase"):
+      output = installer.succeed("""
+        nixos-anywhere \
+          -i /root/.ssh/install_key \
+          --debug \
+          --phases disko \
+          --store-paths /etc/nixos-anywhere/disko /etc/nixos-anywhere/system-to-install \
+          installed >&2
+    """)
+
+    with subtest("Install Phase"):
+      installer.succeed("""
+        nixos-anywhere \
+          -i /root/.ssh/install_key \
+          --debug \
+          --phases install \
+          --store-paths /etc/nixos-anywhere/disko /etc/nixos-anywhere/system-to-install \
+          root@installed >&2 
+      """)
+  '';
+}