krops: enable evaluating nodes with non-native systems

docs/install.md

@@ -238,6 +238,24 @@ You can also build Nix from source by following the instructions at https://nixo
     cp -r ../nix-bitcoin/examples/{nix-bitcoin-release.nix,configuration.nix,shell.nix,krops,.gitignore} .
     ```
 
+#### Optional: Specify the system of your node
+   This enables evaluating your node config on a machine that has a different system platform
+   than your node.\
+   Examples: Deploying from macOS or deploying from a x86 desktop PC to a Raspberry Pi.
+
+    ```
+    # Run this when your node has a 64-Bit x86 CPU (e.g., an Intel or AMD CPU)
+    echo "x86_64-linux" > krops/system
+
+    # Run this when your node has a 64-Bit ARM CPU (e.g., Raspberry Pi 4 B, Pine64)
+    echo "aarch64-linux" > krops/system
+    ```
+    Other available systems:
+    - `i686-linux` (`x86`)
+    - `armv7l-linux` (`ARMv7`)\
+      This platform is untested and has no binary caches.
+      [See here](https://nixos.wiki/wiki/NixOS_on_ARM) for details.
+
 ## 4. Deploy with krops
 
 1. Edit your ssh config

helper/makeShell.nix

@@ -109,11 +109,25 @@ pkgs.stdenv.mkDerivation {
       $(nix-build --no-out-link "${cfgDir}/krops/deploy.nix")
     )}
 
-    eval-config() {
+    eval-config() {(
+      set -euo pipefail
+      system=$(getNodeSystem)
       NIXOS_CONFIG="${cfgDir}/krops/krops-configuration.nix" \
-        nix-instantiate --eval ${nixpkgs}/nixos -A system.outPath | tr -d '"'
+        nix-instantiate --eval ${nixpkgs}/nixos $system -A system.outPath | tr -d '"'
       echo
-    }
+    )}
+
+    getNodeSystem() {
+      if [[ -e '${cfgDir}/krops/system' ]]; then
+        echo -n "--argstr system "; cat '${cfgDir}/krops/system'
+      elif [[ $OSTYPE == darwin* ]]; then
+        # On macOS, `builtins.currentSystem` (`*-darwin`) can never equal
+        # the node system (`*-linux`), so we can always provide a helpful error message:
+        >&2 echo "Error, node system not set. See here how to fix this:"
+        >&2 echo "https://github.com/fort-nix/nix-bitcoin/blob/master/docs/install.md#optional-specify-the-system-of-your-node"
+        return 1
+      fi
+    };
 
     pidClosure() {
       echo "$1"