Initial attempt at configuring from EC2 userdata (with input from cstrahan). Now with VM tests!

nixos/modules/virtualisation/amazon-init.nix

@@ -0,0 +1,52 @@
+{ config, pkgs, modulesPath, ... }:
+
+# This attempts to pull a nix expression from this EC2 instance's user-data.
+
+let
+  bootScript = pkgs.writeScript "bootscript.sh" ''
+    #!${pkgs.stdenv.shell} -eux
+
+    echo "attempting to fetch configuration from user-data..."
+
+    export PATH=${pkgs.nix}/bin:${pkgs.wget}/bin:${pkgs.systemd}/bin:${pkgs.gnugrep}/bin:${pkgs.gnused}/bin:${config.system.build.nixos-rebuild}/bin:$PATH
+    export NIX_PATH=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels
+
+    userData="$(mktemp)"
+    wget -q --wait=1 --tries=0 --retry-connrefused -O - http://169.254.169.254/2011-01-01/user-data > "$userData"
+
+    if [[ $? -eq 0 ]]; then
+      echo "user-data fetched"
+      # If the user-data looks like it could be a nix expression,
+      # copy it over. Also, look for a magic three-hash comment and set
+      # that as the channel.
+      if sed '/^\(#\|SSH_HOST_.*\)/d' < "$userData" | grep -q '\S'; then
+        channels="$(grep '^###' "$userData" | sed 's|###\s*||')"
+        printf "%s" "$channels" | while read channel; do
+          echo "writing channel: $channel"
+        done
+
+        if [[ -n "$channels" ]]; then
+          printf "%s" "$channels" > /root/.nix-channels
+          nix-channel --update
+        fi
+
+        echo "setting configuration"
+        cp "$userData" /etc/nixos/configuration.nix
+      else
+        echo "user-data does not appear to be a nix expression; ignoring"
+      fi
+    else
+      echo "failed to fetch user-data"
+    fi
+
+    type -f nixos-rebuild
+
+    nixos-rebuild switch
+  '';
+in {
+  imports = [ "${modulesPath}/virtualisation/amazon-image.nix" ];
+  ec2.metadata = true;
+  boot.postBootCommands = ''
+    ${bootScript} &
+  '';
+}