modules/microvm-host: add `update-microvm` command

modules/microvm-host.nix

@@ -67,5 +67,43 @@
         "microvm.cachix.org-1:oXnBc6hRE3eX5rSYdRyMYXnfzcCxC7yKPTbZXALsqys="
       ];
     };
+
+    environment.systemPackages = [ (
+      # Provide a manual updating script that fetches the latest
+      # updated+built system from Hydra
+      pkgs.writeScriptBin "update-microvm" ''
+        #! ${pkgs.runtimeShell} -e
+
+        if [ $# -lt 1 ]; then
+          NAMES="$(ls -1 /var/lib/microvms)"
+        else
+          NAMES="$@"
+        fi
+
+        for NAME in $NAMES; do
+          echo MicroVM $NAME
+          cd /var/lib/microvms/$NAME
+          if [ "$(cat flake)" = "git+https://gitea.c3d2.de/c3d2/nix-config?ref=flake-update" ]; then
+            NEW=$(curl -sLH "Accept: application/json" https://hydra.hq.c3d2.de/job/c3d2/nix-config/$NAME/latest | ${pkgs.jq}/bin/jq -r .buildoutputs.out.path)
+            nix copy --from https://hydra.hq.c3d2.de $NEW
+
+            if [ -e booted ]; then
+              nix store diff-closures $(readlink booted) $NEW
+            else
+              echo "NOT BOOTED?"
+            fi
+            if [ "$(readlink current)" != $NEW ]; then
+              rm -f old
+              mv current old
+              ln -s $NEW current
+            fi
+          fi
+
+          echo Run at your own peril:
+          echo systemctl restart microvm@$NAME
+          echo
+        done
+      ''
+    ) ];
   };
 }