3 files changed, 379 insertions, 0 deletions
diff --git a/modules/by-name/ni/nixos-shell/module.nix b/modules/by-name/ni/nixos-shell/module.nix
new file mode 100644
index 00000000..219f080d
--- /dev/null
+++ b/modules/by-name/ni/nixos-shell/module.nix
@@ -0,0 +1,128 @@
+# nixos-config - My current NixOS configuration
+#
+# Copyright (C) 2025 Jörg Thalheim and contributors
+# SPDX-License-Identifier: MIT
+#
+# This file is part of my nixos-config.
+#
+# You should have received a copy of the License along with this program.
+# If not, see <https://www.gnu.org/licenses/gpl-3.0.txt>.
+{
+  lib,
+  config,
+  pkgs,
+  self,
+  ...
+}: let
+  cfg = config.soispha.nixos-shell;
+in {
+  options.soispha.nixos-shell = {
+    enable = lib.mkEnableOption "nixos-shell";
+
+    user_name = lib.mkOption {
+      type = lib.types.str;
+      default = "soispha";
+      description = "The user to auto-login into the vm.";
+    };
+
+    configuration = {
+      specialArgs = lib.mkOption {
+        type = lib.types.attrsOf lib.types.anything;
+        default = {};
+        description = ''
+          The arguments to pass to the `specialArgs` attribute set.
+        '';
+      };
+      value = lib.mkOption {
+        type = lib.types.deferredModule;
+        default = {};
+        description = ''
+          Additional NixOS configuration to load into the VM's config.
+        '';
+      };
+    };
+
+    mounts = lib.mkOption {
+      type = lib.types.attrsOf (lib.types.submodule ({config, ...}: {
+        options = {
+          target = lib.mkOption {
+            type = lib.types.path;
+            description = "Target on the guest.";
+          };
+
+          cache_mode = lib.mkOption {
+            type = lib.types.enum ["none" "loose" "fscache" "mmap"];
+            default = "loose"; # bad idea? Well, at least it is fast!1!!
+            description = "9p caching policy";
+          };
+
+          readOnly =
+            (lib.mkEnableOption "mount this disk in read-only mode")
+            // {
+              default = true;
+            };
+
+          tag = lib.mkOption {
+            type = lib.types.str;
+            internal = true;
+          };
+        };
+
+        config.tag = lib.mkDefault (
+          builtins.substring 0 31 ( # tags must be shorter than 32 bytes
+            "a"
+            + # tags must not begin with a digit
+            builtins.hashString "md5" config._module.args.name
+          )
+        );
+      }));
+      default = {};
+      description = ''
+        Extra paths to make available in the vm.
+        These will be mounted ro to their `target.`
+      '';
+    };
+  };
+
+  config = let
+    vmSystem = self.inputs.nixpkgs.lib.nixosSystem {
+      inherit (cfg.configuration) specialArgs;
+
+      modules = [
+        {
+          # TODO(@bpeetz): This should be bumped each release. <2025-05-17>
+          system.stateVersion = "25.05";
+        }
+
+        cfg.configuration.value
+
+        (import ./shell_setup.nix {inherit cfg;})
+      ];
+    };
+
+    nixos-shell = pkgs.writeShellApplication {
+      name = "nixos-shell";
+      text = builtins.readFile ./nixos-shell.sh;
+
+      # We need to keep the PATH, as we otherwise can't pass it along.
+      inheritPath = true;
+
+      runtimeInputs = [
+        vmSystem.config.system.build.vm
+        pkgs.mktemp
+        pkgs.coreutils
+        pkgs.moreutils # for sponge
+      ];
+      runtimeEnv = {
+        HOST_NAME = vmSystem.config.system.name;
+      };
+    };
+  in
+    lib.mkIf cfg.enable {
+      environment.systemPackages = [
+        nixos-shell
+      ];
+
+      system.build.nixos-shell = vmSystem.config.system.build.vm;
+    };
+}
diff --git a/modules/by-name/ni/nixos-shell/nixos-shell.sh b/modules/by-name/ni/nixos-shell/nixos-shell.sh
new file mode 100755
index 00000000..390e60b1
--- /dev/null
+++ b/modules/by-name/ni/nixos-shell/nixos-shell.sh
@@ -0,0 +1,60 @@
+#!/usr/bin/env dash
+
+# nixos-config - My current NixOS configuration
+#
+# Copyright (C) 2025 Jörg Thalheim and contributors
+# SPDX-License-Identifier: MIT
+#
+# This file is part of my nixos-config.
+#
+# You should have received a copy of the License along with this program.
+# If not, see <https://www.gnu.org/licenses/gpl-3.0.txt>.
+
+SHARED_DIR="$(mktemp --directory)"
+cleanup() {
+    rm --recursive "$SHARED_DIR"
+}
+trap cleanup EXIT
+export SHARED_DIR
+
+TMPDIR="$SHARED_DIR"
+export TMPDIR
+
+cat <<EOF >"$SHARED_DIR/.env_variables"
+{
+    "pwd": "$PWD",
+    "term": "$TERM",
+    "path": "$PATH"
+}
+EOF
+
+mk_tag() {
+    additional_path="$1"
+
+    # tags must be shorter than 32 bytes
+    # and must not begin with a digit.
+    {
+        printf "a"
+        echo "$additional_path" | sha256sum | head -c 30
+    }
+}
+
+for raw_additional_path in "$@"; do
+    additional_path="$(realpath "$raw_additional_path")"
+    tag="$(mk_tag "$additional_path")"
+
+    if [ "$(jq --arg mount "$tag" '.mount.[$mount]' "$SHARED_DIR/.env_variables")" != "null" ]; then
+        echo "Path '$additional_path' alread added."
+        shift 1
+        continue
+    fi
+
+    jq --arg mount "$tag" --arg target "$additional_path" \
+        '. + {mount: {$mount: $target}}' "$SHARED_DIR/.env_variables" | sponge "$SHARED_DIR/.env_variables"
+
+    set -- "$@" -virtfs "local,path=$additional_path,security_model=none,readonly=on,mount_tag=$tag"
+    shift 1
+done
+
+# Do not exec here, as we don't want to lose our cleanup hooks.
+"run-$HOST_NAME-vm" "$@"
diff --git a/modules/by-name/ni/nixos-shell/shell_setup.nix b/modules/by-name/ni/nixos-shell/shell_setup.nix
new file mode 100644
index 00000000..75af0678
--- /dev/null
+++ b/modules/by-name/ni/nixos-shell/shell_setup.nix
@@ -0,0 +1,191 @@
+# nixos-config - My current NixOS configuration
+#
+# Copyright (C) 2025 Jörg Thalheim and contributors
+# SPDX-License-Identifier: MIT
+#
+# This file is part of my nixos-config.
+#
+# You should have received a copy of the License along with this program.
+# If not, see <https://www.gnu.org/licenses/gpl-3.0.txt>.
+# This file contains the code that is used to setup the VM shell.
+{cfg}: {
+  lib,
+  config,
+  pkgs,
+  modulesPath,
+  ...
+}: let
+  mkVMDefault = lib.mkOverride 970;
+
+  env_file = "/tmp/shared/.env_variables";
+in {
+  imports = [
+    (modulesPath + "/profiles/qemu-guest.nix")
+    (modulesPath + "/virtualisation/qemu-vm.nix")
+  ];
+
+  config = {
+    assertions = [
+      {
+        assertion = builtins.elem cfg.user_name (lib.mapAttrsToList (name: value: name) config.users.users);
+        message = ''
+          Your user ${cfg.user_name} is not a recorded user in `config.users.users`.
+          Auto-login will not work.
+        '';
+      }
+    ];
+
+    # Lock root login.
+    users.users.root.hashedPassword = "";
+
+    # Remove unneeded clutter.
+    system.switch.enable = false;
+
+    virtualisation = {
+      # See https://wiki.qemu.org/Documentation/9psetup#Performance_Considerations.
+      # It is effectively a balance between ram and IO speed.
+      msize = let
+        kib = x: x * 1024;
+      in
+        mkVMDefault (kib 512);
+
+      graphics = mkVMDefault false;
+      memorySize = mkVMDefault 1700; # in MB
+      cores = mkVMDefault 16;
+
+      # Do not persist this VM.
+      diskImage = mkVMDefault null;
+
+      fileSystems =
+        lib.mapAttrs'
+        (_: mount: {
+          name = mount.target;
+
+          value = {
+            device = mount.tag;
+            fsType = "9p";
+            options =
+              [
+                "trans=virtio"
+                "version=9p2000.L"
+                "cache=${mount.cache_mode}"
+                "msize=${toString config.virtualisation.msize}"
+              ]
+              ++ lib.optionals mount.readOnly ["ro"];
+          };
+        })
+        cfg.mounts;
+
+      qemu = {
+        consoles = lib.mkIf (!config.virtualisation.graphics) ["tty0" "hvc0"];
+
+        options = let
+          mkMount = options: "-virtfs " + (builtins.concatStringsSep "," options);
+        in
+          lib.optionals (!config.virtualisation.graphics) [
+            "-serial null"
+            "-device virtio-serial"
+            "-chardev stdio,mux=on,id=char0,signal=off"
+            "-mon chardev=char0,mode=readline"
+            "-device virtconsole,chardev=char0,nr=0"
+          ]
+          ++ (lib.mapAttrsToList
+            (hostPath: mount:
+              mkMount [
+                "local"
+                "path=${builtins.toString hostPath}"
+                "security_model=none"
+                "readonly=on"
+                "mount_tag=${mount.tag}"
+              ])
+            cfg.mounts);
+      };
+    };
+
+    services = {
+      getty.helpLine = ''
+        If you are connect via serial console:
+        Type Ctrl-a c to switch to the qemu console
+        and `quit` to stop the VM.
+      '';
+
+      getty.autologinUser = cfg.user_name;
+    };
+
+    system.activationScripts = {
+      mountAdditionalPaths =
+        # bash
+        ''
+          PATH="${pkgs.jq}/bin:${pkgs.util-linux}/bin:$PATH"
+          export PATH
+
+          max_index="$(jq '.mount | keys | length' --raw-output ${env_file})"
+          index=0
+
+          mount --mkdir --type=tmpfs none "/.rw" --options=rw,relatime,mode=0755
+          while [ "$index" -lt "$max_index" ]; do
+            what="$(jq --argjson index "$index" '.mount | keys | map(.)[$index]' --raw-output ${env_file})"
+            where="$(jq --argjson index "$index" '.mount | map(.)[$index]' --raw-output ${env_file})"
+
+            mkdir "/.rw/$what"
+            mount --mkdir "$what" "/.ro/$what" \
+              --type=9p \
+              --options=ro,trans=virtio,version=9p2000.L,msize=${toString config.virtualisation.msize},x-systemd.requires=modprobe@9pnet_virtio.service
+
+            mkdir "/.rw/work-$what"
+            mount --mkdir --type=overlay overlay \
+              --options=rw,relatime,lowerdir="/.ro/$what",upperdir="/.rw/$what",workdir="/.rw/work-$what",uuid=on \
+              "/.ov/$what"
+
+            index="$((index + 1))"
+          done
+        '';
+    };
+
+    systemd.services.mountAdditionalPaths = {
+      after = ["local-fs.target"];
+      wantedBy = ["multi-user.target"];
+      path = [pkgs.jq];
+      script =
+        # bash
+        ''
+          max_index="$(jq '.mount | keys | length' --raw-output ${env_file})"
+          index=0
+
+          while [ "$index" -lt "$max_index" ]; do
+            what="$(jq --argjson index "$index" '.mount | keys | map(.)[$index]' --raw-output ${env_file})"
+            where="$(jq --argjson index "$index" '.mount | map(.)[$index]' --raw-output ${env_file})"
+
+            systemd-mount --type none "/.ov/$what" "$where" --options=bind
+
+            # HACK(@bpeetz): Nearly all of the paths are in $HOME anyways. So simply avoid
+            # the permission issue.
+            # Ideally, we would pass the original owners along with the mount. <2025-05-17>
+            chown --recursive soispha:users "/home/soispha"
+
+            index="$((index + 1))"
+          done
+        '';
+    };
+
+    environment = {
+      systemPackages = [
+        pkgs.jq
+      ];
+
+      sessionVariables = {
+        IN_NIXOS_SHELL = "true";
+      };
+
+      loginShellInit =
+        # bash
+        ''
+          cd "$(jq '.pwd' --raw-output ${env_file})"
+          export TERM="$(jq '.term' --raw-output ${env_file})"
+          export PATH="$(jq '.path' --raw-output ${env_file}):$PATH"
+        '';
+    };
+
+    networking.firewall.enable = mkVMDefault true;
+  };
+}