nixos/cups-pdf: init

Some implementation notes:

* cups-pdf, and cups-pdf-to-pdf,
  support multiple instances with differing configurations.
  This can be accomplished by creating multiple configuration
  files with names `cups-pdf-{instance-name}.conf`.
  The Nixos module supports this feature by providing
  the option `instances` which is an attrset
  mapping instance names to instance configurations.
  To simplify module usage,
  an instance `pdf` is created by default.

* To use a cups-pdf instance, one also needs
  a cups queue that connects to the backend.
  The module does this automatically by default,
  using the `hardware.printers.ensurePrinters`.
  It uses one of the ppd files which is
  included in the cups-pdf package.
  If this isn't desired (e.g. because printer queues
  should be created by hand, or configured differently),
  the `installPrinter` option can be turned off
  (for each instance separately).

* In our configuration, cups calls external programs
  using the `cups` account and the `lp` group.
  cups-pdf refuses to operate without root privileges,
  likely because it needs to change the
  ownership of it output pdf files so that
  (only) the print job's owner can access them.

  The module installs a suid root wrapper for the backend
  program that can only be called by the `lp` group.
  The cups-pdf package is replaced by a wrapper
  package which calls the suid root wrapper.
  So cups can call its backend programs as usual.

2 files changed, 186 insertions, 0 deletions

diff --git a/nixos/modules/module-list.nix b/nixos/modules/module-list.nix
index 24dd30e157501..56f3b98556b79 100644
--- a/nixos/modules/module-list.nix
+++ b/nixos/modules/module-list.nix
@@ -1018,6 +1018,7 @@
   ./services/networking/znc/default.nix
   ./services/printing/cupsd.nix
   ./services/printing/ipp-usb.nix
+  ./services/printing/cups-pdf.nix
   ./services/scheduling/atd.nix
   ./services/scheduling/cron.nix
   ./services/scheduling/fcron.nix
diff --git a/nixos/modules/services/printing/cups-pdf.nix b/nixos/modules/services/printing/cups-pdf.nix
new file mode 100644
index 0000000000000..07f24367132f5
--- /dev/null
+++ b/nixos/modules/services/printing/cups-pdf.nix
@@ -0,0 +1,185 @@
+{ config, lib, pkgs, ... }:
+
+let
+
+  # cups calls its backends as user `lp` (which is good!),
+  # but cups-pdf wants to be called as `root`, so it can change ownership of files.
+  # We add a suid wrapper and a wrapper script to trick cups into calling the suid wrapper.
+  # Note that a symlink to the suid wrapper alone wouldn't suffice, cups would complain
+  # > File "/nix/store/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-cups-progs/lib/cups/backend/cups-pdf" has insecure permissions (0104554/uid=0/gid=20)
+
+  # wrapper script that redirects calls to the suid wrapper
+  cups-pdf-wrapper = pkgs.writeTextFile {
+    name = "${pkgs.cups-pdf-to-pdf.name}-wrapper.sh";
+    executable = true;
+    destination = "/lib/cups/backend/cups-pdf";
+    checkPhase = ''
+      ${pkgs.stdenv.shellDryRun} "$target"
+      ${lib.getExe pkgs.shellcheck} "$target"
+    '';
+    text = ''
+      #! ${pkgs.runtimeShell}
+      exec "${config.security.wrapperDir}/cups-pdf" "$@"
+    '';
+  };
+
+  # wrapped cups-pdf package that uses the suid wrapper
+  cups-pdf-wrapped = pkgs.buildEnv {
+    name = "${pkgs.cups-pdf-to-pdf.name}-wrapped";
+    # using the wrapper as first path ensures it is used
+    paths = [ cups-pdf-wrapper pkgs.cups-pdf-to-pdf ];
+    ignoreCollisions = true;
+  };
+
+  instanceSettings = name: {
+    freeformType = with lib.types; nullOr (oneOf [ int str path package ]);
+    # override defaults:
+    # inject instance name into paths,
+    # also avoid conflicts between user names and special dirs
+    options.Out = lib.mkOption {
+      type = with lib.types; nullOr singleLineStr;
+      default = "/var/spool/cups-pdf-${name}/users/\${USER}";
+      defaultText = "/var/spool/cups-pdf-{instance-name}/users/\${USER}";
+      example = "\${HOME}/cups-pdf";
+      description = lib.mdDoc ''
+        output directory;
+        `''${HOME}` will be expanded to the user's home directory,
+        `''${USER}` will be expanded to the user name.
+      '';
+    };
+    options.AnonDirName = lib.mkOption {
+      type = with lib.types; nullOr singleLineStr;
+      default = "/var/spool/cups-pdf-${name}/anonymous";
+      defaultText = "/var/spool/cups-pdf-{instance-name}/anonymous";
+      example = "/var/lib/cups-pdf";
+      description = lib.mdDoc "path for anonymously created PDF files";
+    };
+    options.Spool = lib.mkOption {
+      type = with lib.types; nullOr singleLineStr;
+      default = "/var/spool/cups-pdf-${name}/spool";
+      defaultText = "/var/spool/cups-pdf-{instance-name}/spool";
+      example = "/var/lib/cups-pdf";
+      description = lib.mdDoc "spool directory";
+    };
+    options.Anonuser = lib.mkOption {
+      type = lib.types.singleLineStr;
+      default = "root";
+      description = lib.mdDoc ''
+        User for anonymous PDF creation.
+        An empty string disables this feature.
+      '';
+    };
+    options.GhostScript = lib.mkOption {
+      type = with lib.types; nullOr path;
+      default = lib.getExe pkgs.ghostscript;
+      defaultText = lib.literalExpression "lib.getExe pkgs.ghostscript";
+      example = lib.literalExpression ''''${pkgs.ghostscript}/bin/ps2pdf'';
+      description = lib.mdDoc "location of GhostScript binary";
+    };
+  };
+
+  instanceConfig = { name, config, ... }: {
+    options = {
+      enable = (lib.mkEnableOption (lib.mdDoc "this cups-pdf instance")) // { default = true; };
+      installPrinter = (lib.mkEnableOption (lib.mdDoc ''
+        a CUPS printer queue for this instance.
+        The queue will be named after the instance and will use the {file}`CUPS-PDF_opt.ppd` ppd file.
+        If this is disabled, you need to add the queue yourself to use the instance
+      '')) // { default = true; };
+      confFileText = lib.mkOption {
+        type = lib.types.lines;
+        description = lib.mdDoc ''
+          This will contain the contents of {file}`cups-pdf.conf` for this instance, derived from {option}`settings`.
+          You can use this option to append text to the file.
+        '';
+      };
+      settings = lib.mkOption {
+        type = lib.types.submodule (instanceSettings name);
+        default = {};
+        example = {
+          Out = "\${HOME}/cups-pdf";
+          UserUMask = "0033";
+        };
+        description = lib.mdDoc ''
+          Settings for a cups-pdf instance, see the descriptions in the template config file in the cups-pdf package.
+          The key value pairs declared here will be translated into proper key value pairs for {file}`cups-pdf.conf`.
+          Setting a value to `null` disables the option and removes it from the file.
+        '';
+      };
+    };
+    config.confFileText = lib.pipe config.settings [
+      (lib.filterAttrs (key: value: value != null))
+      (lib.mapAttrs (key: builtins.toString))
+      (lib.mapAttrsToList (key: value: "${key} ${value}\n"))
+      lib.concatStrings
+    ];
+  };
+
+  cupsPdfCfg = config.services.printing.cups-pdf;
+
+  copyConfigFileCmds = lib.pipe cupsPdfCfg.instances [
+    (lib.filterAttrs (name: lib.getAttr "enable"))
+    (lib.mapAttrs (name: lib.getAttr "confFileText"))
+    (lib.mapAttrs (name: pkgs.writeText "cups-pdf-${name}.conf"))
+    (lib.mapAttrsToList (name: confFile: "ln --symbolic --no-target-directory ${confFile} /var/lib/cups/cups-pdf-${name}.conf\n"))
+    lib.concatStrings
+  ];
+
+  printerSettings = lib.pipe cupsPdfCfg.instances [
+    (lib.filterAttrs (name: lib.getAttr "enable"))
+    (lib.filterAttrs (name: lib.getAttr "installPrinter"))
+    (lib.mapAttrsToList (name: instance: (lib.mapAttrs (key: lib.mkDefault) {
+      inherit name;
+      model = "CUPS-PDF_opt.ppd";
+      deviceUri = "cups-pdf:/${name}";
+      description = "virtual printer for cups-pdf instance ${name}";
+      location = instance.settings.Out;
+    })))
+  ];
+
+in
+
+{
+
+  options.services.printing.cups-pdf = {
+    enable = lib.mkEnableOption (lib.mdDoc ''
+      the cups-pdf virtual pdf printer backend.
+      By default, this will install a single printer `pdf`.
+      but this can be changed/extended with {option}`services.printing.cups-pdf.instances`
+    '');
+    instances = lib.mkOption {
+      type = lib.types.attrsOf (lib.types.submodule instanceConfig);
+      default.pdf = {};
+      example.pdf.settings = {
+        Out = "\${HOME}/cups-pdf";
+        UserUMask = "0033";
+      };
+      description = lib.mdDoc ''
+        Permits to raise one or more cups-pdf instances.
+        Each instance is named by an attribute name, and the attribute's values control the instance' configuration.
+      '';
+    };
+  };
+
+  config = lib.mkIf cupsPdfCfg.enable {
+    services.printing.enable = true;
+    services.printing.drivers = [ cups-pdf-wrapped ];
+    hardware.printers.ensurePrinters = printerSettings;
+    # the cups module will install the default config file,
+    # but we don't need it and it would confuse cups-pdf
+    systemd.services.cups.preStart = lib.mkAfter ''
+      rm -f /var/lib/cups/cups-pdf.conf
+      ${copyConfigFileCmds}
+    '';
+    security.wrappers.cups-pdf = {
+      group = "lp";
+      owner = "root";
+      permissions = "+r,ug+x";
+      setuid = true;
+      source = "${pkgs.cups-pdf-to-pdf}/lib/cups/backend/cups-pdf";
+    };
+  };
+
+  meta.maintainers = [ lib.maintainers.yarny ];
+
+}