nixos: doc: move non-service parts of service.nixosManual to documentation.nixos

nixos/modules/misc/documentation.nix

@@ -1,8 +1,72 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, baseModules, ... }:
 
 with lib;
 
-let cfg = config.documentation; in
+let
+
+  cfg = config.documentation;
+
+  /* For the purpose of generating docs, evaluate options with each derivation
+    in `pkgs` (recursively) replaced by a fake with path "\${pkgs.attribute.path}".
+    It isn't perfect, but it seems to cover a vast majority of use cases.
+    Caveat: even if the package is reached by a different means,
+    the path above will be shown and not e.g. `${config.services.foo.package}`. */
+  manual = import ../../doc/manual rec {
+    inherit pkgs config;
+    version = config.system.nixos.release;
+    revision = "release-${version}";
+    options =
+      let
+        scrubbedEval = evalModules {
+          modules = [ { nixpkgs.localSystem = config.nixpkgs.localSystem; } ] ++ baseModules;
+          args = (config._module.args) // { modules = [ ]; };
+          specialArgs = { pkgs = scrubDerivations "pkgs" pkgs; };
+        };
+        scrubDerivations = namePrefix: pkgSet: mapAttrs
+          (name: value:
+            let wholeName = "${namePrefix}.${name}"; in
+            if isAttrs value then
+              scrubDerivations wholeName value
+              // (optionalAttrs (isDerivation value) { outPath = "\${${wholeName}}"; })
+            else value
+          )
+          pkgSet;
+      in scrubbedEval.options;
+  };
+
+  helpScript = pkgs.writeScriptBin "nixos-help"
+    ''
+      #! ${pkgs.runtimeShell} -e
+      # Finds first executable browser in a colon-separated list.
+      # (see how xdg-open defines BROWSER)
+      browser="$(
+        IFS=: ; for b in $BROWSER; do
+          [ -n "$(type -P "$b" || true)" ] && echo "$b" && break
+        done
+      )"
+      if [ -z "$browser" ]; then
+        browser="$(type -P xdg-open || true)"
+        if [ -z "$browser" ]; then
+          browser="$(type -P w3m || true)"
+          if [ -z "$browser" ]; then
+            echo "$0: unable to start a web browser; please set \$BROWSER"
+            exit 1
+          fi
+        fi
+      fi
+      exec "$browser" ${manual.manualHTMLIndex}
+    '';
+
+  desktopItem = pkgs.makeDesktopItem {
+    name = "nixos-manual";
+    desktopName = "NixOS Manual";
+    genericName = "View NixOS documentation in a web browser";
+    icon = "nix-snowflake";
+    exec = "${helpScript}/bin/nixos-help";
+    categories = "System";
+  };
+
+in
 
 {
 
@@ -66,6 +130,22 @@ let cfg = config.documentation; in
         '';
       };
 
+      nixos.enable = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Whether to install NixOS's own documentation.
+          <itemizedlist>
+          <listitem><para>This includes man pages like
+                    <citerefentry><refentrytitle>configuration.nix</refentrytitle>
+                    <manvolnum>5</manvolnum></citerefentry> if <option>man.enable</option> is
+                    set.</para></listitem>
+          <listitem><para>This includes the HTML manual and the <command>nixos-help</command> command if
+                    <option>doc.enable</option> is set.</para></listitem>
+          </itemizedlist>
+        '';
+      };
+
     };
 
   };
@@ -99,6 +179,21 @@ let cfg = config.documentation; in
       environment.extraOutputsToInstall = [ "doc" ] ++ optional cfg.dev.enable "devdoc";
     })
 
+    (mkIf cfg.nixos.enable {
+      system.build.manual = manual;
+
+      environment.systemPackages = []
+        ++ optional cfg.man.enable manual.manpages
+        ++ optionals cfg.doc.enable ([ manual.manualHTML helpScript ]
+           ++ optionals config.services.xserver.enable [ desktopItem pkgs.nixos-icons ]);
+
+      services.mingetty.helpLine = mkIf cfg.doc.enable (
+          "\nRun `nixos-help` "
+        + optionalString config.services.nixosManual.showManual "or press <Alt-F${toString config.services.nixosManual.ttyNumber}> "
+        + "for the NixOS manual."
+      );
+    })
+
   ]);
 
 }