module(documentation): Add documentation configuration to avoid to build manCache

nixos/modules/default.nix

@@ -10,5 +10,6 @@
     ./programs
     ./hardware.nix
     ./virtualisation
+    ./documentation.nix
   ];
 }

nixos/modules/documentation.nix

@@ -0,0 +1,124 @@
+{
+  pkgs,
+  config,
+  lib,
+  ...
+}:
+lib.mkMerge [
+  {
+    environment.systemPackages = with pkgs; [man-pages man-pages-posix];
+    documentation = {
+      info.enable = false;
+      nixos.enable = false;
+      dev.enable = true;
+    };
+  }
+  (let
+    inherit (pkgs.writers) writeFish;
+    cfg = config.documentation.man.man-db;
+    cachePath = "/var/cache/man/nixos";
+  in {
+    documentation.man.generateCaches = false;
+
+    systemd.services."man-db" = {
+      requires = ["sysinit-reactivation.target"];
+      after = ["sysinit-reactivation.target"];
+      partOf = ["sysinit-reactivation.target"];
+      wantedBy = ["default.target"];
+      path = [
+        cfg.package
+        pkgs.gawk
+      ];
+
+      serviceConfig = {
+        Nice = 19;
+        IOSchedulingClass = "idle";
+        IOSchedulingPrioriry = 7;
+        ExecStart =
+          writeFish "mandbsvc" # fish
+          
+          ''
+            set -l SystemManLoc "/run/current-system/sw/share/man"
+            set -l ContentRecord "${cachePath}/man-db-state"
+
+            if [ ! -d "${cachePath}" ]
+              mkdir -pv "${cachePath}" || exit 1
+            end
+
+            if [ ! -f "$ContentRecord" ]
+              touch "$ContentRecord" || exit 1
+            end
+            # 1) Collect list of all manpage files and calculate hashes
+            # of them
+            #
+            # man1/ls.1.gz
+            # man3/func.3.gz
+            #
+            # hash ->
+            #
+            # bbbbbbbbbbbb (man1/ls.1.gz)
+            # aaaaaaaaaaaa (man3/func.3.gz)
+            set -l hashes "$(
+                find -L "$SystemManLoc" -type f -iname "*.gz" \
+                    -exec sha256sum "{}" "+" \
+                | awk '{ print $1 }'
+                or exit 1
+            )"
+
+            # 2) Sort the hashes to make them "stable",
+            # and then join them toghther into a big long string,
+            # and then hash this big string to get the hash of the directory
+            #
+            # bbbbbbbbbbbb
+            # aaaaaaaaaaaa
+            #
+            # sort ->
+            #
+            # aaaaaaaaaaaa
+            # bbbbbbbbbbbb
+            #
+            # join ->
+            #
+            # aaaaaaaaaaaabbbbbbbbbbbb
+            #
+            # hash ->
+            #
+            # cccccccccccc
+            set -l ultimate_hash (
+                echo $hashes \
+                | sort \
+                | string join "" \
+                | sha256sum - \
+                | awk '{ print $1 }'
+                or exit 1
+            )
+
+            set -l old_hash "$( string collect < "$ContentRecord" )"
+
+            echo "Old hash: $old_hash"
+            echo "New hash: $ultimate_hash"
+
+            if [ "$old_hash" != "$ultimate_hash" ]
+                echo "Hash changed, do a full man-db rebuild"
+                mandb -psc || exit 1
+                echo "Write new hash"
+                echo "$ultimate_hash" > "$ContentRecord"
+            else
+                echo "Hash not changed, skip"
+            end
+          '';
+      };
+    };
+
+    environment.extraSetup =
+      # bash
+      ''
+        find "$out/share/man" \
+          -mindepth 1 -maxdepth 1 \
+          -not -name "man[1-8]" \
+          -exec rm -r "{}" ";"
+
+        rm -r "$out/share/man/man3"
+      '';
+  })
+]