feat(users/Profpatsch): add a example for omega search

Uses xapian under the hood to index the contents, then makes it
searchable with a CGI binary on http://localhost:8080

We could in theory index every -doc output this way to get local
documentation search for the current system (similar to `man-db`).

Change-Id: I2588c8f100841cfbed570bb65d376b79747c06ee
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12710
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/omega.nix

@@ -0,0 +1,62 @@
+{ depot, pkgs, lib, ... }:
+let
+
+  # fix: https://github.com/NixOS/nixpkgs/pull/352144
+  omega-templates = pkgs.stdenv.mkDerivation {
+    name = "omega-templates";
+    phases = [ "unpackPhase" "installPhase" ];
+    src = pkgs.xapian-omega.src;
+    installPhase = ''
+      mkdir -p $out/share/omega
+      mv templates $out/share/omega
+    '';
+  };
+
+  omega-test = pkgs.stdenv.mkDerivation {
+    name = "omega-test";
+    phases = [ "installPhase" ];
+    installPhase = ''
+        mkdir localhost
+        ln -sT localhost localhost:8080
+        mkdir ./default
+        cp -r ${lib.getOutput "doc" pkgs.tipidee}/share/doc/tipidee ./default/tipidee
+        cp -r ${lib.getOutput "doc" pkgs.tipidee}/share/doc/tipidee ./localhost/tipidee
+
+        ln -sT ${pkgs.writers.writeText "omega.conf" ''
+          database_dir ./omega.db
+          log_dir /tmp/omega.log
+          template_dir ${omega-templates}/share/omega/templates
+        ''} omega.conf
+
+        ${pkgs.tipidee}/bin/tipidee-config -o tipidee.cdb -i ${pkgs.writers.writeText "tipidee.conf" ''
+          domain localhost
+          cgi /omega
+        ''}
+
+      cp ${pkgs.xapian-omega}/lib/xapian-omega/bin/omega ./localhost
+
+      mkdir omega.db
+      ${pkgs.xapian-omega}/bin/omindex --db ./omega.db/default --url / ./default
+
+      mkdir -p $out
+      cp -r ./* $out
+    '';
+  };
+
+  run-omega = pkgs.writers.writeBash "run-omega" ''
+    cd ${omega-test}
+    emptyenv -p \
+      ${pkgs.s6-networking}/bin/s6-tcpserver 127.0.0.1 8080 \
+      ${pkgs.s6-networking}/bin/s6-tcpserver-access \
+      ${pkgs.tipidee}/bin/tipideed -f tipidee.cdb
+  '';
+
+
+in
+{
+  inherit
+    run-omega
+    omega-test
+    ;
+}
+