wiki-js: use nodejs18

Part of #229910.

Unfortunately this is a little hacky because upstream doesn't intend to
support it for 2.5, but only for 3.0 which isn't out yet, however nodejs-16
will get out of maintenance during the support-span of NixOS 23.05[1].

The only breaking change is that `extract-files` uses a deprecated way
of exposing modules, I went through the list of other breaking
changes in v17 and v18[2][3] and couldn't spot any usage of removed
features, also local testing didn't reveal further issues.

Unfortunately fixing that breakage turned out to be non-trivial.
Currently, `extract-files@9.0.0` is used with the problematic portions
in its `package.json`, however it's only a transitive dependency of
`@graphql-tools/url-loader` & `apollo-upload-client`. Unfortunately, the
versions of that in use require v9 and don't work with a newer version of
`extract-files` with the problem fixed[4]. Also, upgrading the
dependencies in question is not a feasible option because `graphql-tools`
was split up into multiple smaller packages in v8 and also some of the
APIs in use in `wiki.js` were dropped there[5], so this would also be
very time-consuming and non-trivial to fix.

Since this was the only issue, I decided to go down the hacky route and
patch the problem in `package.json` of `extract-files` manually during
our `patchPhase`.

[1] https://github.com/requarks/wiki/discussions/6388
[2] https://nodejs.org/en/blog/release/v17.0.0
[3] https://nodejs.org/en/blog/release/v18.0.0
[4] Upon local testing, this broke with the following error:

        Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './public/extractFiles' is not defined by "exports" in /wiki/node_modules/extract-files/package.json
[5] For instance `SchemaDirectiveVisitor` in
  `server/graph/directives/auth`.

nixos/modules/services/web-apps/wiki-js.nix

@@ -133,7 +133,7 @@ in {
         WorkingDirectory = "/var/lib/${cfg.stateDirectoryName}";
         DynamicUser = true;
         PrivateTmp = true;
-        ExecStart = "${pkgs.nodejs_16}/bin/node ${pkgs.wiki-js}/server";
+        ExecStart = "${pkgs.nodejs_18}/bin/node ${pkgs.wiki-js}/server";
       };
     };
   };

pkgs/servers/web-apps/wiki-js/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, lib, nixosTests }:
+{ stdenv, fetchurl, lib, nixosTests, jq, moreutils }:
 
 stdenv.mkDerivation rec {
   pname = "wiki-js";
@@ -9,6 +9,40 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-O7KQ134zh9ullYyQZimmxfdRwXeHkD8aAhy/pRzIjxo=";
   };
 
+  # Implements nodejs 18 support as it's not planned to fix this before
+  # the release of v3[1] which is planned to happen in 2023, but not before
+  # NixOS 23.05. However, in the lifespan of 23.05 v16 will get EOLed, so
+  # we have to hack this on our own.
+  #
+  # The problem we fix here is that `exports."/public/"` in a `package.json`
+  # is prohibited, i.e. you cannot export full directories anymore.
+  #
+  # Unfortunately it's non-trivial to fix this because v10 of `extract-files`
+  # (where the problem is fixed) doesn't work for graphql-tools (which depends
+  # on this). Updating this as well is also quite complex because in later
+  # versions the package was split up into multiple smaller packages and
+  # thus a lot of parts of the code-base would need to be changed accordingly.
+  #
+  # Since this is the only breaking change of nodejs 17/18[2][3], this workaround
+  # will be necessary until we can upgrade to v3.
+  #
+  # [1] https://github.com/requarks/wiki/discussions/6388
+  # [2] https://nodejs.org/en/blog/release/v17.0.0
+  # [3] https://nodejs.org/en/blog/release/v18.0.0
+  nativeBuildInputs = [ jq moreutils ];
+  postPatch = ''
+    # Dirty hack to implement nodejs-18 support.
+    <./node_modules/extract-files/package.json jq '
+      # error out loud if the structure has changed and we need to change
+      # this expression
+      if .exports|has("./public/")|not then
+        halt_error(1)
+      else
+        .exports."./public/*" = "./public/*.js" | del(.exports."./public/")
+      end
+    ' | sponge ./node_modules/extract-files/package.json
+  '';
+
   sourceRoot = ".";
 
   dontBuild = true;