nixos/caddy: Add rfc42 settings option

nixos/modules/services/web-servers/caddy/default.nix

···
         }
       '';
 
+  settingsFormat = pkgs.formats.json { };
+
   configFile =
-    let
-      Caddyfile = pkgs.writeTextDir "Caddyfile" ''
-        {
-          ${cfg.globalConfig}
-        }
-        ${cfg.extraConfig}
-      '';
+    if cfg.settings != { } then
+      settingsFormat.generate "caddy.json" cfg.settings
+    else
+      let
+        Caddyfile = pkgs.writeTextDir "Caddyfile" ''
+          {
+            ${cfg.globalConfig}
+          }
+          ${cfg.extraConfig}
+        '';
 
-      Caddyfile-formatted = pkgs.runCommand "Caddyfile-formatted" { nativeBuildInputs = [ cfg.package ]; } ''
-        mkdir -p $out
-        cp --no-preserve=mode ${Caddyfile}/Caddyfile $out/Caddyfile
-        caddy fmt --overwrite $out/Caddyfile
-      '';
-    in
+        Caddyfile-formatted = pkgs.runCommand "Caddyfile-formatted" { nativeBuildInputs = [ cfg.package ]; } ''
+          mkdir -p $out
+          cp --no-preserve=mode ${Caddyfile}/Caddyfile $out/Caddyfile
+          caddy fmt --overwrite $out/Caddyfile
+        '';
+      in
       "${if pkgs.stdenv.buildPlatform == pkgs.stdenv.hostPlatform then Caddyfile-formatted else Caddyfile}/Caddyfile";
 
   etcConfigFile = "caddy/caddy_config";
···
         to a non-infinite value in {option}`services.caddy.globalConfig`
         to prevent Caddy waiting for active connections to finish,
         which could delay the reload essentially indefinitely.
+      '';
+    };
+
+    settings = mkOption {
+      type = settingsFormat.type;
+      default = {};
+      description = lib.mdDoc ''
+        Structured configuration for Caddy to generate a Caddy JSON configuration file.
+        See <https://caddyserver.com/docs/json/> for available options.
+
+        ::: {.warning}
+        Using a [Caddyfile](https://caddyserver.com/docs/caddyfile) instead of a JSON config is highly recommended by upstream.
+        There are only very few exception to this.
+
+        Please use a Caddyfile via {option}`services.caddy.configFile`, {option}`services.caddy.virtualHosts` or
+        {option}`services.caddy.extraConfig` with {option}`services.caddy.globalConfig` instead.
+        :::
+
+        ::: {.note}
+        Takes presence over most `services.caddy.*` options, such as {option}`services.caddy.configFile` and {option}`services.caddy.virtualHosts`, if specified.
+        :::
       '';
     };
   };

nixos/tests/caddy.nix

···
           "http://localhost:8081" = { };
         };
       };
+      specialisation.rfc42.configuration = {
+        services.caddy.settings = {
+          apps.http.servers.default = {
+            listen = [ ":80" ];
+            routes = [{
+              handle = [{
+                body = "hello world";
+                handler = "static_response";
+                status_code = 200;
+              }];
+            }];
+          };
+        };
+      };
     };
   };
 
···
       etagSystem = "${nodes.webserver.system.build.toplevel}/specialisation/etag";
       justReloadSystem = "${nodes.webserver.system.build.toplevel}/specialisation/config-reload";
       multipleConfigs = "${nodes.webserver.system.build.toplevel}/specialisation/multiple-configs";
+      rfc42Config = "${nodes.webserver.system.build.toplevel}/specialisation/rfc42";
     in
     ''
       url = "http://localhost/example.html"
···
           )
           webserver.wait_for_open_port(8080)
           webserver.wait_for_open_port(8081)
+
+      with subtest("rfc42 settings config"):
+          webserver.succeed(
+              "${rfc42Config}/bin/switch-to-configuration test >&2"
+          )
+          webserver.wait_for_open_port(80)
+          webserver.succeed("curl http://localhost | grep hello")
     '';
 })