fluentd: add simple test

nixos/tests/all-tests.nix

@@ -85,6 +85,7 @@ in
   flannel = handleTestOn ["x86_64-linux"] ./flannel.nix {};
   flatpak = handleTest ./flatpak.nix {};
   flatpak-builder = handleTest ./flatpak-builder.nix {};
+  fluentd = handleTest ./fluentd.nix {};
   fsck = handleTest ./fsck.nix {};
   fwupd = handleTestOn ["x86_64-linux"] ./fwupd.nix {}; # libsmbios is unsupported on aarch64
   gdk-pixbuf = handleTest ./gdk-pixbuf.nix {};

nixos/tests/fluentd.nix

@@ -0,0 +1,46 @@
+import ./make-test.nix ({ pkgs, lib, ... }: {
+  name = "fluentd";
+
+  machine = { pkgs, ... }: {
+    services.fluentd = {
+      enable = true;
+      config = ''
+        <source>
+          @type http
+          port 9880
+        </source>
+
+        <match **>
+          type copy
+          <store>
+            @type file
+            format json
+            path /tmp/fluentd
+            symlink_path /tmp/current-log
+          </store>
+          <store>
+            @type stdout
+          </store>
+        </match>
+      '';
+    };
+  };
+
+  testScript = let
+    testMessage = "an example log message";
+
+    payload = pkgs.writeText "test-message.json" (builtins.toJSON {
+      inherit testMessage;
+    });
+  in ''
+    $machine->start;
+    $machine->waitForUnit('fluentd.service');
+    $machine->waitForOpenPort(9880);
+
+    $machine->succeed("curl -fsSL -X POST -H 'Content-type: application/json' -d @${payload} http://localhost:9880/test.tag");
+
+    $machine->succeed("systemctl stop fluentd"); # blocking flush
+
+    $machine->succeed("grep '${testMessage}' /tmp/current-log");
+  '';
+})