nixos/journalbeat: support journalbeat >= 6 & add test

nixos/modules/services/logging/journalbeat.nix

@@ -5,11 +5,13 @@ with lib;
 let
   cfg = config.services.journalbeat;
 
+  lt6 = builtins.compareVersions cfg.package.version "6" < 0;
+
   journalbeatYml = pkgs.writeText "journalbeat.yml" ''
     name: ${cfg.name}
     tags: ${builtins.toJSON cfg.tags}
 
-    journalbeat.cursor_state_file: ${cfg.stateDir}/cursor-state
+    ${optionalString lt6 "journalbeat.cursor_state_file: /var/lib/${cfg.stateDir}/cursor-state"}
 
     ${cfg.extraConfig}
   '';
@@ -22,6 +24,16 @@ in
 
       enable = mkEnableOption "journalbeat";
 
+      package = mkOption {
+        type = types.package;
+        default = pkgs.journalbeat;
+        defaultText = "pkgs.journalbeat";
+        example = literalExample "pkgs.journalbeat7";
+        description = ''
+          The journalbeat package to use
+        '';
+      };
+
       name = mkOption {
         type = types.str;
         default = "journalbeat";
@@ -36,13 +48,17 @@ in
 
       stateDir = mkOption {
         type = types.str;
-        default = "/var/lib/journalbeat";
-        description = "The state directory. Journalbeat's own logs and other data are stored here.";
+        default = "journalbeat";
+        description = ''
+          Directory below <literal>/var/lib/</literal> to store journalbeat's
+          own logs and other data. This directory will be created automatically
+          using systemd's StateDirectory mechanism.
+        '';
       };
 
       extraConfig = mkOption {
         type = types.lines;
-        default = ''
+        default = optionalString lt6 ''
           journalbeat:
             seek_position: cursor
             cursor_seek_fallback: tail
@@ -61,7 +77,16 @@ in
 
   config = mkIf cfg.enable {
 
-    systemd.services.journalbeat = with pkgs; {
+    assertions = [
+      {
+        assertion = !hasPrefix "/" cfg.stateDir;
+        message =
+          "The option services.journalbeat.stateDir shouldn't be an absolute directory." +
+          " It should be a directory relative to /var/lib/.";
+      }
+    ];
+
+    systemd.services.journalbeat = {
       description = "Journalbeat log shipper";
       wantedBy = [ "multi-user.target" ];
       preStart = ''
@@ -69,7 +94,13 @@ in
         mkdir -p ${cfg.stateDir}/logs
       '';
       serviceConfig = {
-        ExecStart = "${pkgs.journalbeat}/bin/journalbeat -c ${journalbeatYml} -path.data ${cfg.stateDir}/data -path.logs ${cfg.stateDir}/logs";
+        StateDirectory = cfg.stateDir;
+        ExecStart = ''
+          ${cfg.package}/bin/journalbeat \
+            -c ${journalbeatYml} \
+            -path.data /var/lib/${cfg.stateDir}/data \
+            -path.logs /var/lib/${cfg.stateDir}/logs'';
+        Restart = "always";
       };
     };
   };