atuin: configure daemon using systemd and launchd

This configures the atuin daemon for Linux and Darwin systems using
systemd and launchd, respectively. For systemd, a socket is also
automatically configured to exist at atuin's default socket location.

modules/programs/atuin.nix

@@ -8,6 +8,7 @@ let
 
   tomlFormat = pkgs.formats.toml { };
 
+  inherit (pkgs.stdenv) isLinux isDarwin;
 in {
   meta.maintainers = [ maintainers.hawkw ];
 
@@ -94,11 +95,13 @@ in {
         Whether to enable Nushell integration.
       '';
     };
+
+    daemon.enable = mkEnableOption "atuin daemon";
   };
 
   config = let flagsStr = escapeShellArgs cfg.flags;
-  in mkIf cfg.enable {
-
+  in mkIf cfg.enable (mkMerge [
+    {
       # Always add the configured `atuin` package.
       home.packages = [ cfg.package ];
 
@@ -138,5 +141,63 @@ in {
           source ${config.xdg.cacheHome}/atuin/init.nu
         '';
       };
-  };
+    }
+
+    (mkIf cfg.daemon.enable (mkMerge [
+      {
+        assertions = [{
+          assertion = isLinux || isDarwin;
+          message =
+            "The atuin daemon can only be configured on either Linux or macOS.";
+        }];
+
+        programs.atuin.settings = { daemon = { enabled = true; }; };
+      }
+      (mkIf isLinux {
+        programs.atuin.settings = { daemon = { systemd_socket = true; }; };
+
+        systemd.user.services.atuin-daemon = {
+          Unit = {
+            Description = "atuin daemon";
+            Requires = [ "atuin-daemon.socket" ];
+          };
+          Install = {
+            Also = [ "atuin-daemon.socket" ];
+            WantedBy = [ "default.target" ];
+          };
+          Service = {
+            ExecStart = "${lib.getExe cfg.package} daemon";
+            Environment = [ "ATUIN_LOG=info" ];
+            Restart = "on-failure";
+            RestartSteps = 3;
+            RestartMaxDelaySec = 6;
+          };
+        };
+
+        systemd.user.sockets.atuin-daemon = {
+          Unit = { Description = "atuin daemon socket"; };
+          Install = { WantedBy = [ "sockets.target" ]; };
+          Socket = {
+            ListenStream = "%h/.local/share/atuin/atuin.sock";
+            SocketMode = "0600";
+            RemoveOnStop = true;
+          };
+        };
+      })
+      (mkIf isDarwin {
+        launchd.agents.atuin-daemon = {
+          enable = true;
+          config = {
+            ProgramArguments = [ "${lib.getExe cfg.package}" "daemon" ];
+            EnvironmentVariables = { ATUIN_LOG = "info"; };
+            KeepAlive = {
+              Crashed = true;
+              SuccessfulExit = false;
+            };
+            ProcessType = "Background";
+          };
+        };
+      })
+    ]))
+  ]);
 }