unison: better retry/restart reporting failures

The service was never marked with a failed state with the previous
approach, which could lead broken synchronisation pair states to go
undetected.

The module now uses a timer instead of unlimited restarts, which does
not have this issue.

modules/services/unison.nix

@@ -65,9 +65,10 @@ let
 
   serialiseArgs = args: concatStringsSep " " (mapAttrsToList serialiseArg args);
 
+  unitName = name: "unison-pair-${name}";
+
   makeDefs = gen:
-    mapAttrs'
+    mapAttrs' (name: pairCfg: nameValuePair (unitName name) (gen name pairCfg))
-    (name: pairCfg: nameValuePair "unison-pair-${name}" (gen name pairCfg))
     cfg.pairs;
 
 in {
@@ -106,19 +107,10 @@ in {
     ];
 
     systemd.user.services = makeDefs (name: pairCfg: {
-      Unit = {
+      Unit.Description = "Unison pair sync (${name})";
-        Description = "Unison pair sync (${name})";
-        # Retry forever, useful in case of network disruption.
-        StartLimitIntervalSec = 0;
-      };
-
       Service = {
-        Restart = "always";
-        RestartSec = 60;
-
         CPUSchedulingPolicy = "idle";
         IOSchedulingClass = "idle";
-
         Environment = [ "UNISON='${toString pairCfg.stateDirectory}'" ];
         ExecStart = ''
           ${cfg.package}/bin/unison \
@@ -126,8 +118,16 @@ in {
             ${strings.concatMapStringsSep " " escapeShellArg pairCfg.roots}
         '';
       };
+    });
 
-      Install = { WantedBy = [ "default.target" ]; };
+    systemd.user.timers = makeDefs (name: pairCfg: {
+      Unit.Description = "Unison pair sync auto-restart (${name})";
+      Install.WantedBy = [ "timers.target" ];
+      Timer = {
+        Unit = "${unitName name}.service";
+        OnActiveSec = 1;
+        OnUnitInactiveSec = 60;
+      };
     });
   };
 }