feat: add reload semantics for cluster and jetstream options

src/NATS.Server/Configuration/ConfigReloader.cs

@@ -11,7 +11,8 @@ namespace NATS.Server.Configuration;
 public static class ConfigReloader
 {
     // Non-reloadable options (match Go server — Host, Port, ServerName require restart)
-    private static readonly HashSet<string> NonReloadable = ["Host", "Port", "ServerName"];
+    private static readonly HashSet<string> NonReloadable =
+        ["Host", "Port", "ServerName", "Cluster", "JetStream.StoreDir"];
 
     // Logging-related options
     private static readonly HashSet<string> LoggingOptions =
@@ -102,6 +103,13 @@ public static class ConfigReloader
         CompareAndAdd(changes, "NoSystemAccount", oldOpts.NoSystemAccount, newOpts.NoSystemAccount);
         CompareAndAdd(changes, "SystemAccount", oldOpts.SystemAccount, newOpts.SystemAccount);
 
+        // Cluster and JetStream (restart-required boundaries)
+        if (!ClusterEquivalent(oldOpts.Cluster, newOpts.Cluster))
+            changes.Add(new ConfigChange("Cluster", isNonReloadable: true));
+
+        if (JetStreamStoreDirChanged(oldOpts.JetStream, newOpts.JetStream))
+            changes.Add(new ConfigChange("JetStream.StoreDir", isNonReloadable: true));
+
         return changes;
     }
 
@@ -338,4 +346,35 @@ public static class ConfigReloader
                 isNonReloadable: NonReloadable.Contains(name)));
         }
     }
+
+    private static bool ClusterEquivalent(ClusterOptions? oldCluster, ClusterOptions? newCluster)
+    {
+        if (oldCluster is null && newCluster is null)
+            return true;
+
+        if (oldCluster is null || newCluster is null)
+            return false;
+
+        if (!string.Equals(oldCluster.Name, newCluster.Name, StringComparison.Ordinal))
+            return false;
+
+        if (!string.Equals(oldCluster.Host, newCluster.Host, StringComparison.Ordinal))
+            return false;
+
+        if (oldCluster.Port != newCluster.Port)
+            return false;
+
+        return oldCluster.Routes.SequenceEqual(newCluster.Routes, StringComparer.Ordinal);
+    }
+
+    private static bool JetStreamStoreDirChanged(JetStreamOptions? oldJetStream, JetStreamOptions? newJetStream)
+    {
+        if (oldJetStream is null && newJetStream is null)
+            return false;
+
+        if (oldJetStream is null || newJetStream is null)
+            return true;
+
+        return !string.Equals(oldJetStream.StoreDir, newJetStream.StoreDir, StringComparison.Ordinal);
+    }
 }

src/NATS.Server/NatsServer.cs

@@ -1025,10 +1025,22 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     /// the changes, and applies reloadable settings. CLI overrides are preserved.
     /// </summary>
     public void ReloadConfig()
+    {
+        ReloadConfigCore(throwOnError: false);
+    }
+
+    public void ReloadConfigOrThrow()
+    {
+        ReloadConfigCore(throwOnError: true);
+    }
+
+    private void ReloadConfigCore(bool throwOnError)
     {
         if (_options.ConfigFile == null)
         {
             _logger.LogWarning("No config file specified, cannot reload");
+            if (throwOnError)
+                throw new InvalidOperationException("No config file specified.");
             return;
         }
 
@@ -1054,6 +1066,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             {
                 foreach (var err in errors)
                     _logger.LogError("Config reload error: {Error}", err);
+                if (throwOnError)
+                    throw new InvalidOperationException(string.Join("; ", errors));
                 return;
             }
 
@@ -1065,6 +1079,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         catch (Exception ex)
         {
             _logger.LogError(ex, "Failed to reload config file: {ConfigFile}", _options.ConfigFile);
+            if (throwOnError)
+                throw;
         }
     }