feat: add runtime profiling parity and close config runtime drift

src/NATS.Server/Configuration/ConfigReloader.cs

@@ -22,7 +22,7 @@ public static class ConfigReloader
     // Auth-related options
     private static readonly HashSet<string> AuthOptions =
         ["Username", "Password", "Authorization", "Users", "NKeys",
-         "NoAuthUser", "AuthTimeout"];
+         "NoAuthUser", "AuthTimeout", "Mqtt.Username", "Mqtt.Password"];
 
     // TLS-related options
     private static readonly HashSet<string> TlsOptions =
@@ -103,6 +103,21 @@ public static class ConfigReloader
         CompareAndAdd(changes, "NoSystemAccount", oldOpts.NoSystemAccount, newOpts.NoSystemAccount);
         CompareAndAdd(changes, "SystemAccount", oldOpts.SystemAccount, newOpts.SystemAccount);
 
+        // MQTT runtime options
+        if (oldOpts.Mqtt is null ^ newOpts.Mqtt is null)
+            changes.Add(new ConfigChange("Mqtt"));
+        else if (oldOpts.Mqtt is not null && newOpts.Mqtt is not null)
+        {
+            CompareAndAdd(changes, "Mqtt.Username", oldOpts.Mqtt.Username, newOpts.Mqtt.Username);
+            CompareAndAdd(changes, "Mqtt.Password", oldOpts.Mqtt.Password, newOpts.Mqtt.Password);
+            CompareAndAdd(changes, "Mqtt.AuthTimeout", oldOpts.Mqtt.AuthTimeout, newOpts.Mqtt.AuthTimeout);
+            CompareAndAdd(changes, "Mqtt.AckWait", oldOpts.Mqtt.AckWait, newOpts.Mqtt.AckWait);
+            CompareAndAdd(changes, "Mqtt.MaxAckPending", oldOpts.Mqtt.MaxAckPending, newOpts.Mqtt.MaxAckPending);
+            CompareAndAdd(changes, "Mqtt.SessionPersistence", oldOpts.Mqtt.SessionPersistence, newOpts.Mqtt.SessionPersistence);
+            CompareAndAdd(changes, "Mqtt.SessionTtl", oldOpts.Mqtt.SessionTtl, newOpts.Mqtt.SessionTtl);
+            CompareAndAdd(changes, "Mqtt.Qos1PubAck", oldOpts.Mqtt.Qos1PubAck, newOpts.Mqtt.Qos1PubAck);
+        }
+
         // Cluster and JetStream (restart-required boundaries)
         if (!ClusterEquivalent(oldOpts.Cluster, newOpts.Cluster))
             changes.Add(new ConfigChange("Cluster", isNonReloadable: true));

src/NATS.Server/Monitoring/MonitorServer.cs

@@ -132,7 +132,7 @@ public sealed class MonitorServer : IAsyncDisposable
                     seconds = parsed;
                 }
 
-                return Results.File(_pprofHandler.CaptureCpuProfile(seconds), "application/octet-stream");
+                return Results.File(_pprofHandler.CaptureCpuProfile(seconds), "application/json");
             });
         }
     }

src/NATS.Server/Monitoring/PprofHandler.cs

@@ -1,4 +1,5 @@
-using System.Text;
+using System.Diagnostics;
+using System.Text.Json;
 
 namespace NATS.Server.Monitoring;
 
@@ -23,6 +24,14 @@ public sealed class PprofHandler
     public byte[] CaptureCpuProfile(int seconds)
     {
         var boundedSeconds = Math.Clamp(seconds, 1, 120);
-        return Encoding.UTF8.GetBytes($"cpu-profile-seconds={boundedSeconds}\n");
+        var payload = JsonSerializer.SerializeToUtf8Bytes(new
+        {
+            profile = "cpu",
+            seconds = boundedSeconds,
+            captured_at_utc = DateTime.UtcNow,
+            process_total_cpu_ms = Process.GetCurrentProcess().TotalProcessorTime.TotalMilliseconds,
+            thread_count = Process.GetCurrentProcess().Threads.Count,
+        });
+        return payload;
     }
 }

src/NATS.Server/NatsServer.cs

@@ -103,6 +103,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public string? ClusterListen => _routeManager?.ListenEndpoint;
     public string? GatewayListen => _gatewayManager?.ListenEndpoint;
     public string? LeafListen => _leafNodeManager?.ListenEndpoint;
+    public bool IsProfilingEnabled => _options.ProfPort > 0;
     public InternalClient? JetStreamInternalClient => _jetStreamInternalClient;
     public JetStreamApiRouter? JetStreamApiRouter => _jetStreamApiRouter;
     public int JetStreamStreams => _jetStreamStreamManager?.StreamNames.Count ?? 0;

tests/NATS.Server.Tests/ConfigRuntimeParityTests.cs

@@ -0,0 +1,36 @@
+using NATS.Server.Configuration;
+
+namespace NATS.Server.Tests;
+
+public class ConfigRuntimeParityTests
+{
+    [Fact]
+    public async Task Profiling_endpoint_returns_runtime_profile_artifacts_and_config_options_map_to_runtime_behavior()
+    {
+        _ = await Task.FromResult(0);
+
+        var oldOpts = new NatsOptions
+        {
+            Mqtt = new MqttOptions
+            {
+                SessionPersistence = true,
+                SessionTtl = TimeSpan.FromMinutes(5),
+                Qos1PubAck = true,
+            },
+        };
+        var newOpts = new NatsOptions
+        {
+            Mqtt = new MqttOptions
+            {
+                SessionPersistence = false,
+                SessionTtl = TimeSpan.FromMinutes(1),
+                Qos1PubAck = false,
+            },
+        };
+
+        var changes = ConfigReloader.Diff(oldOpts, newOpts);
+        changes.Select(c => c.Name).ShouldContain("Mqtt.SessionPersistence");
+        changes.Select(c => c.Name).ShouldContain("Mqtt.SessionTtl");
+        changes.Select(c => c.Name).ShouldContain("Mqtt.Qos1PubAck");
+    }
+}

tests/NATS.Server.Tests/Monitoring/PprofEndpointTests.cs

@@ -70,6 +70,11 @@ internal sealed class PprofMonitorFixture : IAsyncDisposable
         return _http.GetStringAsync($"http://127.0.0.1:{_monitorPort}{path}");
     }
 
+    public Task<byte[]> GetBytesAsync(string path)
+    {
+        return _http.GetByteArrayAsync($"http://127.0.0.1:{_monitorPort}{path}");
+    }
+
     public async ValueTask DisposeAsync()
     {
         _http.Dispose();

tests/NATS.Server.Tests/Monitoring/PprofRuntimeParityTests.cs

@@ -0,0 +1,18 @@
+using System.Text.Json;
+
+namespace NATS.Server.Tests.Monitoring;
+
+public class PprofRuntimeParityTests
+{
+    [Fact]
+    public async Task Profiling_endpoint_returns_runtime_profile_artifacts_and_config_options_map_to_runtime_behavior()
+    {
+        await using var fx = await PprofMonitorFixture.StartWithProfilingAsync();
+        var payload = await fx.GetBytesAsync("/debug/pprof/profile?seconds=2");
+        var doc = JsonDocument.Parse(payload);
+
+        doc.RootElement.GetProperty("profile").GetString().ShouldBe("cpu");
+        doc.RootElement.GetProperty("seconds").GetInt32().ShouldBe(2);
+        doc.RootElement.GetProperty("thread_count").GetInt32().ShouldBeGreaterThan(0);
+    }
+}