From 89465450a11c5a7a07275611c5845be830af7d98 Mon Sep 17 00:00:00 2001
From: Joseph Doherty <dohejw01@gmail.com>
Date: Mon, 23 Feb 2026 05:38:10 -0500
Subject: [PATCH] fix: use per-SID callback dictionary in SysSubscribe to
 support multiple subscriptions

---
 src/NATS.Server/Events/InternalEventSystem.cs | 29 ++++++++++++-------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/src/NATS.Server/Events/InternalEventSystem.cs b/src/NATS.Server/Events/InternalEventSystem.cs
index 96f84d8..b5707f8 100644
--- a/src/NATS.Server/Events/InternalEventSystem.cs
+++ b/src/NATS.Server/Events/InternalEventSystem.cs
@@ -58,6 +58,7 @@ public sealed class InternalEventSystem : IAsyncDisposable
 
     private ulong _sequence;
     private int _subscriptionId;
+    private readonly ConcurrentDictionary<string, SystemMessageHandler> _callbacks = new();
 
     public Account SystemAccount { get; }
     public InternalClient SystemClient { get; }
@@ -100,20 +101,26 @@ public sealed class InternalEventSystem : IAsyncDisposable
             Client = SystemClient,
         };
 
-        // Wrap callback in noInlineCallback pattern: enqueue to receive loop
+        // Store callback keyed by SID so multiple subscriptions work
+        _callbacks[sid] = callback;
+
+        // Set a single routing callback on the system client that dispatches by SID
         SystemClient.MessageCallback = (subj, s, reply, hdr, msg) =>
         {
-            _receiveQueue.Writer.TryWrite(new InternalSystemMessage
+            if (_callbacks.TryGetValue(s, out var cb))
             {
-                Sub = sub,
-                Client = SystemClient,
-                Account = SystemAccount,
-                Subject = subj,
-                Reply = reply,
-                Headers = hdr,
-                Message = msg,
-                Callback = callback,
-            });
+                _receiveQueue.Writer.TryWrite(new InternalSystemMessage
+                {
+                    Sub = sub,
+                    Client = SystemClient,
+                    Account = SystemAccount,
+                    Subject = subj,
+                    Reply = reply,
+                    Headers = hdr,
+                    Message = msg,
+                    Callback = cb,
+                });
+            }
         };
 
         SystemAccount.SubList.Insert(sub);