From 0caa1d22940448d50664838d5c8b74285132c2c4 Mon Sep 17 00:00:00 2001
From: mgravell <marc.gravell@gmail.com>
Date: Sat, 10 Oct 2026 12:26:07 +0100
Subject: [PATCH] ConnectionAttemptCompleted: take the sequence number and
 timestamp together

The sequence number was taken with Interlocked.Increment and the timestamp read
separately afterwards, so two attempts completing at once - the interactive and
subscription connections failing together - could interleave: one takes the
earlier sequence, the other takes the later sequence but reads the clock first,
and a later SequenceNumber carries an earlier CompletedTimeUtc. Handlers that
order outcomes by sequence (as ConnectionAttemptUnitTests.EveryFailedReconnectIsReported
does) then see time going backwards.

Both are now taken under one lock. The event is rare - once per physical
connection attempt - so the lock costs nothing that matters. Found on the v4
branch, where the two connections are dialled concurrently and the test failed
about 2 runs in 5.
---
 .../ConnectionMultiplexer.Events.cs               | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/StackExchange.Redis/ConnectionMultiplexer.Events.cs b/src/StackExchange.Redis/ConnectionMultiplexer.Events.cs
index 91c0497e2..83de215bc 100644
--- a/src/StackExchange.Redis/ConnectionMultiplexer.Events.cs
+++ b/src/StackExchange.Redis/ConnectionMultiplexer.Events.cs
@@ -25,7 +25,17 @@ internal void OnConnectionAttemptCompleted(
         var handler = RawConfig.ConnectionAttemptCompletedHandler;
         if (handler != null)
         {
-            var sequenceNumber = Interlocked.Increment(ref _connectionAttemptSequence);
+            // the sequence and the timestamp are taken together, so that they agree on the order: taken apart, two
+            // attempts completing at once (the interactive and subscription connections failing together) could each
+            // take one first and the other second, and a later sequence number would carry an earlier time
+            long sequenceNumber;
+            DateTime completedTimeUtc;
+            lock (_connectionAttemptSequenceLock)
+            {
+                sequenceNumber = ++_connectionAttemptSequence;
+                completedTimeUtc = DateTime.UtcNow;
+            }
+
             CompleteAsWorker(new ConnectionAttemptCompletedEventArgs(
                 handler,
                 this,
@@ -39,12 +49,13 @@ internal void OnConnectionAttemptCompleted(
                 tlsHostName,
                 serverCertificateCheck,
                 sequenceNumber,
-                DateTime.UtcNow,
+                completedTimeUtc,
                 physicalName));
         }
     }
 
     private long _connectionAttemptSequence;
+    private readonly object _connectionAttemptSequenceLock = new();
 
     /// <summary>
     /// Raised whenever a physical connection fails.