fix: Race condition in SentryMessageHandler (#3477)

---------

Co-authored-by: James Crosswell <[email protected]>

Author: bitsandfoxes

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+### Fixes
+- Race condition in `SentryMessageHandler` ([#3477](https://github.com/getsentry/sentry-dotnet/pull/3477))
+
 ## 4.9.0
 
 ### Fixes

src/Sentry/SentryMessageHandler.cs

@@ -10,6 +10,7 @@ public abstract class SentryMessageHandler : DelegatingHandler
 {
     private readonly IHub _hub;
     private readonly SentryOptions? _options;
+    private readonly object _innerHandlerLock = new();
 
     /// <summary>
     /// Constructs an instance of <see cref="SentryMessageHandler"/>.
@@ -124,7 +125,13 @@ private void PropagateTraceHeaders(HttpRequestMessage request, string url)
     {
         // Assign a default inner handler for convenience the first time this is used.
         // We can't do this in a constructor, or it will throw when used with HttpMessageHandlerBuilderFilter.
-        InnerHandler ??= new HttpClientHandler();
+        if (InnerHandler is null)
+        {
+            lock (_innerHandlerLock)
+            {
+                InnerHandler ??= new HttpClientHandler();
+            }
+        }
 
         if (_options?.TracePropagationTargets.ContainsMatch(url) is true or null)
         {