fix: SentryMiddleware re-entrant

src/Sentry.AspNetCore/SentryMiddleware.cs

@@ -97,9 +97,13 @@ public async Task InvokeAsync(HttpContext context, RequestDelegate next)
             var transactionContext = hub.ContinueTrace(traceHeader, baggageHeader);
 
             // Adding the headers and the TransactionContext to the context to be picked up by the Sentry tracing middleware
-            context.Items.Add(TraceHeaderItemKey, traceHeader);
-            context.Items.Add(BaggageHeaderItemKey, baggageHeader);
-            context.Items.Add(TransactionContextItemKey, transactionContext);
+            var didAdd = context.Items.TryAdd(TraceHeaderItemKey, traceHeader);
+            if (!didAdd)
+            {
+                _options.LogDebug("Sentry trace was already added. Did you initialize Sentry twice?");
+            }
+            context.Items.TryAdd(BaggageHeaderItemKey, baggageHeader);
+            context.Items.TryAdd(TransactionContextItemKey, transactionContext);
 
             hub.ConfigureScope(scope =>
             {

test/Sentry.AspNetCore.Tests/SentryMiddlewareTests.cs

@@ -749,6 +749,34 @@
         Assert.NotNull(transactionContext);
     }
 
+    [Fact]
+    public async Task InvokeAsync_InvokingWithTheSameContextTwice_DoesNotThrow()
+    {
+        var sut = _fixture.GetSut();
+        var request = Substitute.For<HttpRequest>();
+        var fakeHeaders = new HeaderDictionary
+        {
+            { "Sentry-Trace", "4b4d2878507b43d3af7dd8c4ab7a96d9-3cc6fd1337d243de"},
+            { "Baggage", "sentry-trace_id=4b4d2878507b43d3af7dd8c4ab7a96d9, sentry-public_key=eb18e953812b41c3aeb042e666fd3b5c"},
+        };
+        var contextItems = new Dictionary<object, object>();
+        _fixture.HttpContext.Items.When(items => items.Add(Arg.Any<object>(), Arg.Any<object>()))
+            .Do(info =>
+            {
+                contextItems.Add(info.Args()[0], info.Args()[1]);
+            });
+        _ = request.Headers.Returns(fakeHeaders);
+        _ = _fixture.HttpContext.Request.Returns(request);
+        _ = request.HttpContext.Returns(_fixture.HttpContext);
+
+        // Faking having added the middleware twice by invoking the request with the same context twice
+        await sut.InvokeAsync(_fixture.HttpContext, _fixture.RequestDelegate);
+        await sut.InvokeAsync(_fixture.HttpContext, _fixture.RequestDelegate);
+
+        _fixture.HttpContext.Items.Received(3); // Sanity check
+        Assert.Equal(3, contextItems.Count);
+    }
+
     [Fact]
     public void PopulateScope_AddEventProcessors()
     {