HttpHandlerDiagnosticListener might emit `HttpRequestOut.Start` 2 times for certain hosts with ActivityIdFormat.W3C

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
38/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
csharp

Research direction

Start with src/libraries/System.Diagnostics.DiagnosticSource/tests/HttpHandlerDiagnosticListenerTests.cs and the failing TestW3CHeaders assertion around line 179. Run that test and compare the HttpClient, ActivityIdFormat.W3C, and endpoint conditions described in the issue, using the linked minimal repro if needed. Done means HttpRequestOut.Start is emitted once for the same HttpWebRequest.

Written by the indexing model from the issue text.

Description

area-System.Diagnostics

#112426 captures 2 test failures: TestW3CHeadersTraceStateAndCorrelationContext is a test bug to be fixed by #112753. However the test TestW3CHeaders also fails on this assertion, which seems to be a product bug.

The reason behind the failure is that when

  1. HttpClient is used (vs HttpWebRequest)
  2. With ActivityIdFormat.W3C (vs the default Hierarchical format)
  3. To certain endpoints (eg. to http://corefx-net-http11.azurewebsites.net/Echo.ashx but not http://example.com)

the event System.Net.Http.Desktop.HttpRequestOut.Start is emitted 2 times for the same HttpWebRequest. There are no multiple physical requests being sent. All 3 conditions have to be met.

A minimal repro can be found here.

My guess is this bug has been around for a long time, and no user reported it, so I would assess the priority as low, but I'm letting the owners to triage.

Related: #38152

Dominant language
C#
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