fix: Log4Net integration filters SDK-internal log events to prevent infinite loops

[jamescrosswell]

By default, SentryOptions.DiagnosticLogger is either null (when Debug is off) or a ConsoleDiagnosticLogger/DebugDiagnosticLogger that writes to Console.Error or Debug.WriteLine. Neither of those has anything to do with log4net, so _options.LogWarning(...) in LogRateLimited never touches SentryAppender at all. No loop possible.

The problem occurs when the user does something like:

options.Debug = true;
options.DiagnosticLogger = new Log4NetDiagnosticLogger(LogManager.GetLogger("Sentry"));

At that point _options.LogWarning(...) → Log4NetDiagnosticLogger.Log → log4net → SentryAppender.Append → CaptureEvent → background worker → 429 → repeat. The user has literally wired the SDK's own error output back into the SDK's event pipeline.

Solution

I've tried to add this:

sentry-dotnet/src/Sentry/Http/HttpTransportBase.cs

Lines 595 to 613 in a62bd4b

	private void LogRateLimited(SentryId? eventId, string responseDetail)
	{
	IsLoggingRateLimited.Value = true;
	try
	{
	_options.LogWarning(
	"{0}: Sentry rejected the envelope '{1}' due to rate limiting. " +
	"This may indicate that you are sending too much data or have exceeded your quota. " +
	"See https://docs.sentry.io/product/accounts/quotas/ for more information. " +
	"Server response: {2}",
	_typeName,
	eventId,
	responseDetail);
	}
	finally
	{
	IsLoggingRateLimited.Value = false;
	}
	}

And then this:

sentry-dotnet/src/Sentry.Log4Net/SentryAppender.cs

Lines 81 to 94 in 99d2647

	if (IsReentrant.Value || _isLoggingRateLimited())
	{
	return;
	}
	IsReentrant.Value = true;
	try
	{
	AppendCore(loggingEvent);
	}
	finally
	{
	IsReentrant.Value = false;
	}

But ultimately I don't think there's any good solution to this problem. See this comment in my discussion with the AI overlords.

Aside (consistency)

Added an AsyncLocal<bool> re-entrance guard to SentryAppender.Append (matching the Serilog and NLog pattern) so that synchronous re-entrant log calls are dropped.

This doesn't address the original issue which stems from the Background Worker. The background worker is started with Task.Run(DoWorkAsync) long before any appender call sets IsReentrant = true. So when LogRateLimited → _options.LogWarning fires on the background worker thread, IsReentrant.Value is always false there.

Closes #2953

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.58%. Comparing base (a9cb3b2) to head (079230c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5238      +/-   ##
==========================================
- Coverage   74.12%   73.58%   -0.55%     
==========================================
  Files         508      494      -14     
  Lines       18282    17893     -389     
  Branches     3574     3486      -88     
==========================================
- Hits        13551    13166     -385     
  Misses       3861     3861              
+ Partials      870      866       -4     

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[sentry-warden]

Append_IsLoggingRateLimited_DoesNotCaptureEvent doesn't isolate the rate-limit guard from the re-entrancy guard

The rate-limit test triggers DoAppend from inside the CaptureEvent callback, so IsReentrant.Value is already true when the inner call hits the guard — both conditions fire simultaneously and the test can't prove the _isLoggingRateLimited() check independently blocks capture. A bug that removed only that check would leave this test green.

Evidence

• SentryAppender.Append sets IsReentrant.Value = true before calling AppendCore, and resets it in finally.
• AppendCore calls _hub.CaptureEvent, which executes the .Do() callback synchronously while still inside the try block.
• At that point IsReentrant.Value is true AND _fixture.IsLoggingRateLimited is set to true, so the guard if (IsReentrant.Value || _isLoggingRateLimited()) is satisfied by either condition alone.
• Removing the _isLoggingRateLimited() branch from the production if would not cause this test to fail.
• A proper isolation test would call sut.DoAppend from outside any active appender frame (e.g., directly before the outer call, with IsLoggingRateLimited = true already set).

_{Identified by Warden code-review · UQV-B4J}