Log an error when an exception bypasses the dead letter queue

[nabisobhi]

Why

Dafda lets you register exception types that bypass the dead letter queue and propagate instead, crashing the consumer — the escape hatch for systemic failures like a database outage, where dead-lettering would silently drain a whole topic.

When that bypass fires, nothing is logged from the Consumer's point of view. The consumer just dies, with no record of which message triggered it or which exception was responsible.

Change

Consumer now takes an optional ILogger<Consumer> (defaulting to NullLogger<Consumer>.Instance, so existing call sites and tests are unaffected) and logs before rethrowing:

Exception of type {ExceptionType} bypassed the dead letter queue for message with key {Key} from topic {SourceTopic}. Failing the consumer

Logged at Error level with the exception attached. The real logger is resolved in both AddConsumer overloads.

To log at that point, the bypass check moved out of the exception filter into the catch body, which now rethrows with a bare throw;. That preserves the original stack trace. The check still runs before the retry/backoff branch, so a bypassed exception fails fast without waiting out a backoff delay.

Keeping the semantics of a throwing predicate

The bypass predicate is user-supplied, and moving it out of the exception filter changed one edge case. C# treats an exception thrown by a filter as a non-match, so previously a predicate that threw meant the original handler exception simply escaped — it was never dead-lettered. Evaluated in a catch body, a throwing predicate would instead replace the original exception and hide the real failure.

ShouldBypassDeadLetterQueue now absorbs that, keeping Dispatch clean:

try
{
    return deadLetterQueueBypass(exception);
}
catch
{
    return true;
}

Returning true routes into the bypass branch, whose bare throw; propagates the original exception — matching the old behavior. (false would not: that dead-letters the message, which the old code never did here.)

Overlap worth a reviewer's opinion

ConsumerHostedService.ConsumeAll already logs any escaping exception as "Unhandled error occurred while consuming messaging", so a bypassed exception will now produce two Error entries: this context-rich one, then the generic boundary one. That's a common pattern (detail at the site, generic at the boundary), but if you'd rather avoid the duplication, dropping this to Warning is an easy alternative.

Notes

• No new NuGet dependencies; Microsoft.Extensions.Logging.Abstractions is already referenced.
• No public API change — Consumer is internal.
• Added LoggerSpy<T> test double rather than Mock<ILogger<Consumer>>, because Moq can't proxy a generic closed over an internal type without InternalsVisibleTo("DynamicProxyGenAssembly2").

Tests

Three new tests: the bypass logs the expected level/exception/message, dead-lettered messages do not log the bypass error, and a throwing BypassWhen predicate still surfaces the original exception without dead-lettering. TestConsumer 42/42, full suite 185/185, build clean with 0 warnings.

[Copilot]

🟡 Changes recommended

Moving the bypass predicate out of the exception filter can replace the original failure when the predicate throws.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Adds contextual error logging when an exception bypasses the dead-letter queue.

Changes:

• Injects an optional consumer logger.
• Logs bypassed exceptions before rethrowing.
• Adds logging tests and a logger spy.

File summaries

File	Description
src/Dafda/Consuming/Consumer.cs	Logs bypassed exceptions.
src/Dafda/Configuration/ConsumerServiceCollectionExtensions.cs	Resolves consumer loggers.
src/Dafda.Tests/TestDoubles/LoggerSpy.cs	Adds a logging test double.
src/Dafda.Tests/Consuming/TestConsumer.cs	Tests bypass logging behavior.
src/Dafda.Tests/Builders/ConsumerBuilder.cs	Supports logger injection in tests.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Balanced

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

🟢 Approval recommended

The implementation preserves prior behavior and includes focused regression coverage for the new logging path.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0 new
• Review effort level: Balanced