Design an Asynchronous C++ Logger Using a Lock-Free Queue

Context

You are asked to design and implement an asynchronous logger suitable for a technical screen in a systems/engineering interview. The logger must:

• Accept variadic arguments using fmt-style format strings.
• Use a background thread to perform formatting and writing.
• Pass log events through a provided lock-free queue.
• Not optimize for heap usage or memory footprint (allocations are acceptable).

Assume a generic lock-free MPMC queue is provided with a minimal try_enqueue/try_dequeue API. You should:

• Specify the public API.
• Define the log event structure.
• Describe the producer–consumer flow.
• Demonstrate shutdown/flush behavior.
• Describe error handling and overflow policy.

Requirements

1. Public API that supports:
   • Variadic, fmt-style logging (compile-time format checking preferred).
   • Flush and shutdown semantics.
   • Configurable sink (e.g., file or stderr) and error handling hooks.
2. Log events must be enqueued by producers and formatted/written by a background consumer thread.
3. Messages should be passed as events through the lock-free queue; formatting happens only on the consumer side.
4. Show robust shutdown/flush that drains the queue and avoids message loss.
5. Explain error handling (enqueue overflow, formatting errors, I/O failures).