Objective-C Runtime +resolveInstanceMethod: Concurrency Guarantees Unclear

Objective-C Runtime +resolveInstanceMethod: Concurrency Guarantees Unclear

The goal is to understand whether the Objective‑C runtime serializes concurrent invocations of +resolveInstanceMethod: when multiple threads send the same unknown selector, and how this affects latency.

Current documentation does not specify if the runtime guarantees a single call per selector or allows parallel execution. This uncertainty creates a potential performance bottleneck in multithreaded environments, as lock contention may occur while the method list is modified.

Key constraints:

• Concurrent threads may simultaneously trigger +resolveInstanceMethod: for the same selector.
• The runtime may employ internal locking that is not documented.
• Latency is only observable under heavy concurrent loads.

Unresolved questions:

1. Is +resolveInstanceMethod: guaranteed to be invoked only once per selector when multiple threads send the same missing selector concurrently?
2. Does the runtime serialize concurrent invocations of +resolveInstanceMethod: to protect the method list, or can they proceed in parallel?
3. What best‑practice patterns can developers adopt to mitigate potential lock‑contention latency in performance‑critical paths?