Debugging a closure capture leak in a third-party library

WHAT THIS TESTS The interviewer wants a systematic approach to an opaque leak crossing into Objective-C, where guessing wastes time and may not even fix the cycle.

A GOOD ANSWER COVERS Start with the Memory Graph Debugger in Xcode. Capture the graph, select the leaked instance, and inspect its incoming references to see precisely which objects retain it and trace the strong-reference cycle. Purple runtime-issue badges flag detected leaks and reference cycles. Complement this with the Leaks instrument, which periodically scans for unreachable-but-retained memory, and the Allocations instrument to watch the object's lifetime and generation. Enable malloc stack logging so each allocation carries a backtrace, letting you see where the leaked object and the retaining block were created. For an Objective-C library, the retaining edge is frequently a stored block that strongly captures self or a delegate.

BEYOND WEAK SELF Rather than blanket [weak self], map the cycle and break the specific edge: make the stored block capture weakly, nil out a retained callback when done, or hold the library object weakly if its API allows. Sometimes the fix is to release a registered observer or invalidate a timer the library keeps.

COMMON WRONG ANSWERS Adding [weak self] everywhere and hoping; this can break legitimate strong references or miss the real cycle that lives entirely inside the library. Trusting the Leaks instrument alone, which can miss cycles that are still reachable from a root.

LIKELY FOLLOW-UPS What is the difference between a leak and an abandoned but reachable object? How does malloc stack logging help? When is unowned safer than weak? How do you break a cycle you do not own?

ONE CONCRETE EXAMPLE A view controller never deallocates. The Memory Graph shows a third-party uploader holding a completion block that strongly captures the controller, while the controller holds the uploader. You break the edge by capturing the controller weakly in the block you pass in, or by setting the uploader's delegate to nil in viewWillDisappear, and the graph confirms the controller now releases.