An Xcode project full of intentionally buggy Swift code. Each exercise presents real code with a real concurrency problem — your job is to read it, find the bug, and fix it.
Companion to the Concurrency Bugs Every iOS Dev Should Know series on pixelper.com.
Requires: Xcode 15+, iOS 17+
git clone https://github.com/christopherkmoore/ConcurrencyBugHunt.git
cd ConcurrencyBugHunt
open ConcurrencyBugHunt.xcodeprojOr regenerate with XcodeGen:
brew install xcodegen && xcodegen generateRace Condition — A ShoppingCart with an unprotected dictionary. Concurrent addItem() calls race on reads and writes, producing inconsistent totals. A DispatchGroup spawns the concurrent operations so you can watch the results vary between runs.
Race Conditions: When Threads Collide · Thread-Safe Collections in Swift
Main Thread Violation — A UserProfileLoader that fetches data on DispatchQueue.global() and then writes directly to @Published properties from that background thread. Watch the console for SwiftUI threading warnings.
Main Thread Violations: The Silent Crasher · DispatchQueue.main vs @MainActor
Deadlock — Two scenarios: a DataCache that calls queue.sync on the same serial queue it's already running on, and a BankAccount with a lock ordering problem where transfer() and logTransaction() try to acquire the same two locks in opposite order.
Deadlocks: Sync to Same Queue · Lock Ordering and Circular Dependencies
Task Cancellation — A search bar where each keystroke fires a new Task but old tasks aren't cancelled. Type quickly and watch stale results arrive out of order. A DataProcessor with no cancellation checks keeps running even after being cancelled.
Actor Reentrancy — A BankAccountActor that checks the balance, then awaits a fraud check, then deducts. Because actors allow re-entrance at await points, two concurrent withdrawals can both pass the balance check and both succeed — even when there isn't enough money.
Unstructured Task Leak — An ImageLoader that creates tasks without tracking them, and a PollingService with an infinite while true loop and no cancellation path. Navigate away and come back — the polling is still running.
Unstructured Task Leaks · Structured vs Unstructured Concurrency
Retain Cycle — A UserSessionManager and a SearchService that both capture self strongly in sink closures. Dismiss the sheet and check the console — deinit never fires.
Missing Cancellable — Three examples: a NotificationListener that creates subscriptions but doesn't store them (immediately freed), a DataFetcher that overwrites its single AnyCancellable with each new request, and an EventSubscriber where the subscription is lost the moment it's created.
The Missing Cancellable Problem · Managing Multiple Subscriptions