Swift 6 callback isolation

A crash pattern that costs more than it should, because the stack trace is trivial to read and the symptom points somewhere else entirely.

The pattern

@MainActor
final class CaptureViewModel {
    private func save(_ image: RenderedImage) async {
        try await PHPhotoLibrary.shared().performChanges {
            PHAssetChangeRequest.creationRequestForAsset(from: image)   // ← SIGTRAP
        }
    }
}

PHPhotoLibrary.performChanges runs its block on its own internal queue. Written inline inside a @MainActor method, the block inherits main-actor isolation, so Swift 6 emits a runtime executor check inside it. The check fires on Photos’ queue and kills the process.

_dispatch_assert_queue_fail
swift_task_isCurrentExecutorWithFlagsImpl
closure #1 in CaptureViewModel.save(image:)
PHPhotoLibrary _performCancellableChanges

The fix

Move the call into a nonisolated function — ideally a separate type, so the mistake is hard to repeat:

enum PhotoLibraryWriter {
    static func write(_ rendered: RenderedImage) async throws {
        guard let image = rendered.makeUIImage() else { throw WriteError.couldNotRender }
        try await PHPhotoLibrary.shared().performChanges {
            PHAssetChangeRequest.creationRequestForAsset(from: image)
        }
    }
}

Where else it lurks

Any framework callback not documented to run on the main queue. Grep before adding one: performChanges, addCompletedHandler, delegate callbacks, completion handlers. Callbacks that are main-queue (Timer.scheduledTimer, CMMotionManager with to: .main) are fine.

The expensive part was not the fix

The app died at the end of every capture session. “It crashes when the capture finishes” pointed at the renderer, so two full rounds of work went into the Metal pipeline. Both found real data races and were worth keeping — but neither was the crash. The third round pulled the crash report and it named the culprit in one frame.

Get the report before theorising. Symptom-shaped reasoning is confident and wrong.

xcrun devicectl device info files --device "$UDID" --domain-type systemCrashLogs | grep -i myapp
xcrun devicectl device copy from --device "$UDID" --domain-type systemCrashLogs \
  --source "MyApp-2026-08-13-015330.ips" --destination /tmp/crash.ips

The .ips is a one-line header followed by JSON; d["threads"][d["faultingThread"]]["frames"] is the stack. EXC_BREAKPOINT / SIGTRAP in Swift means a runtime trap — a failed executor check, a force unwrap, or a precondition.

Related: agent-mistake-fix-harness, project-starlapse.