FB14035001: Tracks loaded from an AVAsset should be marked `sending` to be used from actor-isolated contexts

Tracks.swift

import AVFoundation

/*
 * In Swift 6 it’s not currently possible to directly load tracks from an AVAsset using a method like
 * AVAsset.loadTracks(withMediaType:) from an actor-isolated context, because AVAssetTrack isn’t Sendable. Given
 * that many AVFoundation types are not sendable / thread-safe it’s tempting to use an actor to work with
 * compositions and that currently requires some work-arounds that shouldn’t be necessary and may not be obvious
 * to everyone.
 */

// Non-isolated contexts work fine but if this class were building an AVComposition then it’s preferable to enforce
// isolation / serialization of access to the components of that composition.
class NonisolatedTrackLoader {
    func loadTracks() async throws {
        // ✅ Works in nonisolated contexts
        try await AVAsset().loadTracks(withMediaType: .video)
    }
}

// Since many AVFoundation types aren’t Sendable it’s going to be common to reach for actors to work with them.
// However this is currently a warning in Swift 5 and an error in Swift 6.
actor IsolatedTrackLoader {
    func loadTracks() async throws {
        // ❌ Non-sendable type '[AVAssetTrack]' returned by implicitly asynchronous call to nonisolated function cannot cross actor boundary. Class 'AVAssetTrack' does not conform to the 'Sendable' protocol
        try await AVAsset().loadTracks(withMediaType: .video)
    }

    // Using the extension method below we can load tracks because they’re marked as being sent, which should be the default
    func loadTracksWithWorkaround() async throws {
        // ✅ Works when tracks are sent
        _ = try await AVAsset().sendTracks(withMediaType: .video)
    }
}

extension AVAsset {
    // This works around the issue but we shouldn't need to write this ourselves. The built-in methods for loading
    // tracks should send them to the caller.
    func sendTracks(withMediaType mediaType: AVMediaType) async throws -> sending [AVAssetTrack] {
        try await loadTracks(withMediaType: mediaType)
    }
}