Encapsulate transport state in TransportState struct

[sbooth]

This attempts to minimize the use of locks on UI-facing methods.

[Copilot]

Pull request overview

This PR introduces a detail::TransportSnapshot to provide a consistent, UI-friendly view of transport/playback state without taking activeDecodersMutex_, aiming to reduce lock contention on UI-facing methods.

Changes:

• Adds detail::TransportSnapshot and routes playback position/time queries through a snapshot loaded via a seqlock-like mechanism.
• Refactors seeking to use event-driven seekRequest / seekComplete commands and decouples seek requests from direct decoder-state mutation.
• Publishes transport snapshots at key points in decoder lifecycle and render-event processing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Sources/CSFBAudioEngine/Player/AudioPlayer.mm	Implements snapshot publish/load logic and refactors seeking/event processing to rely on snapshots and new seek events.
Sources/CSFBAudioEngine/Player/AudioPlayer.h	Introduces detail::TransportSnapshot, adds snapshot storage fields, and inlines playback position/time methods to read snapshots.

Comments suppressed due to low confidence (3)

Sources/CSFBAudioEngine/Player/AudioPlayer.mm:2305

• loadTransportSnapshot() relies on copying currentSnapshot_ non-atomically and then uses an acquire fence followed by a relaxed sequence reload. This doesn't provide the usual seqlock acquire semantics and (combined with the plain struct copy) can allow torn/stale reads. Switching the snapshot copy to std::atomic_ref and making the second sequence load memory_order_acquire gives the intended synchronization with the writer’s final memory_order_release store.

        const auto seq = snapshotSequence_.load(std::memory_order_acquire);

        // If seq is odd, a writer is currently updating the snapshot
        if ((seq & 1) != 0) [[unlikely]] {
#if DEBUG
            os_log_debug(log_, "Unable to load playback snapshot: write in progress");
#endif /* DEBUG */
            cpuPause();

Sources/CSFBAudioEngine/Player/AudioPlayer.mm:1203

• This log message still says "decoder seek event" even though the enqueued command is now EventCommand::seekComplete. Updating the wording will make fault reports easier to interpret.

This issue also appears on line 1256 of the same file.

            if (events_.enqueue(EventCommand::seekComplete, decoderState->sequenceNumber_, framePosition.value())) {
                eventSemaphore_.signal();
            } else {
                os_log_fault(log_, "Error writing decoder seek event");
            }

Sources/CSFBAudioEngine/Player/AudioPlayer.mm:1261

• This log message still says "decoder seek event" even though the enqueued command is now EventCommand::seekComplete. Updating the wording will make fault reports easier to interpret.

                                if (events_.enqueue(EventCommand::seekComplete, nextDecoderState->sequenceNumber_,
                                                    framePosition.value())) {
                                    eventSemaphore_.signal();
                                } else {
                                    os_log_fault(log_, "Error writing decoder seek event");
                                }

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.