Decrease render event processing latency [msema]

Sources/CSFBAudioEngine/Player/AudioPlayer.h

@@ -11,6 +11,7 @@
 
 #import <atomic>
 #import <cassert>
+#import <chrono>
 #import <deque>
 #import <memory>
 #import <mutex>
@@ -148,13 +149,16 @@ class AudioPlayer final {
 
     /// Thread used for decoding
     std::jthread decodingThread_;
-    /// Dispatch semaphore used for communication with the decoding thread
-    dsema::Semaphore decodingSemaphore_{0};
+    /// Semaphore used for communication with the decoding thread
+    msema::Semaphore decodingSemaphore_{0};
 
     /// Thread used for event processing
     std::jthread eventThread_;
-    /// Dispatch semaphore used for communication with the event processing thread
-    dsema::Semaphore eventSemaphore_{0};
+    /// Semaphore used for communication with the event processing thread
+    msema::Semaphore eventSemaphore_{0};
+    /// Whether `eventSemaphore_` has been signaled since the event processing thread last began draining events
+    std::atomic_bool eventSignaled_{false};
+    static_assert(std::atomic_bool::is_always_lock_free, "Lock-free std::atomic_bool required");
 
     /// Message queue communicating events to the event processing thread
     mpsc::MessageQueue<256, 32> events_;
@@ -424,7 +428,13 @@ class AudioPlayer final {
     bool decodeIntoRingBuffer(DecoderState *_Nonnull &decoderState, AVAudioPCMBuffer *buffer) noexcept;
 
     /// Returns the appropriate decoding semaphore timeout
-    int64_t decodingTimeout(DecoderState *_Nullable decoderState) const noexcept;
+    std::chrono::nanoseconds decodingTimeout(DecoderState *_Nullable decoderState) const noexcept;
+
+#if DEBUG
+    /// Returns `true` if the decoding thread has a decoder to process and should not be idle
+    /// - note: Used only to detect missed signals when a wait with no decoder to process times out
+    bool shouldBeProcessingDecoders() const noexcept;
+#endif /* DEBUG */
 
     // MARK: - Rendering
 
@@ -433,10 +443,12 @@ class AudioPlayer final {
                     AudioBufferList &outputData) noexcept;
 
     /// Enqueues frames rendered event(s) for rendered audio
-    void enqueueFramesRenderedEvents(uint32_t framesRead, const AudioTimeStamp &timestamp) noexcept;
+    /// - returns: `true` if an event was enqueued
+    [[nodiscard]] bool enqueueFramesRenderedEvents(uint32_t framesRead, const AudioTimeStamp &timestamp) noexcept;
 
     /// Enqueues an empty frames rendered event if an empty decoded chunk descriptor is present
-    void enqueueEmptyFramesRenderedEvent(const AudioTimeStamp &timestamp) noexcept;
+    /// - returns: `true` if an event was enqueued
+    [[nodiscard]] bool enqueueEmptyFramesRenderedEvent(const AudioTimeStamp &timestamp) noexcept;
 
     /// The current rendering chunk descriptor
     std::optional<detail::RenderingChunkDescriptor> renderingChunk_{};
@@ -521,6 +533,9 @@ class AudioPlayer final {
     void handleRenderingWillCompleteEvent(Decoder _Nonnull decoder, uint64_t hostTime,
                                           uint64_t outputGeneration) noexcept;
 
+    /// Signals the event semaphore if required
+    void signalEventSemaphore() noexcept;
+
     // MARK: - Transport Snapshots
 
     /// Publishes a playback snapshot.
@@ -640,6 +655,12 @@ inline AVAudioMixerNode *_Nonnull AudioPlayer::mainMixerNode() const noexcept {
 
 inline AVAudioOutputNode *_Nonnull AudioPlayer::outputNode() const noexcept { return engine_.outputNode; }
 
+inline void AudioPlayer::signalEventSemaphore() noexcept {
+    if (!eventSignaled_.exchange(true, std::memory_order_acq_rel)) {
+        eventSemaphore_.signal();
+    }
+}
+
 // MARK: Transport Snapshot
 
 inline bool detail::TransportSnapshot::getPlaybackPositionAndTime(SFBPlaybackPosition *position,