fix: crash on background timer expired with Metal renderer

VoidLink/Metal/MetalVideoRenderer.h

@@ -23,12 +23,13 @@
 @property (nonatomic) id<CAMetalDrawable> _Nullable nextDrawable;
 @property (atomic) BOOL isStopping;
 @property (nonatomic) BOOL hdrEnabled;
+@property (nonatomic, readonly, nonnull) dispatch_semaphore_t inFlightSemaphore;
 
 - (instancetype _Nonnull )initWithMetalDevice:(id<MTLDevice>_Nonnull)device drawablePixelFormat:(MTLPixelFormat)drawablePixelFormat settings:(TemporarySettings* _Nonnull )currentSettings;
 
 
 - (void)renderFrame:(nonnull Frame *)frame toLayer:(nonnull CAMetalLayer *)layer;
-- (void)waitToRenderTo:(nonnull CAMetalLayer *)layer API_AVAILABLE(ios(13.0));
+- (BOOL)waitToRenderTo:(nonnull CAMetalLayer *)layer API_AVAILABLE(ios(13.0));
 - (void)drawableResize:(CGSize)drawableSize;
 - (void)shutdown;
 

VoidLink/Metal/MetalVideoRenderer.m

@@ -138,6 +138,8 @@ CFStringRef __currentColorSpace;
     dispatch_semaphore_t _inFlightSemaphore;
 }
 
+@synthesize inFlightSemaphore = _inFlightSemaphore;
+
 - (instancetype)initWithMetalDevice:(id<MTLDevice>)device drawablePixelFormat:(MTLPixelFormat)drawablePixelFormat settings:(TemporarySettings* )currentSettings {
     self = [super init];
     if (self) {
@@ -842,13 +844,20 @@ CFStringRef __currentColorSpace;
     }
 }
 
-- (void)waitToRenderTo:(nonnull CAMetalLayer *)layer {
+- (BOOL)waitToRenderTo:(nonnull CAMetalLayer *)layer {
     // Wait to ensure only `MaxFramesInFlight` number of frames are getting processed
     // by any stage in the Metal pipeline (CPU, GPU, Metal, Drivers, etc.).
-    if (!self.isStopping) {
-        dispatch_time_t timeout = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.1f * NSEC_PER_SEC));  // 100ms
-        dispatch_semaphore_wait(_inFlightSemaphore, timeout);
+    if (self.isStopping) {
+        return NO;
     }
+
+    dispatch_time_t timeout = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.1f * NSEC_PER_SEC));  // 100ms
+    if (dispatch_semaphore_wait(_inFlightSemaphore, timeout) != 0) {
+        Log(LOG_W, @"Timed out waiting for in-flight frame buffer.");
+        return NO;
+    }
+
+    return YES;
 }
 
 - (void)shutdown {

VoidLink/Metal/MetalViewController.m

@@ -19,6 +19,7 @@
     MetalVideoRenderer *_renderer;
     MetricsHandler _metricsHandler;
     CADisplayLink *_displayLink;
+    BOOL _frameSlotAcquired;
 }
 
 - (nonnull instancetype)initWithFrame:(CGRect)bounds framerate:(float)framerate settings:(TemporarySettings* )settings metricsHandler:(MetricsHandler)metricsHandler {
@@ -113,17 +114,20 @@
         return;
     }
 
-    // Renderer obtains a nextDrawable, waiting if necessary
     if (@available(iOS 13.0, *)) {
-        [_renderer waitToRenderTo:layer];
+        _frameSlotAcquired = [_renderer waitToRenderTo:layer];
     }
 
-    // If we don't have a frame yet, wait on that too
-    [_frameQueue waitForEnqueue];
+    if (_frameSlotAcquired) {
+        [_frameQueue waitForEnqueue];
+    }
 }
 
 /// Draw frame (used by manual loop)
 - (void)renderTo:(nonnull CAMetalLayer *)layer {
+    if (!_frameSlotAcquired) {
+        return;
+    }
     CFTimeInterval timeout = (1.0f / _framerate) - _renderer.averageGPUTime;
     Frame *frame = [_frameQueue dequeueWithTimeout:timeout];
 
@@ -133,10 +137,13 @@
             //if (@available(iOS 13.0, *)) {
                 [_renderer renderFrame:frame toLayer:layer];
             //}
+        } else {
+            dispatch_semaphore_signal([_renderer inFlightSemaphore]);
         }
     } else {
         // When paused, we still dequeue frames to prevent accumulation
         // but don't render them. Also sleep a bit to reduce CPU usage
+        dispatch_semaphore_signal([_renderer inFlightSemaphore]);
         usleep(100000);
     }
 }