Add overcommitment prevention mechanism (iOS) (#755)

skiko/src/iosMain/kotlin/org/jetbrains/skiko/redrawer/MetalRedrawer.ios.kt

@@ -17,7 +17,7 @@ import platform.Metal.MTLDeviceProtocol
 import platform.Metal.MTLPixelFormatBGRA8Unorm
 import platform.QuartzCore.*
 import platform.UIKit.window
-import platform.darwin.NSObject
+import platform.darwin.*
 
 private enum class DrawSchedulingState {
     AVAILABLE_ON_NEXT_FRAME,
@@ -37,6 +37,9 @@ internal class MetalRedrawer(
     private var currentDrawable: CAMetalDrawableProtocol? = null
     private val metalLayer = MetalLayer()
 
+    // Semaphore for preventing command buffers count more than swapchain size to be scheduled/executed at the same time
+    private val inflightSemaphore = dispatch_semaphore_create(metalLayer.maximumDrawableCount.toLong())
+
     /*
      * Initial value is [DrawSchedulingState.AVAILABLE_ON_NEXT_FRAME] because voluntarily dispatching a frame
      * disregarding CADisplayLink timing (which is not accessible while it's paused) can cause frame drifting in worst
@@ -105,6 +108,9 @@ internal class MetalRedrawer(
     fun makeContext() = DirectContext.makeMetal(device.objcPtr(), queue.objcPtr())
 
     fun makeRenderTarget(width: Int, height: Int): BackendRenderTarget {
+        // If more than swapchain size count of command buffers are inflight
+        // wait until one finishes work
+        dispatch_semaphore_wait(inflightSemaphore, DISPATCH_TIME_FOREVER)
         currentDrawable = metalLayer.nextDrawable()!!
         return BackendRenderTarget.makeMetal(width, height, currentDrawable!!.texture.objcPtr())
     }
@@ -190,6 +196,10 @@ internal class MetalRedrawer(
                 val commandBuffer = queue.commandBuffer()!!
                 commandBuffer.label = "Present"
                 commandBuffer.presentDrawable(it)
+                commandBuffer.addCompletedHandler {
+                    // Signal work finish, allow a new command buffer to be scheduled
+                    dispatch_semaphore_signal(inflightSemaphore)
+                }
                 commandBuffer.commit()
                 currentDrawable = null
             }