0.17.0-rc4: finally actually properly fix song playback

now we use an atomic integer as a generation counter which solves
the problem, and conveniently also solves it in a thread-safe way.

MoeStreamer.xcodeproj/project.pbxproj

@@ -7,6 +7,7 @@
 	objects = {
 
 /* Begin PBXBuildFile section */
+		2B009AC825F3389C001364F0 /* Atomic.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2B009AC725F3389C001364F0 /* Atomic.swift */; };
 		2B125D7D253753D1000C45C5 /* MediaKeyHandler.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2B125D7C253753D1000C45C5 /* MediaKeyHandler.swift */; };
 		2B22659F25412DB600F970B9 /* ListenMoeController.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2B22659E25412DB600F970B9 /* ListenMoeController.swift */; };
 		2B2D2C7D256E9F1700ED6DB5 /* RateLimiter.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2B2D2C7C256E9F1700ED6DB5 /* RateLimiter.swift */; };
@@ -87,6 +88,7 @@
 /* End PBXCopyFilesBuildPhase section */
 
 /* Begin PBXFileReference section */
+		2B009AC725F3389C001364F0 /* Atomic.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Atomic.swift; sourceTree = "<group>"; };
 		2B125D7C253753D1000C45C5 /* MediaKeyHandler.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = MediaKeyHandler.swift; sourceTree = "<group>"; };
 		2B22659E25412DB600F970B9 /* ListenMoeController.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ListenMoeController.swift; sourceTree = "<group>"; };
 		2B2D2C7C256E9F1700ED6DB5 /* RateLimiter.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = RateLimiter.swift; sourceTree = "<group>"; };
@@ -222,6 +224,7 @@
 				2B125D7C253753D1000C45C5 /* MediaKeyHandler.swift */,
 				2BECD3E82577784500FF2544 /* NotifCentreNowPlaying.swift */,
 				2BECD3EA2577852F00FF2544 /* InsomniaInducer.swift */,
+				2B009AC725F3389C001364F0 /* Atomic.swift */,
 			);
 			path = system;
 			sourceTree = "<group>";
@@ -400,7 +403,7 @@
 			isa = PBXProject;
 			attributes = {
 				LastSwiftUpdateCheck = 1130;
-				LastUpgradeCheck = 1200;
+				LastUpgradeCheck = 1240;
 				ORGANIZATIONNAME = zhiayang;
 				TargetAttributes = {
 					2BD486BD23EFE3030066F1C4 = {
@@ -472,6 +475,7 @@
 				2BE995F223FBAB340092023C /* PopupButton.swift in Sources */,
 				2B4077E8256D84400054F4DC /* MainModel.swift in Sources */,
 				2BE995F823FBEA4F0092023C /* LocalAudioController.swift in Sources */,
+				2B009AC825F3389C001364F0 /* Atomic.swift in Sources */,
 				2BE995F023FBA0CE0092023C /* AudioController.swift in Sources */,
 				2BD486D523EFE6C20066F1C4 /* ViewController.swift in Sources */,
 				2B125D7D253753D1000C45C5 /* MediaKeyHandler.swift in Sources */,
@@ -644,7 +648,7 @@
 					"@executable_path/../Frameworks",
 				);
 				MACOSX_DEPLOYMENT_TARGET = 10.15;
-				MARKETING_VERSION = "0.17.0-rc3";
+				MARKETING_VERSION = "0.17.0-rc4";
 				PRODUCT_BUNDLE_IDENTIFIER = com.zhiayang.MoeStreamer;
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SWIFT_OBJC_BRIDGING_HEADER = "";
@@ -678,7 +682,7 @@
 					"@executable_path/../Frameworks",
 				);
 				MACOSX_DEPLOYMENT_TARGET = 10.15;
-				MARKETING_VERSION = "0.17.0-rc3";
+				MARKETING_VERSION = "0.17.0-rc4";
 				PRODUCT_BUNDLE_IDENTIFIER = com.zhiayang.MoeStreamer;
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SWIFT_OBJC_BRIDGING_HEADER = "";

MoeStreamer.xcodeproj/xcshareddata/xcschemes/MoeStreamer (Release).xcscheme

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1200"
+   LastUpgradeVersion = "1240"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"

MoeStreamer.xcodeproj/xcshareddata/xcschemes/MoeStreamer.xcscheme

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1200"
+   LastUpgradeVersion = "1240"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"

MoeStreamer/src/audio/AudioEngine.swift

@@ -16,8 +16,7 @@ class AudioEngine
 	private var mirrorDevice: AudioDevice
 	private var savedElapsedTime: Double = 0
 
-	// super thread unsafe
-	private var didStopPlayer: Bool = false
+	private var songGeneration = Atomic64(0)
 
 	init()
 	{
@@ -58,31 +57,35 @@ class AudioEngine
 			return
 		}
 
+		// store the current gen first, so the closure can capture it,
+		// then increment it. incr() returns the old value.
+		let currentGen = self.songGeneration.incr()
+
 		// if the player was playing, then we should resume it after replacing the song.
 		let shouldResume = self.p1.isPlaying
 
 		// either way, we need to stop in order to clear the current queue
 		// of scheduled buffers/songs in the player.
 		self.p1.stop()
-		self.didStopPlayer = true
 
+		// apparently, when you stop the player, all the completion handlers for all the scheduled songs are
+		// called -- even if they never even got a chance to play. that would explain the repeated calls.
 		self.p1.scheduleFile(file, at: nil, completionCallbackType: .dataPlayedBack, completionHandler: { [weak self] _ in
-			guard let self = self else {
-				return
-			}
 
-			if self.didStopPlayer {
-				self.didStopPlayer = false
+			// if and only if the captured generation is exactly 1 behind the current generation, then we are responsible
+			// for controlling the playback and/or calling the next completion handler. if not, then we quit.
+			guard let self = self, currentGen + 1 == self.songGeneration.value else {
 				return
 			}
 
+			// since onComplete probably ends up calling this function itself, we have to do it from
+			// a separate thread, since apparently calling stop() from inside this handler causes a deadlock
 			DispatchQueue.main.async {
-				// since onComplete probably ends up calling this function itself, we have to do it from
-				// a separate thread, since apparently calling stop() from inside this handler causes a deadlock
 				onComplete()
 			}
 		})
 
+		// if it was playing before, then resume playing.
 		if shouldResume {
 			self.p1.play()
 		}

MoeStreamer/src/system/Atomic.swift

@@ -0,0 +1,20 @@
+// Atomic.swift
+// Copyright (c) 2021, zhiayang
+// Licensed under the Apache License Version 2.0.
+
+import Foundation
+
+struct Atomic64
+{
+	private(set) var value: Int64
+
+	init(_ initial: Int64)
+	{
+		self.value = initial
+	}
+
+	// the OSAtomic functions return the new value; to get the old value we just do an
+	// offset, which is perfectly safe since we know for sure the incr/decr already happened.
+	mutating func incr() -> Int64 { return OSAtomicIncrement64(&self.value) - 1 }
+	mutating func decr() -> Int64 { return OSAtomicDecrement64(&self.value) + 1 }
+}