Improve library scan performance

Source/AudibleUtilities/ApiExtended.cs

@@ -91,20 +91,13 @@ namespace AudibleUtilities
 		{
 			Serilog.Log.Logger.Debug("Beginning library scan.");
 
-			var episodeChannel = Channel.CreateBounded<Task<List<Item>>>(
-				new BoundedChannelOptions(1)
-				{
-					SingleReader = true,
-					SingleWriter = false,
-					FullMode = BoundedChannelFullMode.Wait
-				});
-
 			int count = 0;
 			List<Item> items = new();
 			List<Item> seriesItems = new();
 
 			var sw = Stopwatch.StartNew();
 
+			//Scan the library for all added books, and add ay episode-type items to seriesItems to be scanned for episodes/parents
 			await foreach (var item in Api.GetLibraryItemAsyncEnumerable(libraryOptions, BatchSize, MaxConcurrency))
 			{
 				if ((item.IsEpisodes || item.IsSeriesParent) && importEpisodes)
@@ -119,15 +112,24 @@ namespace AudibleUtilities
 			Serilog.Log.Logger.Debug("Beginning episode scan.");
 
 			count = 0;
-			var episodeDlTask = scanAllSeries(seriesItems, episodeChannel.Writer);
 
-			await foreach (var ep in getAllEpisodesAsync(episodeChannel.Reader, MaxConcurrency))
+			//'get' Tasks are activated when they are written to the channel. To avoid more concurrency than is desired, the
+			//channel is bounded with a capacity of 1. Channel write operations are blocked until the current item is read
+			var episodeChannel = Channel.CreateBounded<Task<List<Item>>>(new BoundedChannelOptions(1) { SingleReader = true });
+
+			//Start scanning for all episodes. Episode batch 'get' Tasks are written to the channel.
+			var scanAllSeriesTask = scanAllSeries(seriesItems, episodeChannel.Writer);
+
+			//Read all episodes from the channel and add them to the import items.
+			//This method blocks until episodeChannel.Writer is closed by scanAllSeries()
+			await foreach (var ep in getAllEpisodesAsync(episodeChannel.Reader))
 			{
 				items.Add(ep);
 				count++;
 			}
 
-			await episodeDlTask;
+			//Be sure to await the scanAllSeries Task so that any exceptions are thrown
+			await scanAllSeriesTask;
 
 			sw.Stop();
 			Serilog.Log.Logger.Debug("Episode scan complete. Found {count} episodes and series.", count);
@@ -164,27 +166,37 @@ namespace AudibleUtilities
 
 		#region episodes and podcasts
 
-		private async IAsyncEnumerable<Item> getAllEpisodesAsync(ChannelReader<Task<List<Item>>> allEpisodes, int maxConcurrency)
+		/// <summary>
+		/// Read get tasks from the <paramref name="channel"/> and await results. This method maintains
+		/// a list of up to <see cref="MaxConcurrency"/> get tasks. When any of the get tasks completes,
+		/// the Items are yielded, that task is removed from the list, and a new get task is read from
+		/// the channel.
+		/// </summary>
+		private async IAsyncEnumerable<Item> getAllEpisodesAsync(ChannelReader<Task<List<Item>>> channel)
 		{
 			List<Task<List<Item>>> concurentGets = new();
 
-			for (int i = 0; i < maxConcurrency && await allEpisodes.WaitToReadAsync(); i++)
-				concurentGets.Add(await allEpisodes.ReadAsync());
+			for (int i = 0; i < MaxConcurrency && await channel.WaitToReadAsync(); i++)
+				concurentGets.Add(await channel.ReadAsync());
 
 			while (concurentGets.Count > 0)
 			{
 				var completed = await Task.WhenAny(concurentGets);
 				concurentGets.Remove(completed);
 
-				if (await allEpisodes.WaitToReadAsync())
-					concurentGets.Add(await allEpisodes.ReadAsync());
+				if (await channel.WaitToReadAsync())
+					concurentGets.Add(await channel.ReadAsync());
 
 				foreach (var item in completed.Result)
 					yield return item;
 			}
 		}
 
-		private async Task scanAllSeries(IEnumerable<Item> seriesItems, ChannelWriter<Task<List<Item>>> allEpisodes)
+		/// <summary>
+		/// Gets all child episodes and episode parents belonging to <paramref name="seriesItems"/> in batches and
+		/// writes the get tasks to <paramref name="channel"/>.
+		/// </summary>
+		private async Task scanAllSeries(IEnumerable<Item> seriesItems, ChannelWriter<Task<List<Item>>> channel)
 		{
 			try
 			{
@@ -193,14 +205,15 @@ namespace AudibleUtilities
 				foreach (var item in seriesItems)
 				{
 					if (item.IsEpisodes)
-						await allEpisodes.WriteAsync(getEpisodeParentAsync(item));
+						await channel.WriteAsync(getEpisodeParentAsync(item));
 					else if (item.IsSeriesParent)
-						episodeScanTasks.Add(getParentEpisodesAsync(allEpisodes, item));
+						episodeScanTasks.Add(getParentEpisodesAsync(item, channel));
 				}
 
+				//episodeScanTasks complete only after all episode batch 'gets' have been written to the channel
 				await Task.WhenAll(episodeScanTasks);
 			}
-			finally { allEpisodes.Complete(); }
+			finally { channel.Complete(); }
 		}
 
 		private async Task<List<Item>> getEpisodeParentAsync(Item episode)
@@ -247,7 +260,11 @@ namespace AudibleUtilities
 			return children;
 		}
 
-		private async Task getParentEpisodesAsync(ChannelWriter<Task<List<Item>>> channel, Item parent)
+		/// <summary>
+		/// Gets all episodes belonging to <paramref name="parent"/> in batches of <see cref="BatchSize"/> and writes the batch get tasks to <paramref name="channel"/>
+		/// This method only completes after all episode batch 'gets' have been written to the channel
+		/// </summary>
+		private async Task getParentEpisodesAsync(Item parent, ChannelWriter<Task<List<Item>>> channel)
 		{
 			Serilog.Log.Logger.Debug("Beginning episode scan for {parent}", parent);