c# - Observable.FromAsync+Repeat+TakeWhile 的组合创建无限循环

Question

有人可以解释为什么即使到达流的末尾，下面的 AsObservable 方法也会创建一个无限循环吗？

public static class StreamExt {
    public static IObservable<byte> AsObservable(this Stream stream, int bufferSize) {
        return Observable
            .FromAsync(cancel => stream.ReadBytes(bufferSize, cancel))
            .Repeat()
            .TakeWhile(bytes => bytes != null) // EndOfStream
            .SelectMany(bytes => bytes);
    }

    private static async Task<byte[]> ReadBytes(this Stream stream, int bufferSize, CancellationToken cancel) {
        var buf = new byte[bufferSize];
        var bytesRead = await stream
            .ReadAsync(buf, 0, bufferSize, cancel)
            .ConfigureAwait(false);

        if (bytesRead < 1) return null; // EndOfStream
        var result_size = Math.Min(bytesRead, bufferSize);
        Array.Resize(ref buf, result_size);
        return buf;
    }
}

快速测试表明它会产生无限循环:

class Program {
    static void Main(string[] args) {
        using (var stream = new MemoryStream(new byte[] { 1, 2, 3 })) {
            var testResult = stream
                .AsObservable(1024)
                .ToEnumerable()
                .ToArray();
            Console.WriteLine(testResult.Length);
        }
    }
}

当然我可以添加一个 .SubscribeOn(TaskPoolScheduler.Default) 但是，无限循环仍然存在(阻止任务池调度程序 + 从 Stream 无限读取) .

[更新 2017-05-09]

Shlomo 发布了一个更好的示例来重现此问题:

int i = 0;
var testResult = Observable.FromAsync(() => Task.FromResult(i++))
    .Repeat()
    .TakeWhile(l => l < 3);
testResult.Subscribe(b => Console.WriteLine(b), e => { }, () => Console.WriteLine("OnCompleted"));
Console.WriteLine("This is never printed.");

Answer 1

对于最终来到这里并需要答案的人，而不仅仅是解释:问题似乎是 FromAsync 的默认调度程序，如 this self-answered question 所示.如果您调整到“当前线程”调度程序 Repeat().TakeWhile(...) 的行为更可预测。例如。 (问题摘录):

.FromAsync(cancel => stream.ReadBytes(bufferSize, cancel), 
    System.Reactive.Concurrency.Scheduler.CurrentThread)
.Repeat()
.TakeWhile(bytes => bytes != null) // EndOfStream