Async RPC call of Android Binder

[StageGuard]

此 PR 旨在消除 service proxy 的 runBlocking，提高 service 吞吐量

目前问题

例如，IRemoteTorrentDownloader.aidl 中的 startRemoteDownload 接口为

IRemoteTorrentSession startRemoteDownload(in PEncodedTorrentInfo data, in String overrideSaveDir);

这个接口对应了 TorrentDownloader 的 startDownload 函数，是一个 suspend 函数

suspend fun startDownload(
    data: EncodedTorrentInfo,
    parentCoroutineContext: CoroutineContext = EmptyCoroutineContext,
    overrideSaveDir: SystemPath? = null,
): TorrentSession

目前在 client 的调用实现为

override suspend fun startDownload(
    data: EncodedTorrentInfo,
    parentCoroutineContext: CoroutineContext,
    overrideSaveDir: SystemPath?
): TorrentSession {
    return remote.startRemoteDownload(data.toParceled(), overrideSaveDir?.absolutePath)
}

在 service 的实现为

override fun startRemoteDownload(data: PEncodedTorrentInfo?, overrideSaveDir: String?): IRemoteTorrentSession? {
    val session = runBlocking { 
        actualServiceTorrentDownloader.startDownload(
            withContext(Dispatchers.IO_) { data.toEncodedTorrentInfo() },
            overrideSaveDir = overrideSaveDir?.run { Path(this).inSystem },
        ) 
    }
    return TorrentSessionProxy(session, this, scope.coroutineContext)
}

client 调用 startRemoteDownload 时需要等待 service 实现中的 runBlocking 执行完成才会返回结果，如果 runBlocking 执行了很长时间，那 client 调用端会一直等待结果而阻塞，service 实现也会因为 runBlocking 而不能使用当前的 binder 线程继续处理其他 client 调用

新变化

此 PR 将 startRemoteDownload AIDL 接口改为如下回调形式

IDisposableHandle startRemoteDownload(in PEncodedTorrentInfo data, in String overrideSaveDir, in RemoteContinuation cont);

interface RemoteContinuation {
    void resume(in IRemoteTorrentSession value);
    void resumeWithException(in RemoteContinuationException exception);
}

其中，IDisposableHandle 用于处理 service 取消协程的动作，client 取消协程后通过 IDisposableHandle 通知 service 同样取消对应处理协程，RemoteContinuation 用于回调结果

如此而来，client 通过 suspendCancellableCoroutine 就可以比较轻易地实现回调 API 的挂起

override suspend fun startDownload(
    data: EncodedTorrentInfo,
    parentCoroutineContext: CoroutineContext,
    overrideSaveDir: SystemPath?
): TorrentSession = suspendCancellableCoroutine { cont ->
    val disposable = remote.startRemoteDownload(
        data.toParceled(), 
        overrideSaveDir?.absolutePath,
        object : ContTorrentDownloaderStartDownload.Stub() {
            override fun resume(value: IRemoteTorrentSession?) {
                cont.resume(value)
            }   

            override fun resumeWithException(exception: RemoteContinuationException?) {
                cont.resumeWithException(exception)
            }

        }
    )
    
    cont.invokeOnCancellation { disposable.dispose() }

service 也可以去除 runBlocking，并且快速返回结果

override fun startRemoteDownload(
    data: PEncodedTorrentInfo?,
    overrideSaveDir: String?,
    cont: RemoteContinuation?
): IDisposableHandle? {
    if (data == null) return null
    if (cont == null) return null

    val job = scope.launch(
        CoroutineExceptionHandler { _, throwable ->
            cont.resumeWithException(throwable.toRemoteContinuationException())
        } + Dispatchers.IO_,
    ) {
        val result = delegate.startDownload(
            withContext(Dispatchers.IO_) { data.toEncodedTorrentInfo() },
            overrideSaveDir = overrideSaveDir?.run { Path(this).inSystem },
        )
        cont.resume(TorrentSessionProxy(result, this@TorrentDownloaderProxy, scope.coroutineContext))
    }

    return DisposableHandleProxy { job.cancel() }
}

[StageGuard]

经测试改了之后大幅度加速了 [resolveDownloadingFile]: Still waiting to get file 的等待时间

[Him188]

RemoteCall 改名叫 RemoteObject 是不是更好一点

remoteObject.callSuspend { foo() } 听起来更合理.

class RemoteTorrentSession(private val aidl: RemoteObject<IRemoteTorrentSession>)