SftpFileReader.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Runtime.ExceptionServices;
using System.Threading;

using Renci.SshNet.Abstractions;
using Renci.SshNet.Common;

namespace Renci.SshNet.Sftp
{
    internal sealed class SftpFileReader : ISftpFileReader
    {
        private const int ReadAheadWaitTimeoutInMilliseconds = 1000;

        private readonly byte[] _handle;
        private readonly ISftpSession _sftpSession;
        private readonly uint _chunkSize;
        private readonly SemaphoreSlim _semaphore;
        private readonly object _readLock;
        private readonly ManualResetEvent _disposingWaitHandle;
        private readonly ManualResetEvent _readAheadCompleted;
        private readonly Dictionary<int, BufferedRead> _queue;
        private readonly WaitHandle[] _waitHandles;

        /// <summary>
        /// Holds the size of the file, when available.
        /// </summary>
        private readonly long? _fileSize;

        private ulong _offset;
        private int _readAheadChunkIndex;
        private ulong _readAheadOffset;
        private int _nextChunkIndex;

        /// <summary>
        /// Holds a value indicating whether EOF has already been signaled by the SSH server.
        /// </summary>
        private bool _endOfFileReceived;

        /// <summary>
        /// Holds a value indicating whether the client has read up to the end of the file.
        /// </summary>
        private bool _isEndOfFileRead;

        private bool _disposingOrDisposed;

        private Exception _exception;

        /// <summary>
        /// Initializes a new instance of the <see cref="SftpFileReader"/> class with the specified handle,
        /// <see cref="ISftpSession"/> and the maximum number of pending reads.
        /// </summary>
        /// <param name="handle">The file handle.</param>
        /// <param name="sftpSession">The SFT session.</param>
        /// <param name="chunkSize">The size of a individual read-ahead chunk.</param>
        /// <param name="maxPendingReads">The maximum number of pending reads.</param>
        /// <param name="fileSize">The size of the file, if known; otherwise, <see langword="null"/>.</param>
        /// <param name="offset">The offset to resume from.</param>
        public SftpFileReader(byte[] handle, ISftpSession sftpSession, uint chunkSize, int maxPendingReads, long? fileSize, ulong offset = 0)
        {
            _handle = handle;
            _sftpSession = sftpSession;
            _chunkSize = chunkSize;
            _fileSize = fileSize;
            _semaphore = new SemaphoreSlim(maxPendingReads);
            _queue = new Dictionary<int, BufferedRead>(maxPendingReads);
            _readLock = new object();
            _readAheadCompleted = new ManualResetEvent(initialState: false);
            _disposingWaitHandle = new ManualResetEvent(initialState: false);
            _waitHandles = _sftpSession.CreateWaitHandleArray(_disposingWaitHandle, _semaphore.AvailableWaitHandle);

            // When resuming a download (offset > 0), set the initial offset of the remote file to
            // the same offset as the local output file. Read-ahead also starts at the same offset.
            _offset = offset;
            _readAheadOffset = offset;

            StartReadAhead();
        }

        public byte[] Read()
        {
#if NET7_0_OR_GREATER
            ObjectDisposedException.ThrowIf(_disposingOrDisposed, this);
#else
            if (_disposingOrDisposed)
            {
                throw new ObjectDisposedException(GetType().FullName);
            }
#endif // NET7_0_OR_GREATER

            if (_exception is not null)
            {
                ExceptionDispatchInfo.Capture(_exception).Throw();
            }

            if (_isEndOfFileRead)
            {
                throw new SshException("Attempting to read beyond the end of the file.");
            }

            BufferedRead nextChunk;

            lock (_readLock)
            {
                // wait until either the next chunk is available, an exception has occurred or the current
                // instance is already disposed
                while (!_queue.TryGetValue(_nextChunkIndex, out nextChunk) && _exception is null)
                {
                    _ = Monitor.Wait(_readLock);
                }

                // throw when exception occured in read-ahead, or the current instance is already disposed
                if (_exception != null)
                {
                    ExceptionDispatchInfo.Capture(_exception).Throw();
                }

                var data = nextChunk.Data;

                if (nextChunk.Offset == _offset)
                {
                    // have we reached EOF?
                    if (data.Length == 0)
                    {
                        // PERF: we do not bother updating all of the internal state when we've reached EOF
                        _isEndOfFileRead = true;
                    }
                    else
                    {
                        // remove processed chunk
                        _ = _queue.Remove(_nextChunkIndex);

                        // update offset
                        _offset += (ulong)data.Length;

                        // move to next chunk
                        _nextChunkIndex++;
                    }

                    // unblock wait in read-ahead
                    _ = _semaphore.Release();

                    return data;
                }

                // When we received an EOF for the next chunk and the size of the file is known, then
                // we only complete the current chunk if we haven't already read up to the file size.
                // This way we save an extra round-trip to the server.
                if (data.Length == 0 && _fileSize.HasValue && _offset == (ulong)_fileSize.Value)
                {
                    // avoid future reads
                    _isEndOfFileRead = true;

                    // unblock wait in read-ahead
                    _ = _semaphore.Release();

                    // signal EOF to caller
                    return nextChunk.Data;
                }
            }

            /*
             * When the server returned less bytes than requested (for the previous chunk)
             * we'll synchronously request the remaining data.
             *
             * Due to the optimization above, we'll only get here in one of the following cases:
             * - an EOF situation for files for which we were unable to obtain the file size
             * - fewer bytes that requested were returned
             *
             * According to the SSH specification, this last case should never happen for normal
             * disk files (but can happen for device files). In practice, OpenSSH - for example -
             * returns less bytes than requested when requesting more than 64 KB.
             *
             * Important:
             * To avoid a deadlock, this read must be done outside of the read lock.
             */

            var bytesToCatchUp = nextChunk.Offset - _offset;

            /*
             * TODO: break loop and interrupt blocking wait in case of exception
             */

            var read = _sftpSession.RequestRead(_handle, _offset, (uint)bytesToCatchUp);
            if (read.Length == 0)
            {
                // process data in read lock to avoid ObjectDisposedException while releasing semaphore
                lock (_readLock)
                {
                    // a zero-length (EOF) response is only valid for the read-back when EOF has
                    // been signaled for the next read-ahead chunk
                    if (nextChunk.Data.Length == 0)
                    {
                        _isEndOfFileRead = true;

                        // ensure we've not yet disposed the current instance
                        if (!_disposingOrDisposed)
                        {
                            // unblock wait in read-ahead
                            _ = _semaphore.Release();
                        }

                        // signal EOF to caller
                        return read;
                    }

                    // move reader to error state
                    _exception = new SshException("Unexpectedly reached end of file.");

                    // ensure we've not yet disposed the current instance
                    if (!_disposingOrDisposed)
                    {
                        // unblock wait in read-ahead
                        _ = _semaphore.Release();
                    }

                    // notify caller of error
                    throw _exception;
                }
            }

            _offset += (uint)read.Length;

            return read;
        }

        ~SftpFileReader()
        {
            Dispose(disposing: false);
        }

        public void Dispose()
        {
            Dispose(disposing: true);
            GC.SuppressFinalize(this);
        }

        /// <summary>
        /// Releases unmanaged and - optionally - managed resources.
        /// </summary>
        /// <param name="disposing"><see langword="true"/> to release both managed and unmanaged resources; <see langword="false"/> to release only unmanaged resources.</param>
        private void Dispose(bool disposing)
        {
            if (_disposingOrDisposed)
            {
                return;
            }

            // transition to disposing state
            _disposingOrDisposed = true;

            if (disposing)
            {
                // record exception to break prevent future Read()
                _exception = new ObjectDisposedException(GetType().FullName);

                // signal that we're disposing to interrupt wait in read-ahead
                _ = _disposingWaitHandle.Set();

                // wait until the read-ahead thread has completed
                _ = _readAheadCompleted.WaitOne();

                // unblock the Read()
                lock (_readLock)
                {
                    // dispose semaphore in read lock to ensure we don't run into an ObjectDisposedException
                    // in Read()
                    _semaphore.Dispose();

                    // awake Read
                    Monitor.PulseAll(_readLock);
                }

                _readAheadCompleted.Dispose();
                _disposingWaitHandle.Dispose();

                if (_sftpSession.IsOpen)
                {
                    try
                    {
                        var closeAsyncResult = _sftpSession.BeginClose(_handle, callback: null, state: null);
                        _sftpSession.EndClose(closeAsyncResult);
                    }
                    catch (Exception ex)
                    {
                        DiagnosticAbstraction.Log("Failure closing handle: " + ex);
                    }
                }
            }
        }

        private void StartReadAhead()
        {
            ThreadAbstraction.ExecuteThread(() =>
            {
                while (!_endOfFileReceived && _exception is null)
                {
                    // check if we should continue with the read-ahead loop
                    // note that the EOF and exception check are not included
                    // in this check as they do not require Read() to be
                    // unblocked (or have already done this)
                    if (!ContinueReadAhead())
                    {
                        // unblock the Read()
                        lock (_readLock)
                        {
                            Monitor.PulseAll(_readLock);
                        }

                        // break the read-ahead loop
                        break;
                    }

                    // attempt to obtain the semaphore; this may time out when all semaphores are
                    // in use due to pending read-aheads (which in turn can happen when the server
                    // is slow to respond or when the session is broken)
                    if (!_semaphore.Wait(ReadAheadWaitTimeoutInMilliseconds))
                    {
                        // re-evaluate whether an exception occurred, and - if not - wait again
                        continue;
                    }

                    // don't bother reading any more chunks if we received EOF, an exception has occurred
                    // or the current instance is disposed
                    if (_endOfFileReceived || _exception != null)
                    {
                        break;
                    }

                    // start reading next chunk
                    var bufferedRead = new BufferedRead(_readAheadChunkIndex, _readAheadOffset);

                    try
                    {
                        // even if we know the size of the file and have read up to EOF, we still want
                        // to keep reading (ahead) until we receive zero bytes from the remote host as
                        // we do not want to rely purely on the reported file size
                        //
                        // if the offset of the read-ahead chunk is greater than that file size, then
                        // we can expect to be reading the last (zero-byte) chunk and switch to synchronous
                        // mode to avoid having multiple read-aheads that read beyond EOF
                        if (_fileSize != null && (long)_readAheadOffset > _fileSize.Value)
                        {
                            var asyncResult = _sftpSession.BeginRead(_handle, _readAheadOffset, _chunkSize, callback: null, bufferedRead);
                            var data = _sftpSession.EndRead(asyncResult);
                            ReadCompletedCore(bufferedRead, data);
                        }
                        else
                        {
                            _ = _sftpSession.BeginRead(_handle, _readAheadOffset, _chunkSize, ReadCompleted, bufferedRead);
                        }
                    }
                    catch (Exception ex)
                    {
                        HandleFailure(ex);
                        break;
                    }

                    // advance read-ahead offset
                    _readAheadOffset += _chunkSize;

                    // increment index of read-ahead chunk
                    _readAheadChunkIndex++;
                }

                _ = _readAheadCompleted.Set();
            });
        }

        /// <summary>
        /// Returns a value indicating whether the read-ahead loop should be continued.
        /// </summary>
        /// <returns>
        /// <see langword="true"/> if the read-ahead loop should be continued; otherwise, <see langword="false"/>.
        /// </returns>
        private bool ContinueReadAhead()
        {
            try
            {
                var waitResult = _sftpSession.WaitAny(_waitHandles, _sftpSession.OperationTimeout);
                switch (waitResult)
                {
                    case 0: // disposing
                        return false;
                    case 1: // semaphore available
                        return true;
                    default:
                        throw new NotImplementedException(string.Format(CultureInfo.InvariantCulture, "WaitAny return value '{0}' is not implemented.", waitResult));
                }
            }
            catch (Exception ex)
            {
                _ = Interlocked.CompareExchange(ref _exception, ex, comparand: null);
                return false;
            }
        }

        private void ReadCompleted(IAsyncResult result)
        {
            if (_disposingOrDisposed)
            {
                // skip further processing if we're disposing the current instance
                // to avoid accessing disposed members
                return;
            }

            var readAsyncResult = (SftpReadAsyncResult)result;

            byte[] data;

            try
            {
                data = readAsyncResult.EndInvoke();
            }
            catch (Exception ex)
            {
                HandleFailure(ex);
                return;
            }

            // a read that completes with a zero-byte result signals EOF
            // but there may be pending reads before that read
            var bufferedRead = (BufferedRead)readAsyncResult.AsyncState;
            ReadCompletedCore(bufferedRead, data);
        }

        private void ReadCompletedCore(BufferedRead bufferedRead, byte[] data)
        {
            bufferedRead.Complete(data);

            lock (_readLock)
            {
                // add item to queue
                _queue.Add(bufferedRead.ChunkIndex, bufferedRead);

                // Signal that a chunk has been read or EOF has been reached.
                // In both cases, Read() will eventually also unblock the "read-ahead" thread.
                Monitor.PulseAll(_readLock);
            }

            // check if server signaled EOF
            if (data.Length == 0)
            {
                // set a flag to stop read-aheads
                _endOfFileReceived = true;
            }
        }

        private void HandleFailure(Exception cause)
        {
            _ = Interlocked.CompareExchange(ref _exception, cause, comparand: null);

            // unblock read-ahead
            _ = _semaphore.Release();

            // unblock Read()
            lock (_readLock)
            {
                Monitor.PulseAll(_readLock);
            }
        }

        internal sealed class BufferedRead
        {
            public int ChunkIndex { get; }

            public byte[] Data { get; private set; }

            public ulong Offset { get; }

            public BufferedRead(int chunkIndex, ulong offset)
            {
                ChunkIndex = chunkIndex;
                Offset = offset;
            }

            public void Complete(byte[] data)
            {
                Data = data;
            }
        }
    }
}