Add support for resuming file upload/download

Author: Pedro Fonseca

src/Renci.SshNet/IServiceFactory.cs

@@ -83,17 +83,18 @@ internal partial interface IServiceFactory
         /// <exception cref="SshConnectionException">No key exchange algorithm is supported by both client and server.</exception>
         IKeyExchange CreateKeyExchange(IDictionary<string, Type> clientAlgorithms, string[] serverAlgorithms);
 
-        /// <summary>
+                /// <summary>
         /// Creates an <see cref="ISftpFileReader"/> for the specified file and with the specified
         /// buffer size.
         /// </summary>
         /// <param name="fileName">The file to read.</param>
         /// <param name="sftpSession">The SFTP session to use.</param>
         /// <param name="bufferSize">The size of buffer.</param>
+        /// <param name="offset">The offset to resume from.</param>
         /// <returns>
         /// An <see cref="ISftpFileReader"/>.
         /// </returns>
-        ISftpFileReader CreateSftpFileReader(string fileName, ISftpSession sftpSession, uint bufferSize);
+        ISftpFileReader CreateSftpFileReader(string fileName, ISftpSession sftpSession, uint bufferSize, ulong offset = 0);
 
         /// <summary>
         /// Creates a new <see cref="ISftpResponseFactory"/> instance.

src/Renci.SshNet/ServiceFactory.cs

@@ -138,10 +138,11 @@ public INetConfSession CreateNetConfSession(ISession session, int operationTimeo
         /// <param name="fileName">The file to read.</param>
         /// <param name="sftpSession">The SFTP session to use.</param>
         /// <param name="bufferSize">The size of buffer.</param>
+        /// <param name="offset">The offset to resume from.</param>
         /// <returns>
         /// An <see cref="ISftpFileReader"/>.
         /// </returns>
-        public ISftpFileReader CreateSftpFileReader(string fileName, ISftpSession sftpSession, uint bufferSize)
+        public ISftpFileReader CreateSftpFileReader(string fileName, ISftpSession sftpSession, uint bufferSize, ulong offset = 0)
         {
             const int defaultMaxPendingReads = 3;
 
@@ -163,7 +164,7 @@ public ISftpFileReader CreateSftpFileReader(string fileName, ISftpSession sftpSe
             {
                 var fileAttributes = sftpSession.EndLStat(statAsyncResult);
                 fileSize = fileAttributes.Size;
-                maxPendingReads = Math.Min(10, (int) Math.Ceiling((double) fileAttributes.Size / chunkSize) + 1);
+                maxPendingReads = Math.Min(10, (int) Math.Ceiling((double)(fileSize - (long)offset) / chunkSize) + 1);
             }
             catch (SshException ex)
             {
@@ -173,7 +174,7 @@ public ISftpFileReader CreateSftpFileReader(string fileName, ISftpSession sftpSe
                 DiagnosticAbstraction.Log(string.Format("Failed to obtain size of file. Allowing maximum {0} pending reads: {1}", maxPendingReads, ex));
             }
 
-            return sftpSession.CreateFileReader(handle, sftpSession, chunkSize, maxPendingReads, fileSize);
+            return sftpSession.CreateFileReader(handle, sftpSession, chunkSize, maxPendingReads, fileSize, offset);
         }
 
         /// <summary>

src/Renci.SshNet/Sftp/ISftpSession.cs

@@ -494,10 +494,11 @@ void RequestWrite(byte[] handle,
         /// <param name="chunkSize">The maximum number of bytes to read with each chunk.</param>
         /// <param name="maxPendingReads">The maximum number of pending reads.</param>
         /// <param name="fileSize">The size of the file or <see langword="null"/> when the size could not be determined.</param>
+        /// <param name="offset">The offset to resume from.</param>
         /// <returns>
         /// An <see cref="ISftpFileReader"/> for reading the content of the file represented by the
         /// specified <paramref name="handle"/>.
         /// </returns>
-        ISftpFileReader CreateFileReader(byte[] handle, ISftpSession sftpSession, uint chunkSize, int maxPendingReads, long? fileSize);
+        ISftpFileReader CreateFileReader(byte[] handle, ISftpSession sftpSession, uint chunkSize, int maxPendingReads, long? fileSize, ulong offset = 0);
     }
 }

src/Renci.SshNet/Sftp/SftpFileReader.cs

@@ -56,12 +56,15 @@ internal sealed class SftpFileReader : ISftpFileReader
         /// <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>
-        public SftpFileReader(byte[] handle, ISftpSession sftpSession, uint chunkSize, int maxPendingReads, long? fileSize)
+        /// <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;
+            _offset = offset;
+            _readAheadOffset = offset;
             _semaphore = new SemaphoreLight(maxPendingReads);
             _queue = new Dictionary<int, BufferedRead>(maxPendingReads);
             _readLock = new object();