Implement ValueTask extensibility

src/mscorlib/shared/System.Private.CoreLib.Shared.projitems

@@ -551,6 +551,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Tasks\TaskToApm.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Tasks\TaskSchedulerException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Tasks\ValueTask.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Tasks\Sources\IValueTaskSource.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadAbortException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadPriority.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\ThreadStart.cs" />

src/mscorlib/shared/System/IO/FileStream.Unix.cs

@@ -635,12 +635,12 @@ private unsafe void WriteNative(ReadOnlySpan<byte> source)
         /// <param name="source">The buffer to write data from.</param>
         /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
         /// <returns>A task that represents the asynchronous write operation.</returns>
-        private Task WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken cancellationToken)
+        private ValueTask WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken cancellationToken)
         {
             Debug.Assert(_useAsyncIO);
 
             if (cancellationToken.IsCancellationRequested)
-                return Task.FromCanceled(cancellationToken);
+                return new ValueTask(Task.FromCanceled(cancellationToken));
 
             if (_fileHandle.IsClosed)
                 throw Error.GetFileNotOpen();
@@ -667,11 +667,11 @@ private Task WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken c
                         source.Span.CopyTo(new Span<byte>(GetBuffer(), _writePos, source.Length));
                         _writePos += source.Length;
 
-                        return Task.CompletedTask;
+                        return default;
                     }
                     catch (Exception exc)
                     {
-                        return Task.FromException(exc);
+                        return new ValueTask(Task.FromException(exc));
                     }
                     finally
                     {
@@ -682,7 +682,7 @@ private Task WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken c
 
             // Otherwise, issue the whole request asynchronously.
             _asyncState.ReadOnlyMemory = source;
-            return waitTask.ContinueWith((t, s) =>
+            return new ValueTask(waitTask.ContinueWith((t, s) =>
             {
                 // The options available on Unix for writing asynchronously to an arbitrary file 
                 // handle typically amount to just using another thread to do the synchronous write, 
@@ -702,7 +702,7 @@ private Task WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken c
                     thisRef.WriteSpan(readOnlyMemory.Span);
                 }
                 finally { thisRef._asyncState.Release(); }
-            }, this, CancellationToken.None, TaskContinuationOptions.DenyChildAttach, TaskScheduler.Default);
+            }, this, CancellationToken.None, TaskContinuationOptions.DenyChildAttach, TaskScheduler.Default));
         }
 
         /// <summary>Sets the current position of this stream to the given value.</summary>

src/mscorlib/shared/System/IO/FileStream.Windows.cs

@@ -961,7 +961,7 @@ unsafe private Task<int> ReadNativeAsync(Memory<byte> destination, int numBuffer
             return completionSource.Task;
         }
 
-        private Task WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken cancellationToken)
+        private ValueTask WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken cancellationToken)
         {
             Debug.Assert(_useAsyncIO);
             Debug.Assert((_readPos == 0 && _readLength == 0 && _writePos >= 0) || (_writePos == 0 && _readPos <= _readLength), "We're either reading or writing, but not both.");
@@ -1005,7 +1005,7 @@ private Task WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken c
                     // completely, we want to do the asynchronous flush/write as part of this operation 
                     // rather than waiting until the next write that fills the buffer.
                     if (source.Length != remainingBuffer)
-                        return Task.CompletedTask;
+                        return default;
 
                     Debug.Assert(_writePos == _bufferLength);
                 }
@@ -1051,7 +1051,7 @@ private Task WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken c
                     flushTask.IsFaulted ||
                     flushTask.IsCanceled)
                 {
-                    return flushTask;
+                    return new ValueTask(flushTask);
                 }
             }
 
@@ -1061,10 +1061,10 @@ private Task WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationToken c
             // Finally, issue the write asynchronously, and return a Task that logically
             // represents the write operation, including any flushing done.
             Task writeTask = WriteAsyncInternalCore(source, cancellationToken);
-            return
+            return new ValueTask(
                 (flushTask == null || flushTask.Status == TaskStatus.RanToCompletion) ? writeTask :
                 (writeTask.Status == TaskStatus.RanToCompletion) ? flushTask :
-                Task.WhenAll(flushTask, writeTask);
+                Task.WhenAll(flushTask, writeTask));
         }
 
         private unsafe Task WriteAsyncInternalCore(ReadOnlyMemory<byte> source, CancellationToken cancellationToken)
@@ -1319,7 +1319,7 @@ private async Task AsyncModeCopyToAsync(Stream destination, int bufferSize, Canc
                 int bufferedBytes = _readLength - _readPos;
                 if (bufferedBytes > 0)
                 {
-                    await destination.WriteAsync(GetBuffer(), _readPos, bufferedBytes, cancellationToken).ConfigureAwait(false);
+                    await destination.WriteAsync(new ReadOnlyMemory<byte>(GetBuffer(), _readPos, bufferedBytes), cancellationToken).ConfigureAwait(false);
                     _readPos = _readLength = 0;
                 }
             }
@@ -1345,7 +1345,6 @@ private async Task AsyncModeCopyToAsync(Stream destination, int bufferSize, Canc
             // Further, typically the CopyToAsync buffer size will be larger than that used by the FileStream, such that
             // we'd likely be unable to use it anyway.  Instead, we rent the buffer from a pool.
             byte[] copyBuffer = ArrayPool<byte>.Shared.Rent(bufferSize);
-            bufferSize = 0; // repurpose bufferSize to be the high water mark for the buffer, to avoid an extra field in the state machine
 
             // Allocate an Overlapped we can use repeatedly for all operations
             var awaitableOverlapped = new PreAllocatedOverlapped(AsyncCopyToAwaitable.s_callback, readAwaitable, copyBuffer);
@@ -1452,13 +1451,6 @@ private async Task AsyncModeCopyToAsync(Stream destination, int bufferSize, Canc
                         {
                             readAwaitable._position += numBytesRead;
                         }
-
-                        // (and keep track of the maximum number of bytes in the buffer we used, to avoid excessive and unnecessary
-                        // clearing of the buffer before we return it to the pool)
-                        if (numBytesRead > bufferSize)
-                        {
-                            bufferSize = numBytesRead;
-                        }
                     }
                     finally
                     {
@@ -1479,7 +1471,7 @@ private async Task AsyncModeCopyToAsync(Stream destination, int bufferSize, Canc
                     }
 
                     // Write out the read data.
-                    await destination.WriteAsync(copyBuffer, 0, (int)readAwaitable._numBytes, cancellationToken).ConfigureAwait(false);
+                    await destination.WriteAsync(new ReadOnlyMemory<byte>(copyBuffer, 0, (int)readAwaitable._numBytes), cancellationToken).ConfigureAwait(false);
                 }
             }
             finally
@@ -1488,8 +1480,7 @@ private async Task AsyncModeCopyToAsync(Stream destination, int bufferSize, Canc
                 cancellationReg.Dispose();
                 awaitableOverlapped.Dispose();
 
-                Array.Clear(copyBuffer, 0, bufferSize);
-                ArrayPool<byte>.Shared.Return(copyBuffer, clearArray: false);
+                ArrayPool<byte>.Shared.Return(copyBuffer);
 
                 // Make sure the stream's current position reflects where we ended up
                 if (!_fileHandle.IsClosed && CanSeek)

src/mscorlib/shared/System/IO/FileStream.cs

@@ -458,10 +458,10 @@ public override Task WriteAsync(byte[] buffer, int offset, int count, Cancellati
             if (IsClosed)
                 throw Error.GetFileNotOpen();
 
-            return WriteAsyncInternal(new ReadOnlyMemory<byte>(buffer, offset, count), cancellationToken);
+            return WriteAsyncInternal(new ReadOnlyMemory<byte>(buffer, offset, count), cancellationToken).AsTask();
         }
 
-        public override Task WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default(CancellationToken))
+        public override ValueTask WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default(CancellationToken))
         {
             if (!_useAsyncIO || GetType() != typeof(FileStream))
             {
@@ -473,7 +473,7 @@ public override Task WriteAsync(byte[] buffer, int offset, int count, Cancellati
 
             if (cancellationToken.IsCancellationRequested)
             {
-                return Task.FromCanceled<int>(cancellationToken);
+                return new ValueTask(Task.FromCanceled<int>(cancellationToken));
             }
 
             if (IsClosed)
@@ -853,7 +853,7 @@ public override IAsyncResult BeginWrite(byte[] array, int offset, int numBytes,
             if (!IsAsync)
                 return base.BeginWrite(array, offset, numBytes, callback, state);
             else
-                return TaskToApm.Begin(WriteAsyncInternal(new ReadOnlyMemory<byte>(array, offset, numBytes), CancellationToken.None), callback, state);
+                return TaskToApm.Begin(WriteAsyncInternal(new ReadOnlyMemory<byte>(array, offset, numBytes), CancellationToken.None).AsTask(), callback, state);
         }
 
         public override int EndRead(IAsyncResult asyncResult)

src/mscorlib/shared/System/IO/MemoryStream.cs

@@ -752,11 +752,11 @@ public override Task WriteAsync(Byte[] buffer, int offset, int count, Cancellati
             }
         }
 
-        public override Task WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default(CancellationToken))
+        public override ValueTask WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default(CancellationToken))
         {
             if (cancellationToken.IsCancellationRequested)
             {
-                return Task.FromCanceled(cancellationToken);
+                return new ValueTask(Task.FromCanceled(cancellationToken));
             }
 
             try
@@ -771,15 +771,15 @@ public override Task WriteAsync(Byte[] buffer, int offset, int count, Cancellati
                 {
                     Write(source.Span);
                 }
-                return Task.CompletedTask;
+                return default;
             }
             catch (OperationCanceledException oce)
             {
-                return Task.FromCancellation<VoidTaskResult>(oce);
+                return new ValueTask(Task.FromCancellation<VoidTaskResult>(oce));
             }
             catch (Exception exception)
             {
-                return Task.FromException(exception);
+                return new ValueTask(Task.FromException(exception));
             }
         }
 

src/mscorlib/shared/System/IO/StreamReader.cs

@@ -1091,7 +1091,7 @@ internal override async ValueTask<int> ReadAsyncInternal(Memory<char> buffer, Ca
                         {
                             Debug.Assert(_bytePos <= _encoding.Preamble.Length, "possible bug in _compressPreamble.  Are two threads using this StreamReader at the same time?");
                             int tmpBytePos = _bytePos;
-                            int len = await tmpStream.ReadAsync(tmpByteBuffer, tmpBytePos, tmpByteBuffer.Length - tmpBytePos, cancellationToken).ConfigureAwait(false);
+                            int len = await tmpStream.ReadAsync(new Memory<byte>(tmpByteBuffer, tmpBytePos, tmpByteBuffer.Length - tmpBytePos), cancellationToken).ConfigureAwait(false);
                             Debug.Assert(len >= 0, "Stream.Read returned a negative number!  This is a bug in your stream class.");
 
                             if (len == 0)
@@ -1127,7 +1127,7 @@ internal override async ValueTask<int> ReadAsyncInternal(Memory<char> buffer, Ca
                         {
                             Debug.Assert(_bytePos == 0, "_bytePos can be non zero only when we are trying to _checkPreamble.  Are two threads using this StreamReader at the same time?");
 
-                            _byteLen = await tmpStream.ReadAsync(tmpByteBuffer, 0, tmpByteBuffer.Length, cancellationToken).ConfigureAwait(false);
+                            _byteLen = await tmpStream.ReadAsync(new Memory<byte>(tmpByteBuffer), cancellationToken).ConfigureAwait(false);
 
                             Debug.Assert(_byteLen >= 0, "Stream.Read returned a negative number!  This is a bug in your stream class.");
 
@@ -1303,7 +1303,7 @@ private async Task<int> ReadBufferAsync()
                 {
                     Debug.Assert(_bytePos <= _encoding.Preamble.Length, "possible bug in _compressPreamble. Are two threads using this StreamReader at the same time?");
                     int tmpBytePos = _bytePos;
-                    int len = await tmpStream.ReadAsync(tmpByteBuffer, tmpBytePos, tmpByteBuffer.Length - tmpBytePos).ConfigureAwait(false);
+                    int len = await tmpStream.ReadAsync(new Memory<byte>(tmpByteBuffer, tmpBytePos, tmpByteBuffer.Length - tmpBytePos)).ConfigureAwait(false);
                     Debug.Assert(len >= 0, "Stream.Read returned a negative number!  This is a bug in your stream class.");
 
                     if (len == 0)
@@ -1325,7 +1325,7 @@ private async Task<int> ReadBufferAsync()
                 else
                 {
                     Debug.Assert(_bytePos == 0, "_bytePos can be non zero only when we are trying to _checkPreamble. Are two threads using this StreamReader at the same time?");
-                    _byteLen = await tmpStream.ReadAsync(tmpByteBuffer, 0, tmpByteBuffer.Length).ConfigureAwait(false);
+                    _byteLen = await tmpStream.ReadAsync(new Memory<byte>(tmpByteBuffer)).ConfigureAwait(false);
                     Debug.Assert(_byteLen >= 0, "Stream.Read returned a negative number!  Bug in stream class.");
 
                     if (_byteLen == 0)  // We're at EOF

src/mscorlib/shared/System/IO/StreamWriter.cs

@@ -963,14 +963,14 @@ private static async Task FlushAsyncInternal(StreamWriter _this, bool flushStrea
                 byte[] preamble = encoding.GetPreamble();
                 if (preamble.Length > 0)
                 {
-                    await stream.WriteAsync(preamble, 0, preamble.Length, cancellationToken).ConfigureAwait(false);
+                    await stream.WriteAsync(new ReadOnlyMemory<byte>(preamble), cancellationToken).ConfigureAwait(false);
                 }
             }
 
             int count = encoder.GetBytes(charBuffer, 0, charPos, byteBuffer, 0, flushEncoder);
             if (count > 0)
             {
-                await stream.WriteAsync(byteBuffer, 0, count, cancellationToken).ConfigureAwait(false);
+                await stream.WriteAsync(new ReadOnlyMemory<byte>(byteBuffer, 0, count), cancellationToken).ConfigureAwait(false);
             }
 
             // By definition, calling Flush should flush the stream, but this is

src/mscorlib/shared/System/IO/UnmanagedMemoryStream.cs

@@ -783,11 +783,11 @@ public override Task WriteAsync(Byte[] buffer, Int32 offset, Int32 count, Cancel
         /// </summary>
         /// <param name="buffer">Buffer that will be written.</param>
         /// <param name="cancellationToken">Token that can be used to cancel the operation.</param>
-        public override Task WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default(CancellationToken))
+        public override ValueTask WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default(CancellationToken))
         {
             if (cancellationToken.IsCancellationRequested)
             {
-                return Task.FromCanceled(cancellationToken);
+                return new ValueTask(Task.FromCanceled(cancellationToken));
             }
 
             try
@@ -802,11 +802,11 @@ public override Task WriteAsync(Byte[] buffer, Int32 offset, Int32 count, Cancel
                 {
                     Write(source.Span);
                 }
-                return Task.CompletedTask;
+                return default;
             }
             catch (Exception ex)
             {
-                return Task.FromException(ex);
+                return new ValueTask(Task.FromException(ex));
             }
         }
 

src/mscorlib/shared/System/IO/UnmanagedMemoryStreamWrapper.cs

@@ -217,7 +217,7 @@ public override Task WriteAsync(Byte[] buffer, Int32 offset, Int32 count, Cancel
             return _unmanagedStream.WriteAsync(buffer, offset, count, cancellationToken);
         }
 
-        public override Task WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default(CancellationToken))
+        public override ValueTask WriteAsync(ReadOnlyMemory<byte> source, CancellationToken cancellationToken = default(CancellationToken))
         {
             return _unmanagedStream.WriteAsync(source, cancellationToken);
         }