Fix StackOverflowException when reading large buffers.

This was similar to #239 but occurred in InsertLargeBlobAsync when reading over a slow connection. The 'ContinueWith' code chained too many continuations together recursively. The compiler-generated 'await' state machine avoids this problem.

Author: bgrainger

src/MySqlConnector/MySqlConnector.csproj

@@ -22,6 +22,7 @@
     <RepositoryType>git</RepositoryType>
     <RepositoryUrl>https://github.com/mysql-net/MySqlConnector.git</RepositoryUrl>
     <DebugType>embedded</DebugType>
+    <LangVersion>latest</LangVersion>
   </PropertyGroup>
 
   <ItemGroup Condition=" '$(OS)' == 'Windows_NT' ">

src/MySqlConnector/Protocol/Serialization/BufferedByteReader.cs

@@ -6,10 +6,7 @@ namespace MySqlConnector.Protocol.Serialization
 {
 	internal sealed class BufferedByteReader
 	{
-		public BufferedByteReader()
-		{
-			m_buffer = new byte[16384];
-		}
+		public BufferedByteReader() => m_buffer = new byte[16384];
 
 		public ValueTask<ArraySegment<byte>> ReadBytesAsync(IByteHandler byteHandler, int count, IOBehavior ioBehavior)
 		{
@@ -32,62 +29,32 @@ public ValueTask<ArraySegment<byte>> ReadBytesAsync(IByteHandler byteHandler, in
 			return ReadBytesAsync(byteHandler, new ArraySegment<byte>(buffer, m_remainingData.Count, buffer.Length - m_remainingData.Count), count, ioBehavior);
 		}
 
-		private ValueTask<ArraySegment<byte>> ReadBytesAsync(IByteHandler byteHandler, ArraySegment<byte> buffer, int totalBytesToRead, IOBehavior ioBehavior)
-		{
-			// keep reading data synchronously while it is available
-			var readBytesTask = byteHandler.ReadBytesAsync(buffer, ioBehavior);
-			while (readBytesTask.IsCompletedSuccessfully)
-			{
-				ValueTask<ArraySegment<byte>> result;
-				if (HasReadAllData(readBytesTask.Result, ref buffer, totalBytesToRead, out result))
-					return result;
-
-				readBytesTask = byteHandler.ReadBytesAsync(buffer, ioBehavior);
-			}
-
-			// call .ContinueWith (as a separate method, so that the temporary class for the lambda is only allocated if necessary)
-			return AddContinuation(readBytesTask, byteHandler, buffer, totalBytesToRead, ioBehavior);
-
-			ValueTask<ArraySegment<byte>> AddContinuation(ValueTask<int> readBytesTask_, IByteHandler byteHandler_, ArraySegment<byte> buffer_, int totalBytesToRead_, IOBehavior ioBehavior_) =>
-				readBytesTask_.ContinueWith(x => HasReadAllData(x, ref buffer_, totalBytesToRead_, out var result_) ? result_ : ReadBytesAsync(byteHandler_, buffer_, totalBytesToRead_, ioBehavior_));
-		}
-
-		/// <summary>
-		/// Returns <c>true</c> if all the required data has been read, then sets <paramref name="result"/> to a <see cref="ValueTask{ArraySegment{byte}}"/> representing that data.
-		/// Otherwise, returns <c>false</c> and updates <paramref name="buffer"/> to where more data should be placed when it's read.
-		/// </summary>
-		/// <param name="readBytesCount">The number of bytes that have just been read into <paramref name="buffer"/>.</param>
-		/// <param name="buffer">The <see cref="ArraySegment{byte}"/> that contains all the data read so far, and that will receive more data read in the future. It is assumed that data is stored
-		/// at the beginning of the array owned by <paramref name="buffer"/> and that <code>Offset</code> indicates where to place future data.</param>
-		/// <param name="totalBytesToRead">The total number of bytes that need to be read.</param>
-		/// <param name="result">On success, a <see cref="ValueTask{ArraySegment{byte}}"/> representing all the data that was read.</param>
-		/// <returns><c>true</c> if all data has been read; otherwise, <c>false</c>.</returns>
-		private bool HasReadAllData(int readBytesCount, ref ArraySegment<byte> buffer, int totalBytesToRead, out ValueTask<ArraySegment<byte>> result)
+		private async ValueTask<ArraySegment<byte>> ReadBytesAsync(IByteHandler byteHandler, ArraySegment<byte> buffer, int totalBytesToRead, IOBehavior ioBehavior)
 		{
-			if (readBytesCount == 0)
-			{
-				var data = m_remainingData;
-				m_remainingData = default(ArraySegment<byte>);
-				result = new ValueTask<ArraySegment<byte>>(data);
-				return true;
-			}
-
-			var bufferSize = buffer.Offset + readBytesCount;
-			if (bufferSize >= totalBytesToRead)
+			while (true)
 			{
-				var bufferBytes = new ArraySegment<byte>(buffer.Array, 0, bufferSize);
-				var requestedBytes = bufferBytes.Slice(0, totalBytesToRead);
-				m_remainingData = bufferBytes.Slice(totalBytesToRead);
-				result = new ValueTask<ArraySegment<byte>>(requestedBytes);
-				return true;
+				var readBytesCount = await byteHandler.ReadBytesAsync(buffer, ioBehavior).ConfigureAwait(false);
+				if (readBytesCount == 0)
+				{
+					var data = m_remainingData;
+					m_remainingData = default;
+					return data;
+				}
+
+				var bufferSize = buffer.Offset + readBytesCount;
+				if (bufferSize >= totalBytesToRead)
+				{
+					var bufferBytes = new ArraySegment<byte>(buffer.Array, 0, bufferSize);
+					var requestedBytes = bufferBytes.Slice(0, totalBytesToRead);
+					m_remainingData = bufferBytes.Slice(totalBytesToRead);
+					return requestedBytes;
+				}
+
+				buffer = buffer.Slice(readBytesCount);
 			}
-
-			buffer = new ArraySegment<byte>(buffer.Array, bufferSize, buffer.Array.Length - bufferSize);
-			result = default(ValueTask<ArraySegment<byte>>);
-			return false;
 		}
 
-		ArraySegment<byte> m_remainingData;
 		readonly byte[] m_buffer;
+		ArraySegment<byte> m_remainingData;
 	}
 }