preallocate

Author: GarrettBeatty

sdk/src/Services/S3/Custom/Transfer/Internal/BufferedPartDataHandler.cs

@@ -280,7 +280,7 @@ private async Task<IPartDataSource> BufferPartFromResponseAsync(
                 _logger.DebugFormat("BufferedPartDataHandler: [Part {0}] Buffering {1} bytes using chunked buffer stream",
                     partNumber, expectedBytes);
 
-                chunkedStream = new ChunkedBufferStream();
+                chunkedStream = new ChunkedBufferStream(expectedBytes);
 
                 _logger.DebugFormat("BufferedPartDataHandler: [Part {0}] Reading response stream into chunked buffers",
                     partNumber);

sdk/src/Services/S3/Custom/Transfer/Internal/ChunkedBufferStream.cs

@@ -78,12 +78,40 @@ internal class ChunkedBufferStream : Stream
         /// </summary>
         private const long MAX_STREAM_SIZE = (long)int.MaxValue * CHUNK_SIZE;
 
-        private readonly List<byte[]> _chunks = new List<byte[]>();
+        private readonly List<byte[]> _chunks;
         private long _length = 0;
         private long _position = 0;
         private bool _isReadMode = false;
         private bool _disposed = false;
 
+        /// <summary>
+        /// Creates a new ChunkedBufferStream with default initial capacity.
+        /// </summary>
+        public ChunkedBufferStream()
+        {
+            _chunks = new List<byte[]>();
+        }
+
+        /// <summary>
+        /// Creates a new ChunkedBufferStream with pre-allocated capacity for the expected size.
+        /// This avoids List resizing during writes, improving performance for known sizes.
+        /// </summary>
+        /// <param name="estimatedSize">The estimated total size in bytes. Used to pre-allocate the chunk list capacity.</param>
+        public ChunkedBufferStream(long estimatedSize)
+        {
+            if (estimatedSize > 0)
+            {
+                // Calculate number of chunks needed and cap at int.MaxValue for List capacity
+                long estimatedChunks = (estimatedSize + CHUNK_SIZE - 1) / CHUNK_SIZE;
+                int capacity = (int)Math.Min(estimatedChunks, int.MaxValue);
+                _chunks = new List<byte[]>(capacity);
+            }
+            else
+            {
+                _chunks = new List<byte[]>();
+            }
+        }
+
         /// <summary>
         /// Throws <see cref="ObjectDisposedException"/> if the stream has been disposed.
         /// </summary>

sdk/src/Services/S3/Custom/Transfer/Internal/ChunkedPartDataSource.cs

@@ -102,12 +102,12 @@ public ChunkedPartDataSource(int partNumber, ChunkedBufferStream stream)
         /// are not currently available, or zero if the end of the stream is reached.
         /// </returns>
         /// <exception cref="ObjectDisposedException">Thrown if the object has been disposed.</exception>
-        public async Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        public Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
         {
             if (_disposed)
                 throw new ObjectDisposedException(nameof(ChunkedPartDataSource));
 
-            return await _stream.ReadAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
+            return _stream.ReadAsync(buffer, offset, count, cancellationToken);
         }
 
         /// <summary>