Fix PooledByteBufferWriter handling of sizeHint <= 0

src/libraries/System.Net.ServerSentEvents/src/System/Net/ServerSentEvents/PooledByteBufferWriter.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Buffers;
@@ -8,19 +8,20 @@ namespace System.Net.ServerSentEvents
 {
     internal sealed class PooledByteBufferWriter : IBufferWriter<byte>, IDisposable
     {
+        private const int MinimumBufferSize = 256;
         private ArrayBuffer _buffer = new(initialSize: 256, usePool: true);
 
         public void Advance(int count) => _buffer.Commit(count);
 
         public Memory<byte> GetMemory(int sizeHint = 0)
         {
-            _buffer.EnsureAvailableSpace(sizeHint);
+            _buffer.EnsureAvailableSpace(Math.Max(sizeHint, MinimumBufferSize));
             return _buffer.AvailableMemory;
         }
 
         public Span<byte> GetSpan(int sizeHint = 0)
         {
-            _buffer.EnsureAvailableSpace(sizeHint);
+            _buffer.EnsureAvailableSpace(Math.Max(sizeHint, MinimumBufferSize));
             return _buffer.AvailableSpan;
         }
 

src/libraries/System.Net.ServerSentEvents/tests/SseFormatterTests.cs

@@ -4,6 +4,7 @@
 using System.Buffers;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Text;
 using System.Text.Json;
@@ -117,6 +118,35 @@ async IAsyncEnumerable<SseItem<string>> GetItemsAsync([EnumeratorCancellation] C
             }
         }
 
+        [Fact]
+        public static async Task WriteLargeItems_DataWrittenSuccessfully()
+        {
+            const int NumberOfItems = 10;
+            byte[] expected = Encoding.UTF8.GetBytes(string.Concat(Enumerable.Repeat("This is a test. This is only a test.", 100)));
+
+            MemoryStream memoryStream = new();
+            await SseFormatter.WriteAsync(GetBuffersAsync(), memoryStream, (item, writer) => writer.Write(item.Data));
+
+            memoryStream.Position = 0;
+            int count = 0;
+            foreach (SseItem<byte[]> item in SseParser.Create(memoryStream, (eventType, data) => data.ToArray()).Enumerate())
+            {
+                Assert.Equal(expected, item.Data);
+                count++;
+            }
+
+            Assert.Equal(NumberOfItems, count);
+
+            async IAsyncEnumerable<SseItem<byte[]>> GetBuffersAsync()
+            {
+                await Task.Yield();
+                for (int i = 0; i < NumberOfItems; i++)
+                {
+                    yield return new SseItem<byte[]>(expected);
+                }
+            }
+        }
+
         [Fact]
         public static async Task WriteAsync_ParserCanRoundtripJsonEvents()
         {