[release/7.0] Ensure ReadBufferState resets any BOM offsets every time the buffer is advanced.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/ReadBufferState.cs

@@ -122,7 +122,6 @@ public void AdvanceBuffer(int bytesConsumed)
                     // Copy the unprocessed data to the new buffer while shifting the processed bytes.
                     Buffer.BlockCopy(oldBuffer, _offset + bytesConsumed, newBuffer, 0, _count);
                     _buffer = newBuffer;
-                    _offset = 0;
                     _maxCount = _count;
 
                     // Clear and return the old buffer
@@ -133,9 +132,10 @@ public void AdvanceBuffer(int bytesConsumed)
                 {
                     // Shift the processed bytes to the beginning of buffer to make more room.
                     Buffer.BlockCopy(_buffer, _offset + bytesConsumed, _buffer, 0, _count);
-                    _offset = 0;
                 }
             }
+
+            _offset = 0;
         }
 
         private void ProcessReadBytes()

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/ContinuationTests.cs

@@ -235,6 +235,31 @@ public static void InvalidJsonShouldFailAtAnyPosition_Sequence(
             Assert.Equal(expectedFailure.Column, ex.BytePositionInLine);
         }
 
+        [Fact]
+        public static async Task BomHandlingRegressionTest()
+        {
+            byte[] utf8Bom = Encoding.UTF8.GetPreamble();
+            byte[] json = """{ "Value" :                    "Hello" }"""u8.ToArray();
+
+            using var stream = new MemoryStream();
+            stream.Write(utf8Bom, 0, utf8Bom.Length);
+            stream.Write(json, 0, json.Length);
+            stream.Position = 0;
+
+            var options = new JsonSerializerOptions
+            {
+                DefaultBufferSize = 32
+            };
+
+            Test result = await JsonSerializer.DeserializeAsync<Test>(stream, options);
+            Assert.Equal("Hello", result.Value);
+        }
+
+        private class Test
+        {
+            public string Value { get; set; }
+        }
+
         private class Chunk : ReadOnlySequenceSegment<byte>
         {
             public Chunk(string json, int firstSegmentLength)

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/JsonSerializerWrapper.Reflection.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
 using System.IO;
 using System.Runtime.CompilerServices;
 using System.Text.Json.Nodes;
@@ -15,9 +16,9 @@ public abstract partial class JsonSerializerWrapper
         public static JsonSerializerWrapper SpanSerializer { get; } = new SpanSerializerWrapper();
         public static JsonSerializerWrapper StringSerializer { get; } = new StringSerializerWrapper();
         public static StreamingJsonSerializerWrapper AsyncStreamSerializer { get; } = new AsyncStreamSerializerWrapper();
-        public static StreamingJsonSerializerWrapper AsyncStreamSerializerWithSmallBuffer { get; } = new AsyncStreamSerializerWrapper(forceSmallBufferInOptions: true);
+        public static StreamingJsonSerializerWrapper AsyncStreamSerializerWithSmallBuffer { get; } = new AsyncStreamSerializerWrapper(forceSmallBufferInOptions: true, forceBomInsertions: true);
         public static StreamingJsonSerializerWrapper SyncStreamSerializer { get; } = new SyncStreamSerializerWrapper();
-        public static StreamingJsonSerializerWrapper SyncStreamSerializerWithSmallBuffer { get; } = new SyncStreamSerializerWrapper(forceSmallBufferInOptions: true);
+        public static StreamingJsonSerializerWrapper SyncStreamSerializerWithSmallBuffer { get; } = new SyncStreamSerializerWrapper(forceSmallBufferInOptions: true, forceBomInsertions: true);
         public static JsonSerializerWrapper ReaderWriterSerializer { get; } = new ReaderWriterSerializerWrapper();
         public static JsonSerializerWrapper DocumentSerializer { get; } = new DocumentSerializerWrapper();
         public static JsonSerializerWrapper ElementSerializer { get; } = new ElementSerializerWrapper();
@@ -120,17 +121,22 @@ public override Task<object> DeserializeWrapper(string json, Type type, JsonSeri
         private class AsyncStreamSerializerWrapper : StreamingJsonSerializerWrapper
         {
             private readonly bool _forceSmallBufferInOptions;
+            private readonly bool _forceBomInsertions;
 
             public override bool IsAsyncSerializer => true;
 
-            public AsyncStreamSerializerWrapper(bool forceSmallBufferInOptions = false)
+            public AsyncStreamSerializerWrapper(bool forceSmallBufferInOptions = false, bool forceBomInsertions = false)
             {
                 _forceSmallBufferInOptions = forceSmallBufferInOptions;
+                _forceBomInsertions = forceBomInsertions;
             }
 
             private JsonSerializerOptions? ResolveOptionsInstance(JsonSerializerOptions? options)
                 => _forceSmallBufferInOptions ? JsonSerializerOptionsSmallBufferMapper.ResolveOptionsInstanceWithSmallBuffer(options) : options;
 
+            private Stream ResolveReadStream(Stream stream)
+                => stream is not null && _forceBomInsertions ? new Utf8BomInsertingStream(stream) : stream;
+
             public override Task SerializeWrapper<T>(Stream utf8Json, T value, JsonSerializerOptions options = null)
             {
                 return JsonSerializer.SerializeAsync<T>(utf8Json, value, ResolveOptionsInstance(options));
@@ -153,38 +159,43 @@ public override Task SerializeWrapper(Stream stream, object value, Type inputTyp
 
             public override async Task<T> DeserializeWrapper<T>(Stream utf8Json, JsonSerializerOptions options = null)
             {
-                return await JsonSerializer.DeserializeAsync<T>(utf8Json, ResolveOptionsInstance(options));
+                return await JsonSerializer.DeserializeAsync<T>(ResolveReadStream(utf8Json), ResolveOptionsInstance(options));
             }
 
             public override async Task<object> DeserializeWrapper(Stream utf8Json, Type returnType, JsonSerializerOptions options = null)
             {
-                return await JsonSerializer.DeserializeAsync(utf8Json, returnType, ResolveOptionsInstance(options));
+                return await JsonSerializer.DeserializeAsync(ResolveReadStream(utf8Json), returnType, ResolveOptionsInstance(options));
             }
 
             public override async Task<T> DeserializeWrapper<T>(Stream utf8Json, JsonTypeInfo<T> jsonTypeInfo)
             {
-                return await JsonSerializer.DeserializeAsync<T>(utf8Json, jsonTypeInfo);
+                return await JsonSerializer.DeserializeAsync<T>(ResolveReadStream(utf8Json), jsonTypeInfo);
             }
 
             public override async Task<object> DeserializeWrapper(Stream utf8Json, Type returnType, JsonSerializerContext context)
             {
-                return await JsonSerializer.DeserializeAsync(utf8Json, returnType, context);
+                return await JsonSerializer.DeserializeAsync(ResolveReadStream(utf8Json), returnType, context);
             }
         }
 
         private class SyncStreamSerializerWrapper : StreamingJsonSerializerWrapper
         {
             private readonly bool _forceSmallBufferInOptions;
+            private readonly bool _forceBomInsertions;
+
+            public override bool IsAsyncSerializer => false;
 
-            public SyncStreamSerializerWrapper(bool forceSmallBufferInOptions = false)
+            public SyncStreamSerializerWrapper(bool forceSmallBufferInOptions = false, bool forceBomInsertions = false)
             {
                 _forceSmallBufferInOptions = forceSmallBufferInOptions;
+                _forceBomInsertions = forceBomInsertions;
             }
 
             private JsonSerializerOptions? ResolveOptionsInstance(JsonSerializerOptions? options)
                 => _forceSmallBufferInOptions ? JsonSerializerOptionsSmallBufferMapper.ResolveOptionsInstanceWithSmallBuffer(options) : options;
 
-            public override bool IsAsyncSerializer => false;
+            private Stream ResolveReadStream(Stream stream)
+                => stream is not null && _forceBomInsertions ? new Utf8BomInsertingStream(stream) : stream;
 
             public override Task SerializeWrapper<T>(Stream utf8Json, T value, JsonSerializerOptions options = null)
             {
@@ -212,25 +223,25 @@ public override Task SerializeWrapper(Stream stream, object value, Type inputTyp
 
             public override Task<T> DeserializeWrapper<T>(Stream utf8Json, JsonSerializerOptions options = null)
             {
-                T result = JsonSerializer.Deserialize<T>(utf8Json, ResolveOptionsInstance(options));
+                T result = JsonSerializer.Deserialize<T>(ResolveReadStream(utf8Json), ResolveOptionsInstance(options));
                 return Task.FromResult(result);
             }
 
             public override Task<object> DeserializeWrapper(Stream utf8Json, Type returnType, JsonSerializerOptions options = null)
             {
-                object result = JsonSerializer.Deserialize(utf8Json, returnType, ResolveOptionsInstance(options));
+                object result = JsonSerializer.Deserialize(ResolveReadStream(utf8Json), returnType, ResolveOptionsInstance(options));
                 return Task.FromResult(result);
             }
 
             public override Task<T> DeserializeWrapper<T>(Stream utf8Json, JsonTypeInfo<T> jsonTypeInfo)
             {
-                T result = JsonSerializer.Deserialize<T>(utf8Json, jsonTypeInfo);
+                T result = JsonSerializer.Deserialize<T>(ResolveReadStream(utf8Json), jsonTypeInfo);
                 return Task.FromResult(result);
             }
 
             public override Task<object> DeserializeWrapper(Stream utf8Json, Type returnType, JsonSerializerContext context)
             {
-                object result = JsonSerializer.Deserialize(utf8Json, returnType, context);
+                object result = JsonSerializer.Deserialize(ResolveReadStream(utf8Json), returnType, context);
                 return Task.FromResult(result);
             }
         }
@@ -653,5 +664,88 @@ public static JsonSerializerOptions ResolveOptionsInstanceWithSmallBuffer(JsonSe
                 return smallBufferCopy;
             }
         }
+
+        private sealed class Utf8BomInsertingStream : Stream
+        {
+            private const int Utf8BomLength = 3;
+            private readonly static byte[] s_utf8Bom = Encoding.UTF8.GetPreamble();
+
+            private readonly Stream _source;
+            private byte[]? _prefixBytes;
+            private int _prefixBytesOffset = 0;
+            private int _prefixBytesCount = 0;
+
+            public Utf8BomInsertingStream(Stream source)
+            {
+                Debug.Assert(source.CanRead);
+                _source = source;
+            }
+
+            public override bool CanRead => _source.CanRead;
+            public override bool CanSeek => false;
+            public override bool CanWrite => false;
+
+            public override int Read(byte[] buffer, int offset, int count)
+            {
+                if (_prefixBytes is null)
+                {
+                    // This is the first read operation; read the first 3 bytes
+                    // from the source to determine if it already includes a BOM.
+                    // Only insert a BOM if it's missing from the source stream.
+
+                    _prefixBytes = new byte[2 * Utf8BomLength];
+                    int bytesRead = ReadExactlyFromSource(_prefixBytes, Utf8BomLength, Utf8BomLength);
+
+                    if (_prefixBytes.AsSpan(Utf8BomLength).SequenceEqual(s_utf8Bom))
+                    {
+                        _prefixBytesOffset = Utf8BomLength;
+                        _prefixBytesCount = Utf8BomLength;
+                    }
+                    else
+                    {
+                        s_utf8Bom.CopyTo(_prefixBytes, 0);
+                        _prefixBytesOffset = 0;
+                        _prefixBytesCount = Utf8BomLength + bytesRead;
+                    }
+                }
+
+                int prefixBytesToWrite = Math.Min(_prefixBytesCount, count);
+                if (prefixBytesToWrite > 0)
+                {
+                    _prefixBytes.AsSpan(_prefixBytesOffset, prefixBytesToWrite).CopyTo(buffer.AsSpan(offset, count));
+                    _prefixBytesOffset += prefixBytesToWrite;
+                    _prefixBytesCount -= prefixBytesToWrite;
+                    offset += prefixBytesToWrite;
+                    count -= prefixBytesToWrite;
+                }
+
+                return prefixBytesToWrite + _source.Read(buffer, offset, count);
+            }
+
+            private int ReadExactlyFromSource(byte[] buffer, int offset, int count)
+            {
+                int totalRead = 0;
+
+                while (totalRead < count)
+                {
+                    int read = _source.Read(buffer, offset + totalRead, count - totalRead);
+                    if (read == 0)
+                    {
+                        break;
+                    }
+
+                    totalRead += read;
+                }
+
+                return totalRead;
+            }
+
+            public override long Length => throw new NotSupportedException();
+            public override long Position { get => throw new NotSupportedException(); set => throw new NotSupportedException(); }
+            public override void Flush() => throw new NotSupportedException();
+            public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();
+            public override void SetLength(long value) => throw new NotSupportedException();
+            public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
+        }
     }
 }