[Json] Avoid ReadOnlySequence with Stream deserializing (#118408)

Author: BrennanConroy

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

@@ -12,7 +12,10 @@ internal interface IReadBufferState<TReadBufferState, TStream> : IDisposable
     {
         public abstract bool IsFinalBlock { get; }
 
+#if DEBUG
+        // Used for Debug.Assert's
         public abstract ReadOnlySequence<byte> Bytes { get; }
+#endif
 
         public abstract ValueTask<TReadBufferState> ReadAsync(
             TStream utf8Json,
@@ -22,5 +25,7 @@ public abstract ValueTask<TReadBufferState> ReadAsync(
         public abstract void Read(TStream utf8Json);
 
         public abstract void Advance(long bytesConsumed);
+
+        public abstract Utf8JsonReader GetReader(JsonReaderState jsonReaderState);
     }
 }

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfoOfT.ReadHelper.cs

@@ -136,29 +136,17 @@ internal bool ContinueDeserialize<TReadBufferState, TStream>(
             out T? value)
             where TReadBufferState : struct, IReadBufferState<TReadBufferState, TStream>
         {
-            Utf8JsonReader reader;
-            if (bufferState.Bytes.IsSingleSegment)
-            {
-                reader = new Utf8JsonReader(
-#if NET
-                    bufferState.Bytes.FirstSpan,
-#else
-                    bufferState.Bytes.First.Span,
-#endif
-                    bufferState.IsFinalBlock, jsonReaderState);
-            }
-            else
-            {
-                reader = new Utf8JsonReader(bufferState.Bytes, bufferState.IsFinalBlock, jsonReaderState);
-            }
+            Utf8JsonReader reader = bufferState.GetReader(jsonReaderState);
 
             try
             {
                 bool success = EffectiveConverter.ReadCore(ref reader, out value, Options, ref readStack);
 
+#if DEBUG
                 Debug.Assert(reader.BytesConsumed <= bufferState.Bytes.Length);
                 Debug.Assert(!bufferState.IsFinalBlock || reader.AllowMultipleValues || reader.BytesConsumed == bufferState.Bytes.Length,
                     "The reader should have thrown if we have remaining bytes.");
+#endif
 
                 jsonReaderState = reader.CurrentState;
                 return success;

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

@@ -27,7 +27,9 @@ public PipeReadBufferState(PipeReader utf8Json)
 
         public readonly bool IsFinalBlock => _isFinalBlock;
 
+#if DEBUG
         public readonly ReadOnlySequence<byte> Bytes => _sequence;
+#endif
 
         public void Advance(long bytesConsumed)
         {
@@ -73,6 +75,24 @@ public async ValueTask<PipeReadBufferState> ReadAsync(PipeReader utf8Json, Cance
 
         public void Read(PipeReader utf8Json) => throw new NotImplementedException();
 
+        public Utf8JsonReader GetReader(JsonReaderState jsonReaderState)
+        {
+            if (_sequence.IsSingleSegment)
+            {
+                return new Utf8JsonReader(
+#if NET
+                    _sequence.FirstSpan,
+#else
+                    _sequence.First.Span,
+#endif
+                    IsFinalBlock, jsonReaderState);
+            }
+            else
+            {
+                return new Utf8JsonReader(_sequence, IsFinalBlock, jsonReaderState);
+            }
+        }
+
         private void ProcessReadBytes()
         {
             if (_isFirstBlock)

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

@@ -51,7 +51,9 @@ public StreamReadBufferState(int initialBufferSize)
 
         public readonly bool IsFinalBlock => _isFinalBlock;
 
+#if DEBUG
         public readonly ReadOnlySequence<byte> Bytes => new(_buffer.AsMemory(_offset, _count));
+#endif
 
         /// <summary>
         /// Read from the stream until either our buffer is filled or we hit EOF.
@@ -155,6 +157,14 @@ public void Advance(long bytesConsumed)
             _offset = 0;
         }
 
+        public Utf8JsonReader GetReader(JsonReaderState jsonReaderState)
+        {
+            return new Utf8JsonReader(
+                _buffer.AsSpan(_offset, _count),
+                IsFinalBlock,
+                jsonReaderState);
+        }
+
         private void ProcessReadBytes()
         {
             if (_count > _maxCount)