From c1e8e581e53e35fe64295cbd743f00341918995d Mon Sep 17 00:00:00 2001
From: Simon Cropp <simon.cropp@gmail.com>
Date: Sat, 5 Sep 2026 09:36:47 +1000
Subject: [PATCH] Add Write(ReadOnlySpan<byte>) override to
 FileBufferingWriteStream

Span-based writers such as System.Text.Json previously went through the
base Stream fallback, which rents a pooled array and copies before the
array overload copies again into the paged buffer. Move the write
implementation onto the span overload, have the array overload delegate
to it, and give PagedByteBuffer a ReadOnlySpan<byte> Add that the
ReadOnlyMemory<byte> overload forwards to.
---
 .../WebUtilities/src/FileBufferingWriteStream.cs  | 15 +++++++++++----
 src/Http/WebUtilities/src/PagedByteBuffer.cs      | 11 +++++++----
 src/Http/WebUtilities/src/PublicAPI.Unshipped.txt |  1 +
 3 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/Http/WebUtilities/src/FileBufferingWriteStream.cs b/src/Http/WebUtilities/src/FileBufferingWriteStream.cs
index 280f583183ff..cf74b402660f 100644
--- a/src/Http/WebUtilities/src/FileBufferingWriteStream.cs
+++ b/src/Http/WebUtilities/src/FileBufferingWriteStream.cs
@@ -107,20 +107,27 @@ public override ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken
     public override void Write(byte[] buffer, int offset, int count)
     {
         ValidateBufferArguments(buffer, offset, count);
+
+        Write(buffer.AsSpan(offset, count));
+    }
+
+    /// <inheritdoc />
+    public override void Write(ReadOnlySpan<byte> buffer)
+    {
         ThrowIfDisposed();
 
-        if (_bufferLimit.HasValue && _bufferLimit - Length < count)
+        if (_bufferLimit.HasValue && _bufferLimit - Length < buffer.Length)
         {
             Dispose();
             throw new IOException("Buffer limit exceeded.");
         }
 
         // Allow buffering in memory if we're below the memory threshold once the current buffer is written.
-        var allowMemoryBuffer = (_memoryThreshold - count) >= PagedByteBuffer.Length;
+        var allowMemoryBuffer = (_memoryThreshold - buffer.Length) >= PagedByteBuffer.Length;
         if (allowMemoryBuffer)
         {
             // Buffer content in the MemoryStream if it has capacity.
-            PagedByteBuffer.Add(buffer, offset, count);
+            PagedByteBuffer.Add(buffer);
             Debug.Assert(PagedByteBuffer.Length <= _memoryThreshold);
         }
         else
@@ -132,7 +139,7 @@ public override void Write(byte[] buffer, int offset, int count)
             // Spool memory content to disk.
             PagedByteBuffer.MoveTo(FileStream);
 
-            FileStream.Write(buffer, offset, count);
+            FileStream.Write(buffer);
         }
     }
 
diff --git a/src/Http/WebUtilities/src/PagedByteBuffer.cs b/src/Http/WebUtilities/src/PagedByteBuffer.cs
index a148275a3a4c..b57a75208696 100644
--- a/src/Http/WebUtilities/src/PagedByteBuffer.cs
+++ b/src/Http/WebUtilities/src/PagedByteBuffer.cs
@@ -44,20 +44,23 @@ public void Add(byte[] buffer, int offset, int count)
         => Add(buffer.AsMemory(offset, count));
 
     public void Add(ReadOnlyMemory<byte> memory)
+        => Add(memory.Span);
+
+    public void Add(ReadOnlySpan<byte> span)
     {
         ThrowIfDisposed();
 
-        while (!memory.IsEmpty)
+        while (!span.IsEmpty)
         {
             var currentPage = CurrentPage;
-            var copyLength = Math.Min(memory.Length, currentPage.Length - _currentPageIndex);
+            var copyLength = Math.Min(span.Length, currentPage.Length - _currentPageIndex);
 
-            memory.Slice(0, copyLength).CopyTo(currentPage.AsMemory(_currentPageIndex, copyLength));
+            span.Slice(0, copyLength).CopyTo(currentPage.AsSpan(_currentPageIndex, copyLength));
 
             Length += copyLength;
             _currentPageIndex += copyLength;
 
-            memory = memory.Slice(copyLength);
+            span = span.Slice(copyLength);
         }
     }
 
diff --git a/src/Http/WebUtilities/src/PublicAPI.Unshipped.txt b/src/Http/WebUtilities/src/PublicAPI.Unshipped.txt
index 7dc5c58110bf..a5e29bcfc4ab 100644
--- a/src/Http/WebUtilities/src/PublicAPI.Unshipped.txt
+++ b/src/Http/WebUtilities/src/PublicAPI.Unshipped.txt
@@ -1 +1,2 @@
 #nullable enable
+override Microsoft.AspNetCore.WebUtilities.FileBufferingWriteStream.Write(System.ReadOnlySpan<byte> buffer) -> void