Refactor test protocol to use slab-like 256B pipe segments

TestMultiSegmentProtocol now uses a custom MemoryPool to simulate Kestrel's slab allocator with 256-byte segments for both writing and reading. This replaces manual multi-segment sequence creation with a real Pipe backed by SlabSimulatingPool, ensuring more realistic segment boundaries and offsets. Old helpers were removed, and comments updated to clarify the improved simulation of production SignalR/Kestrel pipe behavior.

AyCode.Services.Server.Tests/SignalRs/TestMultiSegmentProtocol.cs

@@ -8,24 +8,30 @@ using Microsoft.AspNetCore.SignalR.Protocol;
 namespace AyCode.Services.Server.Tests.SignalRs;
 
 /// <summary>
-/// Test protocol that simulates production Kestrel pipe behavior.
+/// Test protocol that simulates production Kestrel pipe behavior with 256-byte segments.
 ///
-/// Write side: uses Pipe (not ArrayBufferWriter) so GetSpan/GetMemory return stable slab segments
-/// — matching Kestrel's memory pool behavior. This ensures Span back-patching for length prefixes works.
+/// Production: SignalR → WriteMessage(PipeWriter) → Kestrel slab 4096-byte segments → PipeReader → TryParseMessage
+/// Test:       SignalR → WriteMessage(PipeWriter) → FixedSizePool 256-byte segments → PipeReader → TryParseMessage
 ///
-/// Read side: splits the serialized bytes into 256-byte segments before parsing,
-/// exercising SequenceBinaryInput cross-boundary reads at every boundary.
+/// Both sides go through a real Pipe with fixed-size memory segments,
+/// exercising BWO chunk writes and SequenceBinaryInput cross-boundary reads.
 /// </summary>
 internal class TestMultiSegmentProtocol : AyCodeBinaryHubProtocol
 {
     private const int SegmentSize = 256;
 
+    public TestMultiSegmentProtocol()
+    {
+        Options.BufferWriterChunkSize = SegmentSize;
+    }
+
     /// <summary>
-    /// Serialize via Pipe (production-like stable memory blocks) instead of ArrayBufferWriter.
+    /// Write side: WriteMessage → PipeWriter backed by 256-byte pool segments.
+    /// Same code path as production, just smaller segments.
     /// </summary>
     public new ReadOnlyMemory<byte> GetMessageBytes(HubMessage message)
     {
-        var pipe = new Pipe();
+        var pipe = new Pipe(new PipeOptions(pool: new SlabSimulatingPool(SegmentSize)));
         WriteMessage(message, pipe.Writer);
         pipe.Writer.Complete();
         pipe.Reader.TryRead(out var result);
@@ -35,47 +41,62 @@ internal class TestMultiSegmentProtocol : AyCodeBinaryHubProtocol
     }
 
     /// <summary>
-    /// Split input into 256-byte segments before parsing — forces multi-segment ReadOnlySequence
-    /// through SequenceBinaryInput, exercising cross-boundary reads on every test.
+    /// Read side: fill PipeWriter 256 bytes at a time → PipeReader gives multi-segment sequence.
+    /// Same as production Kestrel PipeReader delivering 4096-byte segments.
     /// </summary>
     public override bool TryParseMessage(ref ReadOnlySequence<byte> input, IInvocationBinder binder,
         [NotNullWhen(true)] out HubMessage? message)
     {
-        var multiSegment = CreateMultiSegmentSequence(input, SegmentSize);
-        return base.TryParseMessage(ref multiSegment, binder, out message);
-    }
+        var bytes = input.ToArray();
+        var pipe = new Pipe(new PipeOptions(pool: new SlabSimulatingPool(SegmentSize)));
+        var writer = pipe.Writer;
 
-    private static ReadOnlySequence<byte> CreateMultiSegmentSequence(ReadOnlySequence<byte> source, int chunkSize)
-    {
-        var bytes = source.ToArray();
-
-        // Each segment gets its own byte[] — matching Kestrel pool slab behavior
-        // where each pipe segment is a separate memory block.
-        var firstChunk = new byte[Math.Min(chunkSize, bytes.Length)];
-        Buffer.BlockCopy(bytes, 0, firstChunk, 0, firstChunk.Length);
-        var first = new MemorySegment(firstChunk);
-        var current = first;
-
-        for (var offset = chunkSize; offset < bytes.Length; offset += chunkSize)
+        // Write in chunks — GetMemory may return less than requested (like Kestrel slab)
+        var remaining = bytes.AsSpan();
+        while (remaining.Length > 0)
         {
-            var length = Math.Min(chunkSize, bytes.Length - offset);
-            var chunk = new byte[length];
-            Buffer.BlockCopy(bytes, offset, chunk, 0, length);
-            current = current.Append(chunk);
+            var mem = writer.GetMemory(Math.Min(SegmentSize, remaining.Length));
+            var chunk = Math.Min(mem.Length, remaining.Length);
+            remaining[..chunk].CopyTo(mem.Span);
+            writer.Advance(chunk);
+            remaining = remaining[chunk..];
         }
 
-        return new ReadOnlySequence<byte>(first, 0, current, current.Memory.Length);
+        writer.Complete();
+        pipe.Reader.TryRead(out var result);
+        var seq = result.Buffer;
+        var success = base.TryParseMessage(ref seq, binder, out message);
+        pipe.Reader.Complete();
+        return success;
     }
 
-    private sealed class MemorySegment : ReadOnlySequenceSegment<byte>
+    /// <summary>
+    /// MemoryPool that returns <paramref name="segmentSize"/>-byte blocks at random offsets
+    /// within a larger backing array — simulating Kestrel's slab allocator where segments
+    /// share a large slab and have non-zero offsets.
+    /// </summary>
+    private sealed class SlabSimulatingPool(int segmentSize) : MemoryPool<byte>
     {
-        public MemorySegment(ReadOnlyMemory<byte> memory) => Memory = memory;
+        private readonly Random _rng = new(42); // deterministic seed for reproducibility
 
-        public MemorySegment Append(ReadOnlyMemory<byte> memory)
+        public override int MaxBufferSize => segmentSize;
+
+        public override IMemoryOwner<byte> Rent(int minBufferSize = -1)
         {
-            var next = new MemorySegment(memory) { RunningIndex = RunningIndex + Memory.Length };
-            Next = next;
-            return next;
+            var size = Math.Max(minBufferSize, segmentSize);
+            var offset = _rng.Next(0, segmentSize);       // random slab offset
+            var jitter = _rng.Next(-1, 2);                 // -1, 0, or +1
+            var actualSize = Math.Max(1, size + jitter);   // random segment size variance
+            var array = new byte[actualSize + offset];
+            return new Owner(array, offset, actualSize);
+        }
+
+        protected override void Dispose(bool disposing) { }
+
+        private sealed class Owner(byte[] array, int offset, int length) : IMemoryOwner<byte>
+        {
+            public Memory<byte> Memory { get; } = array.AsMemory(offset, length);
+            public void Dispose() { }
         }
     }
 }