Handle zero size hints in Http1OutputProducer

src/Servers/Kestrel/Core/src/Internal/Http/Http1OutputProducer.cs

@@ -668,6 +668,13 @@ private void WriteCurrentChunkMemoryToPipeWriter(ref BufferWriter<PipeWriter> wr
 
     internal Memory<byte> GetFakeMemory(int minSize)
     {
+        if (minSize == 0)
+        {
+            // MemoryPool.Rent(0) may return empty memory;
+            // use Kestrel's minimum segment size to satisfy the IBufferWriter contract.
+            minSize = _memoryPool.GetMinimumSegmentSize();
+        }
+
         // Try to reuse _fakeMemoryOwner
         if (_fakeMemoryOwner != null)
         {
@@ -735,6 +742,13 @@ private void EnsureCapacity(int sizeHint)
 
     private void AddSegment(int sizeHint = 0)
     {
+        if (sizeHint == 0)
+        {
+            // MemoryPool.Rent(0) may return empty memory;
+            // use Kestrel's minimum segment size to satisfy the IBufferWriter contract.
+            sizeHint = _memoryPool.GetMinimumSegmentSize();
+        }
+
         if (_currentSegment.Length != 0)
         {
             // We're adding a segment to the list

src/Servers/Kestrel/Core/test/Http1/Http1OutputProducerTests.cs

@@ -137,6 +137,35 @@ public void AbortsTransportEvenAfterDispose()
         Assert.Equal(ConnectionEndReason.AbortedByApp, metricsContext.ConnectionEndReason);
     }
 
+    [Fact]
+    public void GetMemoryAndGetSpanWithZeroSizeHintReturnNonEmptyBuffers()
+    {
+        var memoryPool = new Mock<MemoryPool<byte>>();
+        memoryPool.SetupGet(pool => pool.MaxBufferSize).Returns(MemoryPool<byte>.Shared.MaxBufferSize);
+        memoryPool.Setup(pool => pool.Rent(0)).Returns(Mock.Of<IMemoryOwner<byte>>());
+        memoryPool.Setup(pool => pool.Rent(It.Is<int>(size => size > 0)))
+            .Returns((int size) => MemoryPool<byte>.Shared.Rent(size));
+
+        using var output = CreateOutputProducer(memoryPool: memoryPool.Object);
+
+        var beforeStartMemoryLength = output.GetMemory(0).Length;
+        var beforeStartSpanLength = output.GetSpan(0).Length;
+
+        output.Stop();
+
+        var completedMemoryLength = output.GetMemory(0).Length;
+        var completedSpanLength = output.GetSpan(0).Length;
+
+        Assert.All(
+            [
+                (Operation: "GetMemory before response start", Length: beforeStartMemoryLength),
+                (Operation: "GetSpan before response start", Length: beforeStartSpanLength),
+                (Operation: "GetMemory after completion", Length: completedMemoryLength),
+                (Operation: "GetSpan after completion", Length: completedSpanLength),
+            ],
+            result => Assert.True(result.Length > 0, $"{result.Operation} returned an empty buffer."));
+    }
+
     [Fact]
     public void AllocatesFakeMemorySmallerThanMaxBufferSize()
     {
@@ -223,7 +252,8 @@ public void ReusesFakeMemory()
     private TestHttpOutputProducer CreateOutputProducer(
         PipeOptions pipeOptions = null,
         ConnectionContext connectionContext = null,
-        ConnectionMetricsContext metricsContext = null)
+        ConnectionMetricsContext metricsContext = null,
+        MemoryPool<byte> memoryPool = null)
     {
         pipeOptions = pipeOptions ?? new PipeOptions();
         connectionContext = connectionContext ?? Mock.Of<ConnectionContext>();
@@ -234,7 +264,7 @@ private TestHttpOutputProducer CreateOutputProducer(
             pipe,
             "0",
             connectionContext,
-            _memoryPool,
+            memoryPool ?? _memoryPool,
             serviceContext.Log,
             Mock.Of<ITimeoutControl>(),
             Mock.Of<IHttpMinResponseDataRateFeature>(),