if actual bytes to be written is negative, set to zero

src/Servers/Kestrel/Core/src/Internal/Http2/Http2FrameWriter.cs

@@ -198,6 +198,14 @@ private async Task WriteToOutputPipe()
                     // Now check the connection window
                     actual = CheckConnectionWindow(actual);
 
+                    // actual is negative means window size has become negative
+                    // this can usually happen if the receiver decreases window size before receiving the previous data frame
+                    // in this case, reset to 0 and continue, no data will be sent but will wait for window update
+                    if (actual < 0)
+                    {
+                        actual = 0;
+                    }
+
                     // Write what we can
                     if (actual < buffer.Length)
                     {

src/Servers/Kestrel/test/InMemory.FunctionalTests/Http2/Http2ConnectionTests.cs

@@ -4751,6 +4751,52 @@ await ExpectAsync(Http2FrameType.DATA,
         Assert.True(_helloWorldBytes.AsSpan(9, 3).SequenceEqual(dataFrame3.PayloadSequence.ToArray()));
     }
 
+    [Fact]
+    public async Task WINDOW_UPDATE_Received_OnStream_Resumed_WhenInitialWindowSizeNegativeMidStream()
+    {
+        const int windowSize = 3;
+        _clientSettings.InitialWindowSize = windowSize;
+        var tcs = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        await InitializeConnectionAsync(async context =>
+        {
+            var bodyControlFeature = context.Features.Get<IHttpBodyControlFeature>();
+            bodyControlFeature.AllowSynchronousIO = true;
+            await context.Response.Body.WriteAsync(new byte[windowSize - 1], 0, windowSize - 1);
+            await tcs.Task;
+            await context.Response.Body.WriteAsync(new byte[windowSize], 0, windowSize);
+        });
+        await StartStreamAsync(1, _browserRequestHeaders, endStream: true);
+        await ExpectAsync(Http2FrameType.HEADERS,
+            withLength: 32,
+            withFlags: (byte)Http2HeadersFrameFlags.END_HEADERS,
+            withStreamId: 1);
+
+        // Decrease window size after server has already sent the current window - 1 size of data
+        _clientSettings.InitialWindowSize = windowSize - 2;
+        await SendSettingsAsync();
+        await ExpectAsync(Http2FrameType.DATA,
+            withLength: windowSize - 1,
+            withFlags: (byte)Http2DataFrameFlags.NONE,
+            withStreamId: 1);
+        await ExpectAsync(Http2FrameType.SETTINGS,
+            withLength: 0,
+            withFlags: (byte)Http2DataFrameFlags.END_STREAM,
+            withStreamId: 0);
+        tcs.SetResult();
+
+        // send window update to receive the next frame data
+        await SendWindowUpdateAsync(1, windowSize + 1);
+        await ExpectAsync(Http2FrameType.DATA,
+            withLength: windowSize,
+            withFlags: (byte)Http2DataFrameFlags.NONE,
+            withStreamId: 1);
+        await ExpectAsync(Http2FrameType.DATA,
+            withLength: 0,
+            withFlags: (byte)Http2DataFrameFlags.END_STREAM,
+            withStreamId: 1);
+        await StopConnectionAsync(expectedLastStreamId: 1, ignoreNonGoAwayFrames: false);
+    }
+
     [Fact]
     public async Task CONTINUATION_Received_Decoded()
     {