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Allow HTTP2 encoder to split headers across frames #55322

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Allow HTTP2 encoder to split headers across frames #55322

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a10d9ee
Initial implementation
Apr 20, 2024
5cad996
Review feedback
Apr 25, 2024
f53dd7c
Updating a comment
Apr 25, 2024
231c716
Minor perf opt when all headers fit in a single header frame.
Apr 27, 2024
887f505
Adding MaxResponseHeadersLimit to H2 framewriter
May 1, 2024
b91a34c
Protect against overflow
May 2, 2024
a655a38
Validating the header length in HPackHeaderWriter
May 3, 2024
f31c8c8
Removing a test file commented out and adding more tests.
May 4, 2024
6ab7271
Review feedback
May 13, 2024
e21620a
Removing header limits related code because it is up to the service o…
May 15, 2024
9720c3a
A test for 1MB header value
May 16, 2024
f8f8ff9
Review comments - removing an unneeded if condition
May 18, 2024
994b27d
Sketch
May 21, 2024
a77a930
using arraypool instead of arraybufferwriter
May 22, 2024
4f14b0c
Re-using the rented buffers
May 24, 2024
46be03f
Preserving the size of the temporary larger buffer within the framewr…
May 30, 2024
9857dcb
Adjusting the logic of UpdateMaxFrameSize for setting a new _headersE…
May 31, 2024
95c9553
Adjusting the logic of _headersEncodingLargeBufferSize to avoid 0 val…
May 31, 2024
c4b93d6
Extenting Benachmark with headers sized 0, 10KB and 20KB
May 31, 2024
3fe0a73
Remove int.Max on header size increase and using checked arithmetic i…
Jun 1, 2024
0dfc97b
Tests
JamesNK Jun 3, 2024
79be13b
Update src/Servers/Kestrel/Core/src/Internal/Http2/Http2FrameWriter.cs
ladeak Jun 3, 2024
8773062
Review feedback: HeaderWriteResult not an inner type, adding comments…
Jun 3, 2024
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6 changes: 4 additions & 2 deletions src/Servers/Kestrel/Core/src/Internal/Http2/Http2Connection.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -938,7 +938,9 @@ private Task ProcessSettingsFrameAsync(in ReadOnlySequence<byte> payload)
if (_clientSettings.MaxFrameSize != previousMaxFrameSize)
{
// Don't let the client choose an arbitrarily large size, this will be used for response buffers.
_frameWriter.UpdateMaxFrameSize(Math.Min(_clientSettings.MaxFrameSize, _serverSettings.MaxFrameSize));
// Safe cast, MaxFrameSize is limited to 2^24-1 bytes by the protocol and by Http2PeerSettings.
// Ref: https://datatracker.ietf.org/doc/html/rfc7540#section-4.2
_frameWriter.UpdateMaxFrameSize((int)Math.Min(_clientSettings.MaxFrameSize, _serverSettings.MaxFrameSize));
}

// This difference can be negative.
Expand Down Expand Up @@ -1829,4 +1831,4 @@ private static class GracefulCloseInitiator
public const int Server = 1;
public const int Client = 2;
}
}
}
137 changes: 109 additions & 28 deletions src/Servers/Kestrel/Core/src/Internal/Http2/Http2FrameWriter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,8 @@ internal sealed class Http2FrameWriter
/// TODO (https://github.com/dotnet/aspnetcore/issues/51309): eliminate this limit.
private const string MaximumFlowControlQueueSizeProperty = "Microsoft.AspNetCore.Server.Kestrel.Http2.MaxConnectionFlowControlQueueSize";

private const int HeaderBufferSizeMultiplier = 2;

private static readonly int? AppContextMaximumFlowControlQueueSize = GetAppContextMaximumFlowControlQueueSize();

private static int? GetAppContextMaximumFlowControlQueueSize()
Expand Down Expand Up @@ -71,8 +73,9 @@ internal sealed class Http2FrameWriter
// This is only set to true by tests.
private readonly bool _scheduleInline;

private uint _maxFrameSize = Http2PeerSettings.MinAllowedMaxFrameSize;
private int _maxFrameSize = Http2PeerSettings.MinAllowedMaxFrameSize;
private byte[] _headerEncodingBuffer;
private int _headersEncodingLargeBufferSize = Http2PeerSettings.MinAllowedMaxFrameSize * HeaderBufferSizeMultiplier;
private long _unflushedBytes;

private bool _completed;
Expand Down Expand Up @@ -110,7 +113,6 @@ internal sealed class Http2FrameWriter
_headerEncodingBuffer = new byte[_maxFrameSize];

_scheduleInline = serviceContext.Scheduler == PipeScheduler.Inline;

_hpackEncoder = new DynamicHPackEncoder(serviceContext.ServerOptions.AllowResponseHeaderCompression);

_maximumFlowControlQueueSize = AppContextMaximumFlowControlQueueSize is null
Expand Down Expand Up @@ -367,12 +369,15 @@ public void UpdateMaxHeaderTableSize(uint maxHeaderTableSize)
}
}

public void UpdateMaxFrameSize(uint maxFrameSize)
public void UpdateMaxFrameSize(int maxFrameSize)
{
lock (_writeLock)
{
if (_maxFrameSize != maxFrameSize)
{
// Safe multiply, MaxFrameSize is limited to 2^24-1 bytes by the protocol and by Http2PeerSettings.
// Ref: https://datatracker.ietf.org/doc/html/rfc7540#section-4.2
_headersEncodingLargeBufferSize = int.Max(_headersEncodingLargeBufferSize, maxFrameSize * HeaderBufferSizeMultiplier);
_maxFrameSize = maxFrameSize;
_headerEncodingBuffer = new byte[_maxFrameSize];
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}
Expand Down Expand Up @@ -507,11 +512,12 @@ private void WriteResponseHeadersUnsynchronized(int streamId, int statusCode, Ht
{
try
{
// In the case of the headers, there is always a status header to be returned, so BeginEncodeHeaders will not return BufferTooSmall.
_headersEnumerator.Initialize(headers);
_outgoingFrame.PrepareHeaders(headerFrameFlags, streamId);
var buffer = _headerEncodingBuffer.AsSpan();
var done = HPackHeaderWriter.BeginEncodeHeaders(statusCode, _hpackEncoder, _headersEnumerator, buffer, out var payloadLength);
FinishWritingHeadersUnsynchronized(streamId, payloadLength, done);
var writeResult = HPackHeaderWriter.BeginEncodeHeaders(statusCode, _hpackEncoder, _headersEnumerator, _headerEncodingBuffer, out var payloadLength);
Debug.Assert(writeResult != HeaderWriteResult.BufferTooSmall, "This always writes the status as the first header, and it should never be an over the buffer size.");
FinishWritingHeadersUnsynchronized(streamId, payloadLength, writeResult);
}
// Any exception from the HPack encoder can leave the dynamic table in a corrupt state.
// Since we allow custom header encoders we don't know what type of exceptions to expect.
Expand Down Expand Up @@ -548,11 +554,11 @@ private ValueTask<FlushResult> WriteDataAndTrailersAsync(Http2Stream stream, in

try
{
_headersEnumerator.Initialize(headers);
// In the case of the trailers, there is no status header to be written, so even the first call to BeginEncodeHeaders can return BufferTooSmall.
_outgoingFrame.PrepareHeaders(Http2HeadersFrameFlags.END_STREAM, streamId);
var buffer = _headerEncodingBuffer.AsSpan();
var done = HPackHeaderWriter.BeginEncodeHeaders(_hpackEncoder, _headersEnumerator, buffer, out var payloadLength);
FinishWritingHeadersUnsynchronized(streamId, payloadLength, done);
_headersEnumerator.Initialize(headers);
var writeResult = HPackHeaderWriter.BeginEncodeHeaders(_hpackEncoder, _headersEnumerator, _headerEncodingBuffer, out var payloadLength);
FinishWritingHeadersUnsynchronized(streamId, payloadLength, writeResult);
}
// Any exception from the HPack encoder can leave the dynamic table in a corrupt state.
// Since we allow custom header encoders we don't know what type of exceptions to expect.
Expand All @@ -566,32 +572,107 @@ private ValueTask<FlushResult> WriteDataAndTrailersAsync(Http2Stream stream, in
}
}

private void FinishWritingHeadersUnsynchronized(int streamId, int payloadLength, bool done)
private void SplitHeaderFramesToOutput(int streamId, ReadOnlySpan<byte> dataToFrame, bool endOfHeaders, bool isFramePrepared)
{
var buffer = _headerEncodingBuffer.AsSpan();
_outgoingFrame.PayloadLength = payloadLength;
if (done)
var shouldPrepareFrame = !isFramePrepared;
while (dataToFrame.Length > 0)
{
_outgoingFrame.HeadersFlags |= Http2HeadersFrameFlags.END_HEADERS;
}
if (shouldPrepareFrame)
{
_outgoingFrame.PrepareContinuation(Http2ContinuationFrameFlags.NONE, streamId);
}

WriteHeaderUnsynchronized();
_outputWriter.Write(buffer.Slice(0, payloadLength));
// Should prepare continuation frames.
shouldPrepareFrame = true;
var currentSize = Math.Min(dataToFrame.Length, _maxFrameSize);
_outgoingFrame.PayloadLength = currentSize;
if (endOfHeaders && dataToFrame.Length == currentSize)
{
_outgoingFrame.HeadersFlags |= Http2HeadersFrameFlags.END_HEADERS;
}

while (!done)
{
_outgoingFrame.PrepareContinuation(Http2ContinuationFrameFlags.NONE, streamId);
WriteHeaderUnsynchronized();
_outputWriter.Write(dataToFrame[..currentSize]);
dataToFrame = dataToFrame.Slice(currentSize);
}
}

done = HPackHeaderWriter.ContinueEncodeHeaders(_hpackEncoder, _headersEnumerator, buffer, out payloadLength);
private void FinishWritingHeadersUnsynchronized(int streamId, int payloadLength, HeaderWriteResult writeResult)
{
Debug.Assert(payloadLength <= _maxFrameSize, "The initial payload lengths is written to _headerEncodingBuffer with size of _maxFrameSize");
byte[]? largeHeaderBuffer = null;
Span<byte> buffer;
if (writeResult == HeaderWriteResult.Done)
{
// Fast path, only a single HEADER frame.
_outgoingFrame.PayloadLength = payloadLength;

if (done)
_outgoingFrame.HeadersFlags |= Http2HeadersFrameFlags.END_HEADERS;
WriteHeaderUnsynchronized();
_outputWriter.Write(_headerEncodingBuffer.AsSpan(0, payloadLength));
return;
}
else if (writeResult == HeaderWriteResult.MoreHeaders)
{
_outgoingFrame.PayloadLength = payloadLength;
WriteHeaderUnsynchronized();
_outputWriter.Write(_headerEncodingBuffer.AsSpan(0, payloadLength));
}
else
{
// This may happen in case of the TRAILERS after the initial encode operation.
// The _maxFrameSize sized _headerEncodingBuffer was too small.
while (writeResult == HeaderWriteResult.BufferTooSmall)
{
Debug.Assert(payloadLength == 0, "Payload written even though buffer is too small");
largeHeaderBuffer = ArrayPool<byte>.Shared.Rent(_headersEncodingLargeBufferSize);
buffer = largeHeaderBuffer.AsSpan(0, _headersEncodingLargeBufferSize);
writeResult = HPackHeaderWriter.RetryBeginEncodeHeaders(_hpackEncoder, _headersEnumerator, buffer, out payloadLength);
if (writeResult != HeaderWriteResult.BufferTooSmall)
{
SplitHeaderFramesToOutput(streamId, buffer[..payloadLength], endOfHeaders: writeResult == HeaderWriteResult.Done, isFramePrepared: true);
}
else
{
// The size is backed in a local field and not reset for further streams on the connections.
// When a large header is repeatedly returned on a connection, the buffer size won't need to be repeatedly increased.
_headersEncodingLargeBufferSize = checked(_headersEncodingLargeBufferSize * HeaderBufferSizeMultiplier);
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@JamesNK JamesNK Jun 3, 2024 •
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_headersEncodingLargeBufferSize is updated to be bigger when required, but I don't see anywhere that resets it back it a smaller size.

For example, request 1 has a response header that is 10megs in size, so this field and the buffer eventually grows to that size. Then request 2 has a response header that is 100kb in size (larger than max frame size) but the writer rents a 10meg buffer because that is _headersEncodingLargeBufferSize size from last time.

Is this behavior intentional? Is the size preserved to avoid retrying because a connection keeps sending large response headers? Should add a comment if that is the case.

If the behavior isn't intentional, then I believe the field is only used inside FinishWritingHeadersUnsynchronized. It could be changed to a local variable.

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This behavior was intentional, exactly as you said: it might be ie. a cookie that is sent on all streams and that is always large. If you think that is not a desired behavior, I am happy to make it local. In the meantime, I will leave a comment explaining this.

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@JamesNK There's some discussion here: #55322 (comment)

}
ArrayPool<byte>.Shared.Return(largeHeaderBuffer);
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Should returning the buffer be in a finally to ensure its returned when an error occurs? There are definitely ways to throw when encoding headers.

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@ladeak ladeak Jun 3, 2024 •
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My understanding was that in these cases the buffer would be garbage collected.

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That's my default inclination as well, but I understood we (dotnet? aspnetcore?) had guidelines saying rented objects should not be returned in the event of an exception.

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@ladeak ladeak Jun 3, 2024 •
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Let me know, I am happy to change already prepared on my machine.
I used to see try-finally all over for ArrayPool, but lately I see less and less.

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Yes, there's definitely strong precedent for only recycling on success, especially when async operations are possible; as a side-effect this also avoids the need for some try. There might be a caveat here if we expect failure in a significant % (exceptions as flow control), but I don't think that applies here

largeHeaderBuffer = null;
}
if (writeResult == HeaderWriteResult.Done)
{
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_outgoingFrame.ContinuationFlags = Http2ContinuationFrameFlags.END_HEADERS;
return;
}
}

WriteHeaderUnsynchronized();
_outputWriter.Write(buffer.Slice(0, payloadLength));
// HEADERS and zero or more CONTINUATIONS sent - all subsequent frames are (unprepared) CONTINUATIONs
buffer = _headerEncodingBuffer;
while (writeResult != HeaderWriteResult.Done)
{
writeResult = HPackHeaderWriter.ContinueEncodeHeaders(_hpackEncoder, _headersEnumerator, buffer, out payloadLength);
if (writeResult == HeaderWriteResult.BufferTooSmall)
{
if (largeHeaderBuffer != null)
{
ArrayPool<byte>.Shared.Return(largeHeaderBuffer);

// The size is backed in a local field and not reset for further streams on the connections.
// When a large header is repeatedly returned on a connection, the buffer size won't need to be repeatedly increased.
_headersEncodingLargeBufferSize = checked(_headersEncodingLargeBufferSize * HeaderBufferSizeMultiplier);
}
largeHeaderBuffer = ArrayPool<byte>.Shared.Rent(_headersEncodingLargeBufferSize);
buffer = largeHeaderBuffer.AsSpan(0, _headersEncodingLargeBufferSize);
}
else
{
// In case of Done or MoreHeaders: write to output.
SplitHeaderFramesToOutput(streamId, buffer[..payloadLength], endOfHeaders: writeResult == HeaderWriteResult.Done, isFramePrepared: false);
}
}
if (largeHeaderBuffer != null)
{
ArrayPool<byte>.Shared.Return(largeHeaderBuffer);
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Should this be in a finally?

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@ladeak ladeak Jun 3, 2024 •
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My understanding was that in these cases the buffer would be garbage collected.

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Not returning doesn't create a memory leak, but it could cause future rent calls to allocate because buffers haven't been returned.

}
}

Expand Down Expand Up @@ -1023,4 +1104,4 @@ private void EnqueueWaitingForMoreConnectionWindow(Http2OutputProducer producer)
_http2Connection.Abort(new ConnectionAbortedException("HTTP/2 connection exceeded the outgoing flow control maximum queue size."));
}
}
}
}