HTTP2 server occasionally sends SETTINGS ack as first frame
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@normanmaurer is already working on this.
Since Jan 20, 2026.
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Description
Aleph's test suite occasionally flakes with the following error:
io.netty.handler.codec.http2.Http2Exception: First received frame was not SETTINGS. Hex dump for first 5 bytes: 0000000401
The error message is a bit misleading: It is in fact a SETTINGS frame but with the ACK bit set (see 8f334885ef58aa0dda4107391200c29527106df7 - maybe the error message could be improved here). I suspect that it's a race condition in which the server receives the client's initial SETTINGS frame before having sent its own and then immediately sends the ack frame. I've tried to construct a reproducer for this but was unsuccessful so far but wanted to share it already - maybe you can figure this out more quickly 🙏
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