After Http2StreamSinkChannel.awaitWritable invokes super.awaitWritable():

https://github.com/undertow-io/undertow/blob/2.2.16.Final/core/src/main/java/io/undertow/protocols/http2/Http2StreamSinkChannel.java#L186

It just checks for condition, without verifying if we should wait for the next flow control window update as long as we stay inside the await writable timeout before while waiting:

https://github.com/undertow-io/undertow/blob/2.2.16.Final/core/src/main/java/io/undertow/protocols/http2/Http2StreamSinkChannel.java#L188

As a result, we could have an IOException complaining of a flow control that has not been updated sooner than the client can actually send a flow control update.

Notice that the super class awaitWritable just verifies if the channel is ready for flushed and not closed while waiting:

https://github.com/undertow-io/undertow/blob/2.2.16.Final/core/src/main/java/io/undertow/server/protocol/framed/AbstractFramedStreamSinkChannel.java#L288

Naturally, this method is not aware of the flow control mechanism in HTTP2 and cannot verify for that condition. That's why it is important that the subclass method waits on the remaining time inside the timeout period before throwing an exception.