request buffer doesn't work with the new HTTP2 WINDOW_UPDATE sending logic

[zizhong]

With 16e0937, the client can only send up to SETTINGS_INITIAL_WINDOW_SIZE bytes.
@masaori335 can share details of his test that reproduced this issue.

[zizhong]

From @masaori335 ,

CONFIG proxy.config.http.request_buffer_enabled INT 1
CONFIG proxy.config.http.post_copy_size INT 1073741824
- with HTTP/1.1, ATS buffers whole request body
- with HTTP/2, ATS stop accepting 73KB ( this is looks like 64KB + 8 KB ) 

[masaori335]

Summary

After 50441b39e6631389ef95c4133f06bbf94544879c (#6643) change, if the request buffer feature[*1] is enabled, POST requests (over 65535) on HTTP/2 connection are stalled.

*1 : proxy.config.http.request_buffer_enabled

Root cause

ATS sends WINDOW_UPDATE frames only if the buffer is consumed & buffering data is small enough. However, when the request buffer feature is enabled, the buffer is not consumed intentionally.

trafficserver/proxy/http2/Http2ConnectionState.cc

Lines 1303 to 1307 in 50441b3

	// If read_vio is buffering data, do not fully update window
	int64_t data_size = stream->read_vio_read_avail();
	if (data_size >= initial_rwnd) {
	return;
	}

Workaround Fix

A workaround fix is adjust proxy.config.http2.initial_window_size_in to the same value to the proxy.config.http.post_copy_size. For example in above, it will be below

CONFIG proxy.config.http.request_buffer_enabled INT 1
CONFIG proxy.config.http.post_copy_size INT 1073741824
CONFIG proxy.config.http2.initial_window_size_in INT 1073741824

We can use this workaround fix up to buffering 2^31 - 1 (2,147,483,647) bytes, which is the maximum value of HTTP/2 flow-control window size. ( rfc7540#section-6.5.2 )
However, we can't apply this on a specific remap, because proxy.config.http2.initial_window_size_in is a global config.

Another approach to fix the root cause

It's another approach that checking the feature is enabled on the logic that stops updating the window. Probably, we can refer the config through Http2Stream and HttpSM.

TODOs

• Add test cases to cover this (Request buffer AuTest suite lacks of a case with HTTP2 client #6901)