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An interesting issue was met when testing the mux-to-mux forwarding code.
In order to preserve fairness, in h2_snd_buf() if other streams are waiting
in send_list or fctl_list, the stream that is attempting to send also goes
to its list, and will be woken up by h2_process_mux() or h2_send() when
some space is released. But on rare occasions, there are only a few (or
even a single) streams waiting in this list, and these streams are just
quickly removed because of a timeout or a quick h2_detach() that calls
h2s_destroy(). In this case there's no even to wake up the other waiting
stream in its list, and this will possibly resume processing after some
client WINDOW_UPDATE frames or even new streams, so usually it doesn't
last too long and it not much noticeable, reason why it was left that
long. In addition, measures have shown that in heavy network-bound
benchmark, this exact situation happens on less than 1% of the streams
(reached 4% with mux-mux).
The fix here consists in replacing these LIST_DEL_INIT() calls on
h2s->list with a function call that checks if other streams were queued
to the send_list recently, and if so, which also tries to resume them
by calling h2_resume_each_sending_h2s(). The detection of late additions
is made via a new flag on the connection, H2_CF_WAIT_INLIST, which is set
when a stream is queued due to other streams being present, and which is
cleared when this is function is called.
It is particularly difficult to reproduce this case which is particularly
timing-dependent, but in a constrained environment, a test involving 32
conns of 20 streams each, all downloading a 10 MB object previously
showed a limitation of 17 Gbps with lots of idle CPU time, and now
filled the cable at 25 Gbps.
This should be backported to all versions where it applies.
68d02e5fa9