tree 81d4799e410385a08f022cad7bf5df397bae562b
parent c1b0645dac158d58451226ac0b4de105336dc4cc
author Willy Tarreau <w@1wt.eu> 1542035468 +0100
committer Willy Tarreau <w@1wt.eu> 1542045525 +0100
encoding latin1

BUG/MEDIUM: stream-int: change the way buffer room is requested by a stream-int

Subsequent to the recent stream-int updates, we started to consider that
SI_FL_WANT_PUT needs to be set when receipt is enabled, but this is wrong
and results in 100% CPU when an HTTP client stays idle after a keep-alive
request because the stream-int has nothing to provide and nothing to send.

In fact just like for applets this flag should reflect the continuation
of an attempt. So it's si_cs_recv() which should set the flag, and clear
it if it has nothing more to provide. This function is called the first
time in process_stream()), and called again during transfers, so it will
always be up to date during stream_int_update() and stream_int_notify().

As a special case, it should also be set when a connection switches to
the established state. And we should absolutely refrain from calling
si_cs_recv() to re-enable reading, normally just setting this flag
(from within the stream-int's handler or prior to calling si_chk_rcv())
is expected to be OK.

A corner case remains where it was observed that in stream_int_notify() we
can sometimes be called with an empty output channel with SI_FL_WAIT_ROOM
and no CF_WRITE_PARTIAL, so there's no way to detect that we should
re-enable receiving. It's easy to also take care of this condition
there for the time it takes to figure if this situation is expected
or not.

Now it becomes more obvious that relying on a single flag to request
room (or on two flags to arbiter activity) is not workable given the
autonomy of both sides. The mux_h2 has taught us that blocking flags
are much more reliable, require much less condition and are much easier
to deal with. That's probably something to consider quickly in this
area.

No backport is needed.