tree 345667de784ef70da77827c3f36d0c3c801e2d5d
parent e9038438c813a6ff028ded6c4250fd2269c1a215
author Willy Tarreau <w@1wt.eu> 1570796147 +0200
committer Willy Tarreau <w@1wt.eu> 1570804180 +0200
encoding latin1

BUG/MEDIUM: applet: always check a fast running applet's activity before killing

In issue #277 is reported a strange problem related to a fast-spinning
applet which seems to show valid progress being made. It's uncertain how
this can happen, maybe some very specific timing patterns manage to place
just a few bytes in each buffer and result in the peers applet being called
a lot. But it appears possible to artificially cross the spinning threshold
by asking for monster stats page (500 MB) and limiting the send() size to
1 MSS (1460 bytes), causing the stats page to be called for very small
blocks which most often do not leave enough room to place a new chunk.

The idea developed in this patch consists in not crashing for an applet
which reaches a very high call rate if it shows some indication of
progress. Detecting progress on applets is not trivial but in our case
we know that they must at least not claim to wait for a buffer allocation
if this buffer is present, wait for room if the buffer is empty, ask for
more data without polling if such data are still present, nor leave with
an empty input buffer without having written anything nor read anything
from the other side while a shutw is pending.

Doing so doesn't affect normal behaviors nor abuses of our existing
applets and does at least protect against an applet performing an
early return without processing events, or one causing an endless
loop by asking for impossible conditions.

This must be backported to 2.0.

(cherry picked from commit 19920d6fc93c6824923fc4231b8227e0ee9a78ee)
Signed-off-by: Willy Tarreau <w@1wt.eu>