tree 60a9044eb021f7958ff667beac8b2a45e16723bd
parent 4e2d1b273d7d1f44960b30094ee568002f301799
author Willy Tarreau <w@1wt.eu> 1722014673 +0200
committer Willy Tarreau <w@1wt.eu> 1722257739 +0200
encoding latin1

MINOR: queue: add a function to check for TOCTOU after queueing

There's a rare TOCTOU case that happens from time to time with maxconn 1
and multiple threads. Between the moment we see the queue full and the
moment we queue a request, it's possible that the last request on the
server or proxy ended and that no other one is left to offer it its place.

Given that all this code path is performance-critical and we cannot afford
to increase the lock duration, better recheck for the condition after
queueing. For this we need to be able to check for the condition and
cleanly dequeue a request. That's what this patch provides via the new
function pendconn_must_try_again(). It will catch more requests than
absolutely needed though it will catch them all. It may find that around
1/1000 of requests are at risk, though testing shows that in practice,
it's around 1 per million that really gets stuck (other ones benefit
from timing and finishing late requests). Maybe in the future some
conditions might be refined but it's harmless.

What happens to such requests is that they're dequeued and their pendconn
freed, so that the caller can decide to try to LB or queue them again. For
now the function is not used, it's just added separately for easier tracking.

(cherry picked from commit 1a8f3a368f1d212f5c2869d400fb07c78b2e7f45)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 94f85bd64609c52f105e95c8fc6500cdc814d642)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 7a30a2d21dfb79eedbbda52754d6486ed4f5a067)
Signed-off-by: Willy Tarreau <w@1wt.eu>