BUG/MAJOR: server: weight calculation fails for map-based algorithms
A crash was reported by Igor at owind when changing a server's weight
on the CLI. Lukas Tribus could reproduce a related bug where setting
a server's weight would result in the new weight being multiplied by
the initial one. The two bugs are the same.
The incorrect weight calculation results in the total farm weight being
larger than what was initially allocated, causing the map index to be out
of bounds on some hashes. It's easy to reproduce using "balance url_param"
with a variable param, or with "balance static-rr".
It appears that the calculation is made at many places and is not always
right and not always wrong the same way. Thus, this patch introduces a
new function "server_recalc_eweight()" which is dedicated to this task
of computing ->eweight from many other elements including uweight and
current time (for slowstart), and all users now switch to use this
function.
The patch is a bit large but the code was not trivially fixable in a way
that could guarantee this situation would not occur anymore. The fix is
much more readable and has been verified to work with all algorithms,
with both consistent and map-based hashes, and even with static-rr.
Slowstart was tested as well, just like enable/disable server.
The same bug is very likely present in 1.4 as well, so the patch will
probably need to be backported eventhough it will not apply as-is.
Thanks to Lukas and Igor for the information they provided to reproduce it.
diff --git a/src/lb_fas.c b/src/lb_fas.c
index daff666..b4cfc79 100644
--- a/src/lb_fas.c
+++ b/src/lb_fas.c
@@ -252,7 +252,8 @@
p->lbprm.wdiv = BE_WEIGHT_SCALE;
for (srv = p->srv; srv; srv = srv->next) {
- srv->prev_eweight = srv->eweight = srv->uweight * BE_WEIGHT_SCALE;
+ srv->eweight = (srv->uweight * p->lbprm.wdiv + p->lbprm.wmult - 1) / p->lbprm.wmult;
+ srv->prev_eweight = srv->eweight;
srv->prev_state = srv->state;
}