Change evutil_weakrand_() to avoid platform random()

This change allows us to avoid perturbing the platform's random(), and
to avoid hitting locks on random() in the platform's libc.

evutil_weakrand_() is, well, weak, so we choose here an algorithm that
favors speed over a number of other possibly desirable properties.
We're using a linear congruential generator, and taking our parameters
from those shared by the OpenBSD random() implementation, and
Glibc's fastest random() implementation.

The low bits of a LCG of modulus 2^32 are (notoriously) less random
than the higher bits.  So to generate a random value in a range, using
the % operator is no good; we ought to divide.  We add an
evutil_weakrand_range_() function to do that.

This code also changes the interface of evutil_weakrand_() so that it
now manipulates an explicit seed, rather than having the seed in a
static variable.  This change enables us to use existing locks to
achieve thread-safety, rather than having to rely on an additional lock.

(Patch by Nicholas Marriott; commit message by Nick Mathewson.)

1 files changed, 1 insertions, 1 deletions

diff --git a/bufferevent_ratelim.c b/bufferevent_ratelim.c
index 02c50228..4272c0da 100644
--- a/bufferevent_ratelim.c
+++ b/bufferevent_ratelim.c
@@ -452,7 +452,7 @@ bev_group_random_element_(struct bufferevent_rate_limit_group *group)
 
 	EVUTIL_ASSERT(! LIST_EMPTY(&group->members));
 
-	which = evutil_weakrand_() % group->n_members;
+	which = evutil_weakrand_range_(&group->weakrand_seed, group->n_members);
 
 	bev = LIST_FIRST(&group->members);
 	while (which--)