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/select.c b/select.c
index e1d4987c..f9a0c206 100644
--- a/select.c
+++ b/select.c
@@ -186,7 +186,7 @@ select_dispatch(struct event_base *base, struct timeval *tv)
 	event_debug(("%s: select reports %d", __func__, res));
 
 	check_selectop(sop);
-	i = random() % nfds;
+	i = evutil_weakrand_range_(&base->weakrand_seed, nfds);
 	for (j = 0; j < nfds; ++j) {
 		if (++i >= nfds)
 			i = 0;