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, 2 insertions, 1 deletions

diff --git a/test/regress_buffer.c b/test/regress_buffer.c
index dfb680b5..2e4ef176 100644
--- a/test/regress_buffer.c
+++ b/test/regress_buffer.c
@@ -699,6 +699,7 @@ test_evbuffer_add_file(void *ptr)
 	struct event *rev=NULL, *wev=NULL;
 	struct event_base *base = testdata->base;
 	evutil_socket_t pair[2] = {-1, -1};
+	static ev_uint32_t seed = 123456789U;
 
 	/* This test is highly parameterized based on substrings of its
 	 * argument.  The strings are: */
@@ -757,7 +758,7 @@ test_evbuffer_add_file(void *ptr)
 		data = malloc(1024*512);
 		tt_assert(data);
 		for (i = 0; i < datalen; ++i)
-			data[i] = (char)evutil_weakrand_();
+			data[i] = (char)evutil_weakrand_(&seed);
 	} else {
 		data = strdup("here is a relatively small string.");
 		tt_assert(data);