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.)

bufferevent-internal.h

@@ -104,6 +104,10 @@ struct bufferevent_rate_limit_group {
 	/** Timeout event that goes off once a tick, when the bucket is ready
 	 * to refill. */
 	struct event master_refill_event;
+
+	/** Seed for weak random number generator. */
+	ev_uint32_t weakrand_seed;
+
 	/** Lock to protect the members of this group.  This lock should nest
 	 * within every bufferevent lock: if you are holding this lock, do
 	 * not assume you can lock another bufferevent. */

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--)

event-internal.h

@@ -290,6 +290,9 @@ struct event_base {
 	struct event th_notify;
 	/** A function used to wake up the main thread from another thread. */
 	int (*th_notify_fn)(struct event_base *base);
+
+	/* Saved seed for weak random number generator. */
+	ev_uint32_t weakrand_seed;
 };
 
 struct event_config_entry {

evutil.c

@@ -2273,14 +2273,23 @@ evutil_getenv_(const char *varname)
 	return getenv(varname);
 }
 
-long
-evutil_weakrand_(void)
+ev_uint32_t
+evutil_weakrand_(ev_uint32_t* seed)
 {
-#ifdef _WIN32
-	return rand();
-#else
-	return random();
-#endif
+	*seed = ((*seed) * 1103515245 + 12345) & 0x7fffffff;
+	return (*seed);
+}
+
+ev_uint32_t
+evutil_weakrand_range_(ev_uint32_t* seed, ev_uint32_t top)
+{
+	ev_uint32_t divisor, result;
+
+	divisor = EV_INT32_MAX / top;
+	do
+		result = evutil_weakrand_(seed) / divisor;
+	while (result > top);
+	return result;
 }
 
 int

poll.c

@@ -183,7 +183,7 @@ poll_dispatch(struct event_base *base, struct timeval *tv)
 	if (res == 0 || nfds == 0)
 		return (0);
 
-	i = random() % nfds;
+	i = evutil_weakrand_range_(&base->weakrand_seed, nfds);
 	for (j = 0; j < nfds; j++) {
 		int what;
 		if (++i == nfds)

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;

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);

test/regress_util.c

@@ -829,6 +829,7 @@ test_evutil_rand(void *arg)
 	char buf2[32];
 	int counts[256];
 	int i, j, k, n=0;
+	static ev_uint32_t seed = 12346789U;
 
 	memset(buf2, 0, sizeof(buf2));
 	memset(counts, 0, sizeof(counts));
@@ -836,8 +837,8 @@ test_evutil_rand(void *arg)
 	for (k=0;k<32;++k) {
 		/* Try a few different start and end points; try to catch
 		 * the various misaligned cases of arc4random_buf */
-		int startpoint = evutil_weakrand_() % 4;
-		int endpoint = 32 - (evutil_weakrand_() % 4);
+		int startpoint = evutil_weakrand_(&seed) % 4;
+		int endpoint = 32 - (evutil_weakrand_(&seed) % 4);
 
 		memset(buf2, 0, sizeof(buf2));
 

util-internal.h

@@ -254,7 +254,8 @@ int evutil_resolve_(int family, const char *hostname, struct sockaddr *sa,
 
 const char *evutil_getenv_(const char *name);
 
-long evutil_weakrand_(void);
+ev_uint32_t evutil_weakrand_(ev_uint32_t* seed);
+ev_uint32_t evutil_weakrand_range_(ev_uint32_t* seed, ev_uint32_t top);
 
 /* Evaluates to the same boolean value as 'p', and hints to the compiler that
  * we expect this value to be false. */

win32select.c

@@ -326,7 +326,8 @@ win32_dispatch(struct event_base *base, struct timeval *tv)
 	}
 
 	if (win32op->readset_out->fd_count) {
-		i = rand() % win32op->readset_out->fd_count;
+		i = evutil_weakrand_range_(&base->weakrand_seed,
+		    win32op->readset_out->fd_count);
 		for (j=0; j<win32op->readset_out->fd_count; ++j) {
 			if (++i >= win32op->readset_out->fd_count)
 				i = 0;
@@ -335,7 +336,8 @@ win32_dispatch(struct event_base *base, struct timeval *tv)
 		}
 	}
 	if (win32op->exset_out->fd_count) {
-		i = rand() % win32op->exset_out->fd_count;
+		i = evutil_weakrand_range_(&base->weakrand_seed,
+		    win32op->exset_out->fd_count);
 		for (j=0; j<win32op->exset_out->fd_count; ++j) {
 			if (++i >= win32op->exset_out->fd_count)
 				i = 0;
@@ -345,7 +347,8 @@ win32_dispatch(struct event_base *base, struct timeval *tv)
 	}
 	if (win32op->writeset_out->fd_count) {
 		SOCKET s;
-		i = rand() % win32op->writeset_out->fd_count;
+		i = evutil_weakrand_range_(&base->weakrand_seed,
+		    win32op->writeset_out->fd_count);
 		for (j=0; j<win32op->writeset_out->fd_count; ++j) {
 			if (++i >= win32op->writeset_out->fd_count)
 				i = 0;