gossipd: increase randomness in route selection.

We have a seed, which is for (future!) unit testing consistency.  This
makes it change every time, so our pay_direct_test is more useful.

I tried restarting the noed around the loop, but it tended to fail
rebinding to the same port for some reason?

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/pseudorand.c

@@ -37,6 +37,13 @@ uint64_t pseudorand(uint64_t max)
 	return isaac64_next_uint(&isaac64, max);
 }
 
+uint64_t pseudorand_u64(void)
+{
+	init_if_needed();
+
+	return isaac64_next_uint64(&isaac64);
+}
+
 const struct siphash_seed *siphash_seed(void)
 {
 	init_if_needed();

common/pseudorand.h

@@ -8,6 +8,11 @@
  */
 uint64_t pseudorand(uint64_t max);
 
+/**
+ * pseudorand - pseudo (guessable!) random number between 0 and UINT64_MAX.
+ */
+uint64_t pseudorand_u64(void);
+
 /**
  * Get the siphash seed for hash tables.
  */

gossipd/gossipd.c

@@ -1922,7 +1922,7 @@ static struct io_plan *getroute_req(struct io_conn *conn, struct daemon *daemon,
 	/* routing.c does all the hard work; can return NULL. */
 	hops = get_route(tmpctx, daemon->rstate, &source, &destination,
 			 msatoshi, riskfactor, final_cltv,
-			 fuzz, siphash_seed(), excluded, max_hops);
+			 fuzz, pseudorand_u64(), excluded, max_hops);
 
 	out = towire_gossip_getroute_reply(NULL, hops);
 	daemon_conn_send(daemon->master, take(out));

gossipd/routing.c

@@ -1496,7 +1496,7 @@ struct route_hop *get_route(const tal_t *ctx, struct routing_state *rstate,
 			    const struct pubkey *destination,
 			    const u64 msatoshi, double riskfactor,
 			    u32 final_cltv,
-			    double fuzz, const struct siphash_seed *base_seed,
+			    double fuzz, u64 seed,
 			    const struct short_channel_id_dir *excluded,
 			    size_t max_hops)
 {
@@ -1507,9 +1507,12 @@ struct route_hop *get_route(const tal_t *ctx, struct routing_state *rstate,
 	struct route_hop *hops;
 	struct node *n;
 	u64 *saved_capacity;
+	struct siphash_seed base_seed;
 
 	saved_capacity = tal_arr(tmpctx, u64, tal_count(excluded));
 
+	base_seed.u.u64[0] = base_seed.u.u64[1] = seed;
+
 	/* Temporarily set excluded channels' capacity to zero. */
 	for (size_t i = 0; i < tal_count(excluded); i++) {
 		struct chan *chan = get_channel(rstate, &excluded[i].scid);
@@ -1522,7 +1525,7 @@ struct route_hop *get_route(const tal_t *ctx, struct routing_state *rstate,
 
 	route = find_route(ctx, rstate, source, destination, msatoshi,
 			   riskfactor / BLOCKS_PER_YEAR / 10000,
-			   fuzz, base_seed, max_hops, &fee);
+			   fuzz, &base_seed, max_hops, &fee);
 
 	/* Now restore the capacity. */
 	for (size_t i = 0; i < tal_count(excluded); i++) {

gossipd/routing.h

@@ -264,7 +264,7 @@ struct route_hop *get_route(const tal_t *ctx, struct routing_state *rstate,
 			    const u64 msatoshi, double riskfactor,
 			    u32 final_cltv,
 			    double fuzz,
-			    const struct siphash_seed *base_seed,
+			    u64 seed,
 			    const struct short_channel_id_dir *excluded,
 			    size_t max_hops);
 /* Disable channel(s) based on the given routing failure. */