chanbackup: unbias our random peer selection.

We picked one node and iterated.  This means we would only sent to 1, not
2 nodes if we picked the last.  But it also means we would generally send
to the same pair of nodes, which isn't great for redundancy.

Rework to be more random.

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

Diff for: plugins/chanbackup.c

@@ -551,13 +551,47 @@ static struct command_result *after_send_scb_single_fail(struct command *cmd,
 	return notification_or_hook_done(cmd);
 }
 
+static bool already_have_node_id(const struct node_id *ids,
+				 const struct node_id *id)
+{
+	for (size_t i = 0; i < tal_count(ids); i++)
+		if (node_id_eq(&ids[i], id))
+			return true;
+
+	return false;
+}
+
+static const struct node_id *random_peers(const tal_t *ctx,
+					  const struct chanbackup *cb)
+{
+	struct peer_map_iter it;
+	const struct node_id *peer;
+	struct node_id *ids = tal_arr(ctx, struct node_id, 0);
+
+	/* Simple case: pick all of them */
+	if (peer_map_count(cb->peers) <= NUM_BACKUP_PEERS) {
+		for (peer = peer_map_first(cb->peers, &it);
+		     peer;
+		     peer = peer_map_next(cb->peers, &it)) {
+			tal_arr_expand(&ids, *peer);
+		}
+	} else {
+		while (peer_map_count(cb->peers) < NUM_BACKUP_PEERS) {
+			peer = peer_map_pick(cb->peers, pseudorand_u64(), &it);
+			if (already_have_node_id(ids, peer))
+				continue;
+			tal_arr_expand(&ids, *peer);
+		}
+	}
+	return ids;
+}
+
 static struct command_result *send_to_peers(struct command *cmd)
 {
         struct out_req *req;
 	size_t *idx = tal(cmd, size_t);
         u8 *serialise_scb, *data;
-	struct peer_map_iter it;
-	const struct node_id *peer;
+	const struct node_id *peers;
 	struct chanbackup *cb = chanbackup(cmd->plugin);
 
 	if (!cb->send_our_peer_backup)
@@ -582,14 +616,13 @@ static struct command_result *send_to_peers(struct command *cmd)
 	}
 	serialise_scb = towire_peer_storage(cmd, data);
 
-	*idx = 0;
-	for (peer = peer_map_pick(cb->peers, pseudorand_u64(), &it);
-	     peer;
-	     peer = peer_map_next(cb->peers, &it)) {
+	peers = random_peers(tmpctx, cb);
+	*idx = tal_count(peers);
+	for (size_t i = 0; i < tal_count(peers); i++) {
 		struct info *info = tal(cmd, struct info);
 
 		info->idx = idx;
-		info->node_id = *peer;
+		info->node_id = peers[i];
 
 		req = jsonrpc_request_start(cmd,
 					    "sendcustommsg",
@@ -599,12 +632,7 @@ static struct command_result *send_to_peers(struct command *cmd)
 
 		json_add_node_id(req->js, "node_id", &info->node_id);
 		json_add_hex_talarr(req->js, "msg", serialise_scb);
-		(*info->idx)++;
 		send_outreq(req);
-
-		/* Only send to NUM_BACKUP_PEERS for each update */
-		if (*info->idx == NUM_BACKUP_PEERS)
-			break;
 	}
 
 	if (*idx == 0)