pay: Consider amounts when selecting a channel to send over

The channel selection did not consider the amounts that we are trying
to transfer, which in a multiplexed channel world could end up always
selecting a channel that is too small for the payment. We also log
which channel was selected based on the selector that is passed in,
allowing us to better follow the decisions.

lightningd/pay.c

@@ -1,3 +1,4 @@
+#include "bitcoin/short_channel_id.h"
 #include "config.h"
 #include <ccan/mem/mem.h>
 #include <ccan/tal/str/str.h>
@@ -830,32 +831,53 @@ static struct command_result *check_invoice_request_usage(struct command *cmd,
 	return NULL;
 }
 
-static struct channel *find_channel_for_htlc_add(struct lightningd *ld,
-						 const struct node_id *node,
-						 const struct short_channel_id *scid_or_alias)
+static struct channel *
+find_channel_for_htlc_add(struct lightningd *ld, const struct node_id *node,
+			  const struct short_channel_id *scid_or_alias,
+			  const struct amount_msat *amount)
 {
 	struct channel *channel;
+	struct short_channel_id *scid;
 	struct peer *peer = peer_by_id(ld, node);
 	if (!peer)
 		return NULL;
 
 	channel = find_channel_by_scid(peer, scid_or_alias);
-	if (channel && channel_can_add_htlc(channel))
-		return channel;
+	if (channel && channel_can_add_htlc(channel)) {
+		goto found;
+	}
 
 	channel = find_channel_by_alias(peer, scid_or_alias, LOCAL);
-	if (channel && channel_can_add_htlc(channel))
-		return channel;
+	if (channel && channel_can_add_htlc(channel)) {
+		goto found;
+	}
 
 	/* We used to ignore scid: now all-zero means "any" */
-	if (!channel && (ld->deprecated_apis || memeqzero(scid_or_alias, sizeof(*scid_or_alias)))) {
+	if (!channel && (ld->deprecated_apis ||
+			 memeqzero(scid_or_alias, sizeof(*scid_or_alias)))) {
 		list_for_each(&peer->channels, channel, list) {
-			if (channel_can_add_htlc(channel)) {
-				return channel;
+			if (channel_can_add_htlc(channel) &&
+			    amount_msat_greater(channel->our_msat, *amount)) {
+				goto found;
 			}
 		}
 	}
+
+	log_debug(ld->log, "No channel found for selector %s (%s)",
+		  short_channel_id_to_str(tmpctx, scid_or_alias),
+		  type_to_string(tmpctx, struct amount_msat, amount));
 	return NULL;
+
+found:
+	scid = channel->scid ? channel->scid : channel->alias[LOCAL];
+	log_debug(
+	    ld->log, "Selected channel %s (%s) for selector %s (%s)",
+	    short_channel_id_to_str(tmpctx, scid),
+	    type_to_string(tmpctx, struct amount_msat, &channel->our_msat),
+	    short_channel_id_to_str(tmpctx, scid_or_alias),
+	    type_to_string(tmpctx, struct amount_msat, amount));
+
+	return channel;
 }
 
 /* destination/route_channels/route_nodes are NULL (and path_secrets may be NULL)
@@ -1048,7 +1070,7 @@ send_payment_core(struct lightningd *ld,
 		return invreq_err;
 
 	channel = find_channel_for_htlc_add(ld, &first_hop->node_id,
-					    &first_hop->scid);
+					    &first_hop->scid, &msat);
 	if (!channel) {
 		struct json_stream *data
 			= json_stream_fail(cmd, PAY_TRY_OTHER_ROUTE,