lightningd: make 'spendable_msatoshi' more realistic.

Take into account the fee we'd have to pay if we're the funder, and
also drop to 0 if the amount is less than the smallest HTLC the peer
will accept.

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

lightningd/Makefile

@@ -31,6 +31,7 @@ LIGHTNINGD_COMMON_OBJS :=			\
 	common/gen_status_wire.o		\
 	common/hash_u5.o			\
 	common/htlc_state.o			\
+	common/htlc_trim.o			\
 	common/htlc_wire.o			\
 	common/key_derive.o			\
 	common/io_lock.o			\

lightningd/peer_control.c

@@ -14,6 +14,7 @@
 #include <channeld/gen_channel_wire.h>
 #include <common/dev_disconnect.h>
 #include <common/features.h>
+#include <common/htlc_trim.h>
 #include <common/initial_commit_tx.h>
 #include <common/json_command.h>
 #include <common/json_helpers.h>
@@ -436,6 +437,7 @@ static void json_add_htlcs(struct lightningd *ld,
 	struct htlc_in_map_iter ini;
 	const struct htlc_out *hout;
 	struct htlc_out_map_iter outi;
+	u32 local_feerate = channel->channel_info.feerate_per_kw[LOCAL];
 
 	/* FIXME: Add more fields. */
 	json_array_start(response, "htlcs");
@@ -455,6 +457,9 @@ static void json_add_htlcs(struct lightningd *ld,
 			     &hin->payment_hash, sizeof(hin->payment_hash));
 		json_add_string(response, "state",
 				htlc_state_name(hin->hstate));
+		if (htlc_is_trimmed(REMOTE, hin->msat, local_feerate,
+				    channel->our_config.dust_limit, LOCAL))
+			json_add_bool(response, "local_trimmed", true);
 		json_object_end(response);
 	}
 
@@ -474,6 +479,9 @@ static void json_add_htlcs(struct lightningd *ld,
 			     &hout->payment_hash, sizeof(hout->payment_hash));
 		json_add_string(response, "state",
 				htlc_state_name(hout->hstate));
+		if (htlc_is_trimmed(LOCAL, hout->msat, local_feerate,
+				    channel->our_config.dust_limit, LOCAL))
+			json_add_bool(response, "local_trimmed", true);
 		json_object_end(response);
 	}
 	json_array_end(response);
@@ -491,6 +499,49 @@ static void json_add_sat_only(struct json_stream *result,
 				type_to_string(tmpctx, struct amount_msat, &msat));
 }
 
+/* This is quite a lot of work to figure out what it would cost us! */
+static struct amount_sat commit_txfee(const struct channel *channel,
+				      struct amount_msat spendable)
+{
+	/* FIXME: make per-channel htlc maps! */
+	const struct htlc_in *hin;
+	struct htlc_in_map_iter ini;
+	const struct htlc_out *hout;
+	struct htlc_out_map_iter outi;
+	struct lightningd *ld = channel->peer->ld;
+	u32 local_feerate = channel->channel_info.feerate_per_kw[LOCAL];
+	size_t num_untrimmed_htlcs = 0;
+
+	/* Assume we tried to spend "spendable" */
+	if (!htlc_is_trimmed(LOCAL, spendable,
+			     local_feerate, channel->our_config.dust_limit,
+			     LOCAL))
+		num_untrimmed_htlcs++;
+
+	for (hin = htlc_in_map_first(&ld->htlcs_in, &ini);
+	     hin;
+	     hin = htlc_in_map_next(&ld->htlcs_in, &ini)) {
+		if (hin->key.channel != channel)
+			continue;
+		if (!htlc_is_trimmed(REMOTE, hin->msat, local_feerate,
+				     channel->our_config.dust_limit,
+				     LOCAL))
+			num_untrimmed_htlcs++;
+	}
+	for (hout = htlc_out_map_first(&ld->htlcs_out, &outi);
+	     hout;
+	     hout = htlc_out_map_next(&ld->htlcs_out, &outi)) {
+		if (hout->key.channel != channel)
+			continue;
+		if (!htlc_is_trimmed(LOCAL, hout->msat, local_feerate,
+				     channel->our_config.dust_limit,
+				     LOCAL))
+			num_untrimmed_htlcs++;
+	}
+
+	return commit_tx_base_fee(local_feerate, num_untrimmed_htlcs);
+}
+
 static void json_add_channel(struct lightningd *ld,
 			     struct json_stream *response, const char *key,
 			     const struct channel *channel)
@@ -605,6 +656,18 @@ static void json_add_channel(struct lightningd *ld,
 				 channel->channel_info.their_config.channel_reserve))
 		spendable = AMOUNT_MSAT(0);
 
+	/* If we're funder, subtract txfees we'll need to spend this */
+	if (channel->funder == LOCAL) {
+		if (!amount_msat_sub_sat(&spendable, spendable,
+					 commit_txfee(channel, spendable)))
+			spendable = AMOUNT_MSAT(0);
+	}
+
+	/* We can't offer an HTLC less than the other side will accept. */
+	if (amount_msat_less(spendable,
+			     channel->channel_info.their_config.htlc_minimum))
+		spendable = AMOUNT_MSAT(0);
+
 	json_add_amount_msat_compat(response, spendable,
 				    "spendable_msatoshi", "spendable_msat");
 	json_add_amount_msat_compat(response,

lightningd/test/run-invoice-select-inchan.c

@@ -112,6 +112,13 @@ const struct chainparams *get_chainparams(const struct lightningd *ld UNNEEDED)
 /* Generated stub for get_log_level */
 enum log_level get_log_level(struct log_book *lr UNNEEDED)
 { fprintf(stderr, "get_log_level called!\n"); abort(); }
+/* Generated stub for htlc_is_trimmed */
+bool htlc_is_trimmed(enum side htlc_owner UNNEEDED,
+		     struct amount_msat htlc_amount UNNEEDED,
+		     u32 feerate_per_kw UNNEEDED,
+		     struct amount_sat dust_limit UNNEEDED,
+		     enum side side UNNEEDED)
+{ fprintf(stderr, "htlc_is_trimmed called!\n"); abort(); }
 /* Generated stub for htlcs_reconnect */
 void htlcs_reconnect(struct lightningd *ld UNNEEDED,
 		     struct htlc_in_map *htlcs_in UNNEEDED,

wallet/test/run-wallet.c

@@ -112,6 +112,13 @@ u32 get_block_height(const struct chain_topology *topo UNNEEDED)
 /* Generated stub for get_chainparams */
 const struct chainparams *get_chainparams(const struct lightningd *ld UNNEEDED)
 { fprintf(stderr, "get_chainparams called!\n"); abort(); }
+/* Generated stub for htlc_is_trimmed */
+bool htlc_is_trimmed(enum side htlc_owner UNNEEDED,
+		     struct amount_msat htlc_amount UNNEEDED,
+		     u32 feerate_per_kw UNNEEDED,
+		     struct amount_sat dust_limit UNNEEDED,
+		     enum side side UNNEEDED)
+{ fprintf(stderr, "htlc_is_trimmed called!\n"); abort(); }
 /* Generated stub for invoices_create */
 bool invoices_create(struct invoices *invoices UNNEEDED,
 		     struct invoice *pinvoice UNNEEDED,