channeld: have commit_tx tell is what output was the other's anchor.

We're going to want this, soon.

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

channeld/commit_tx.c

@@ -128,7 +128,8 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 			     u64 obscured_commitment_number,
 			     bool option_anchor_outputs,
 			     bool option_anchors_zero_fee_htlc_tx,
-			     enum side side)
+			     enum side side,
+			     int *anchor_outnum)
 {
 	struct amount_sat base_fee;
 	struct amount_msat total_pay;
@@ -139,7 +140,8 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 	u32 *cltvs;
 	bool to_local, to_remote;
 	struct htlc *dummy_to_local = (struct htlc *)0x01,
-		*dummy_to_remote = (struct htlc *)0x02;
+		*dummy_to_remote = (struct htlc *)0x02,
+		*dummy_other_anchor = (struct htlc *)0x03;
 	const u8 *funding_wscript = bitcoin_redeem_2of2(tmpctx,
 							local_funding_key,
 							remote_funding_key);
@@ -379,9 +381,11 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 			n++;
 		}
 
+		/* With anchors, the caller really wants to know what
+		 * is the LOCAL anchor for the REMOTE side. */
 		if (to_remote || untrimmed != 0) {
 			tx_add_anchor_output(tx, remote_funding_key);
-			(*htlcmap)[n] = NULL;
+			(*htlcmap)[n] = dummy_other_anchor;
 			n++;
 		}
 	}
@@ -433,17 +437,21 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 	bitcoin_tx_add_input(tx, funding,
 			     sequence, NULL, funding_sats, NULL, funding_wscript);
 
-	/* Identify the direct outputs (to_us, to_them). */
-	if (direct_outputs != NULL) {
+	/* Identify the direct outputs (to_us, to_them), and the local anchor */
+	if (direct_outputs != NULL)
 		direct_outputs[LOCAL] = direct_outputs[REMOTE] = NULL;
-		for (size_t i = 0; i < tx->wtx->num_outputs; i++) {
-			if ((*htlcmap)[i] == dummy_to_local) {
-				(*htlcmap)[i] = NULL;
-				direct_outputs[LOCAL] = tx->wtx->outputs + i;
-			} else if ((*htlcmap)[i] == dummy_to_remote) {
-				(*htlcmap)[i] = NULL;
-				direct_outputs[REMOTE] = tx->wtx->outputs + i;
-			}
+
+	*anchor_outnum = -1;
+	for (size_t i = 0; i < tx->wtx->num_outputs; i++) {
+		if ((*htlcmap)[i] == dummy_to_local) {
+			(*htlcmap)[i] = NULL;
+			direct_outputs[LOCAL] = tx->wtx->outputs + i;
+		} else if ((*htlcmap)[i] == dummy_to_remote) {
+			(*htlcmap)[i] = NULL;
+			direct_outputs[REMOTE] = tx->wtx->outputs + i;
+		} else if ((*htlcmap)[i] == dummy_other_anchor) {
+			(*htlcmap)[i] = NULL;
+			*anchor_outnum = i;
 		}
 	}
 

channeld/commit_tx.h

@@ -63,8 +63,9 @@ bool commit_tx_amount_trimmed(const struct htlc **htlcs,
  * @obscured_commitment_number: number to encode in commitment transaction
  * @direct_outputs: If non-NULL, fill with pointers to the direct (non-HTLC) outputs (or NULL if none).
  * @option_anchor_outputs: does option_anchor_outputs apply to this channel?
+ * @option_anchors_zero_fee_htlc_tx: does option_anchors_zero_fee_htlc_tx apply to this channel?
  * @side: side to generate commitment transaction for.
- * @option_anchor_outputs: does option_anchor_outputs apply to this channel?
+ * @anchor_outnum: set to index of local anchor, or -1 if none.
  *
  * We need to be able to generate the remote side's tx to create signatures,
  * but the BOLT is expressed in terms of generating our local commitment
@@ -90,6 +91,7 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 			     u64 obscured_commitment_number,
 			     bool option_anchor_outputs,
 			     bool option_anchors_zero_fee_htlc_tx,
-			     enum side side);
+			     enum side side,
+			     int *anchor_outnum);
 
 #endif /* LIGHTNING_CHANNELD_COMMIT_TX_H */

channeld/full_channel.c

@@ -315,6 +315,7 @@ struct bitcoin_tx **channel_txs(const tal_t *ctx,
 	const struct htlc **committed;
 	struct keyset keyset;
 	struct amount_msat side_pay, other_side_pay;
+	int local_anchor;
 
 	if (!derive_keyset(per_commitment_point,
 			   &channel->basepoints[side],
@@ -363,7 +364,7 @@ struct bitcoin_tx **channel_txs(const tal_t *ctx,
 	    commitment_number ^ channel->commitment_number_obscurer,
 	    channel_has(channel, OPT_ANCHOR_OUTPUTS),
 	    channel_has(channel, OPT_ANCHORS_ZERO_FEE_HTLC_TX),
-	    side);
+	    side, &local_anchor);
 
 	/* Set the remote/local pubkeys on the commitment tx psbt */
 	psbt_input_add_pubkey(txs[0]->psbt, 0,

channeld/test/run-commit_tx.c

@@ -542,6 +542,7 @@ int main(int argc, const char *argv[])
 	bool option_anchor_outputs = false;
 	bool option_anchors_zero_fee_htlc_tx = false;
 	bool option_static_remotekey = false;
+	int local_anchor;
 
 	/* Allow us to check static-remotekey BOLT 3 vectors, too */
 	if (argv[1] && streq(argv[1], "--static-remotekey"))
@@ -822,7 +823,7 @@ int main(int argc, const char *argv[])
 		       NULL, &htlc_map, NULL, commitment_number ^ cn_obscurer,
 		       option_anchor_outputs,
 		       option_anchors_zero_fee_htlc_tx,
-		       LOCAL);
+		       LOCAL, &local_anchor);
 	print_superverbose = false;
 	tx2 = commit_tx(tmpctx,
 			&funding,
@@ -839,7 +840,7 @@ int main(int argc, const char *argv[])
 			NULL, &htlc_map2, NULL, commitment_number ^ cn_obscurer,
 			option_anchor_outputs,
 			option_anchors_zero_fee_htlc_tx,
-			REMOTE);
+			REMOTE, &local_anchor);
 	tx_must_be_eq(tx, tx2);
 	report(tx, wscript, &x_remote_funding_privkey, &remote_funding_pubkey,
 	       &local_funding_privkey, &local_funding_pubkey,
@@ -891,7 +892,7 @@ int main(int argc, const char *argv[])
 		       htlcs, &htlc_map, NULL, commitment_number ^ cn_obscurer,
 		       option_anchor_outputs,
 		       option_anchors_zero_fee_htlc_tx,
-		       LOCAL);
+		       LOCAL, &local_anchor);
 	print_superverbose = false;
 	tx2 = commit_tx(tmpctx,
 			&funding,
@@ -909,7 +910,7 @@ int main(int argc, const char *argv[])
 			commitment_number ^ cn_obscurer,
 			option_anchor_outputs,
 			option_anchors_zero_fee_htlc_tx,
-			REMOTE);
+			REMOTE, &local_anchor);
 	tx_must_be_eq(tx, tx2);
 	report(tx, wscript, &x_remote_funding_privkey, &remote_funding_pubkey,
 	       &local_funding_privkey, &local_funding_pubkey,
@@ -949,7 +950,7 @@ int main(int argc, const char *argv[])
 				  commitment_number ^ cn_obscurer,
 				  option_anchor_outputs,
 				  option_anchors_zero_fee_htlc_tx,
-				  LOCAL);
+				  LOCAL, &local_anchor);
 		/* This is what it would look like for peer generating it! */
 		tx2 = commit_tx(tmpctx,
 				&funding,
@@ -967,7 +968,7 @@ int main(int argc, const char *argv[])
 				commitment_number ^ cn_obscurer,
 				option_anchor_outputs,
 				option_anchors_zero_fee_htlc_tx,
-				REMOTE);
+				REMOTE, &local_anchor);
 		tx_must_be_eq(newtx, tx2);
 #ifdef DEBUG
 		if (feerate_per_kw % 100000 == 0)
@@ -1011,7 +1012,7 @@ int main(int argc, const char *argv[])
 			       commitment_number ^ cn_obscurer,
 			       option_anchor_outputs,
 			       option_anchors_zero_fee_htlc_tx,
-			       LOCAL);
+			       LOCAL, &local_anchor);
 		report(tx, wscript,
 		       &x_remote_funding_privkey, &remote_funding_pubkey,
 		       &local_funding_privkey, &local_funding_pubkey,
@@ -1063,7 +1064,7 @@ int main(int argc, const char *argv[])
 				  commitment_number ^ cn_obscurer,
 				  option_anchor_outputs,
 				  option_anchors_zero_fee_htlc_tx,
-				  LOCAL);
+				  LOCAL, &local_anchor);
 		report(newtx, wscript,
 		       &x_remote_funding_privkey, &remote_funding_pubkey,
 		       &local_funding_privkey, &local_funding_pubkey,
@@ -1142,7 +1143,7 @@ int main(int argc, const char *argv[])
 			       commitment_number ^ cn_obscurer,
 			       option_anchor_outputs,
 			       option_anchors_zero_fee_htlc_tx,
-			       LOCAL);
+			       LOCAL, &local_anchor);
 		report(tx, wscript,
 		       &x_remote_funding_privkey, &remote_funding_pubkey,
 		       &local_funding_privkey, &local_funding_pubkey,
@@ -1199,7 +1200,7 @@ int main(int argc, const char *argv[])
 		       htlcs, &htlc_map, NULL, commitment_number ^ cn_obscurer,
 		       option_anchor_outputs,
 		       option_anchors_zero_fee_htlc_tx,
-		       LOCAL);
+		       LOCAL, &local_anchor);
 	print_superverbose = false;
 	tx2 = commit_tx(tmpctx,
 			&funding,
@@ -1217,7 +1218,7 @@ int main(int argc, const char *argv[])
 			commitment_number ^ cn_obscurer,
 			option_anchor_outputs,
 			option_anchors_zero_fee_htlc_tx,
-			REMOTE);
+			REMOTE, &local_anchor);
 	tx_must_be_eq(tx, tx2);
 	report(tx, wscript, &x_remote_funding_privkey, &remote_funding_pubkey,
 	       &local_funding_privkey, &local_funding_pubkey,

channeld/test/run-full_channel.c

@@ -364,6 +364,7 @@ int main(int argc, const char *argv[])
 	bool option_anchor_outputs = false;
 	bool option_anchors_zero_fee_htlc_tx = false;
 	u32 blockheight = 0;
+	int local_anchor;
 	size_t i;
 
 	chainparams = chainparams_for_network("bitcoin");
@@ -535,7 +536,8 @@ int main(int argc, const char *argv[])
 			   to_local,
 			   to_remote,
 			   NULL, &htlc_map, NULL, 0x2bb038521914 ^ 42,
-			   option_anchor_outputs, option_anchors_zero_fee_htlc_tx, LOCAL);
+			   option_anchor_outputs, option_anchors_zero_fee_htlc_tx,
+			   LOCAL, &local_anchor);
 
 	txs = channel_txs(tmpctx, &funding, funding_amount,
 			  &htlc_map, NULL, &funding_wscript_alt,
@@ -667,7 +669,8 @@ int main(int argc, const char *argv[])
 		    &keyset, feerate_per_kw[LOCAL], local_config->dust_limit,
 		    to_local, to_remote, htlcs, &htlc_map, NULL,
 		    0x2bb038521914 ^ 42,
-		    option_anchor_outputs, option_anchors_zero_fee_htlc_tx, LOCAL);
+		    option_anchor_outputs, option_anchors_zero_fee_htlc_tx,
+		    LOCAL, &local_anchor);
 
 		txs = channel_txs(tmpctx, &funding, funding_amount,
 				  &htlc_map, NULL, &funding_wscript,