block: Compute the txids only once

We're not going to mutate transactions in a block, so computing the txids
every time we need them is a waste, let's compute them upfront and use them
afterwards.

bitcoin/block.c

@@ -208,9 +208,11 @@ bitcoin_block_from_hex(const tal_t *ctx, const struct chainparams *chainparams,
 
 	num = pull_varint(&p, &len);
 	b->tx = tal_arr(b, struct bitcoin_tx *, num);
+	b->txids = tal_arr(b, struct bitcoin_txid, num);
 	for (i = 0; i < num; i++) {
 		b->tx[i] = pull_bitcoin_tx(b->tx, &p, &len);
 		b->tx[i]->chainparams = chainparams;
+		bitcoin_txid(b->tx[i], &b->txids[i]);
 	}
 
 	/* We should end up not overrunning, nor have extra */

bitcoin/block.h

@@ -36,6 +36,7 @@ struct bitcoin_block {
 	struct bitcoin_block_hdr hdr;
 	/* tal_count shows now many */
 	struct bitcoin_tx **tx;
+	struct bitcoin_txid *txids;
 };
 
 struct bitcoin_block *

lightningd/chaintopology.c

@@ -91,7 +91,7 @@ static void filter_block_txs(struct chain_topology *topo, struct block *b)
 		}
 
 		owned = AMOUNT_SAT(0);
-		bitcoin_txid(tx, &txid);
+		txid = b->txids[i];
 		if (txfilter_match(topo->bitcoind->ld->owned_txfilter, tx)) {
 			wallet_extract_owned_outputs(topo->bitcoind->ld->wallet,
 						     tx->wtx, &b->height, &owned);
@@ -748,7 +748,7 @@ static void topo_update_spends(struct chain_topology *topo, struct block *b)
 		struct bitcoin_txid txid;
 		struct amount_sat inputs_total = AMOUNT_SAT(0);
 
-		bitcoin_txid(tx, &txid);
+		txid = b->txids[i];
 
 		for (size_t j = 0; j < tx->wtx->num_inputs; j++) {
 			const struct wally_tx_input *input = &tx->wtx->inputs[j];
@@ -843,6 +843,7 @@ static struct block *new_block(struct chain_topology *topo,
 	b->hdr = blk->hdr;
 
 	b->full_txs = tal_steal(b, blk->tx);
+	b->txids = tal_steal(b, blk->txids);
 
 	return b;
 }

lightningd/chaintopology.h

@@ -62,6 +62,7 @@ struct block {
 
 	/* Full copy of txs (freed in filter_block_txs) */
 	struct bitcoin_tx **full_txs;
+	struct bitcoin_txid *txids;
 };
 
 /* Hash blocks by sha */