bitcoin/tx: (optional) input amount.

We need this for signing segwitness txs.  Unfortunately, we don't have it
for transactions we received as hex, only ones we created; to make this safe
we use a pointer which is NULL if we don't know, and those will crash if
we try to sign or check their sigs.

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

bitcoin/tx.c

@@ -226,6 +226,7 @@ struct bitcoin_tx *bitcoin_tx(const tal_t *ctx, varint_t input_count,
 		/* We assume NULL is a zero bitmap */
 		assert(tx->input[i].script == NULL);
 		tx->input[i].sequence_number = 0xFFFFFFFF;
+		tx->input[i].amount = NULL;
 		tx->input[i].witness = NULL;
 	}
 	tx->lock_time = 0;

bitcoin/tx.h

@@ -31,6 +31,9 @@ struct bitcoin_tx_input {
 	u8 *script;
 	u32 sequence_number;
 
+	/* Value of the output we're spending (NULL if unknown). */
+	u64 *amount;
+
 	/* Only if BIP141 used. */
 	u8 **witness;
 };

close_tx.c

@@ -24,6 +24,7 @@ struct bitcoin_tx *create_close_tx(secp256k1_context *secpctx,
 	/* Our input spends the anchor tx output. */
 	tx->input[0].txid = *anchor_txid;
 	tx->input[0].index = anchor_index;
+	tx->input[0].amount = tal_dup(tx->input, u64, &anchor_satoshis);
 
 	/* One output is to us. */
 	tx->output[0].amount = to_us;

commit_tx.c

@@ -56,6 +56,7 @@ struct bitcoin_tx *create_commit_tx(const tal_t *ctx,
 	/* Our input spends the anchor tx output. */
 	tx->input[0].txid = *anchor_txid;
 	tx->input[0].index = anchor_index;
+	tx->input[0].amount = tal_dup(tx->input, u64, &anchor_satoshis);
 
 	/* First output is a P2SH to a complex redeem script (usu. for me) */
 	redeemscript = bitcoin_redeem_secret_or_delay(tx, our_final,

daemon/peer.c

@@ -1125,6 +1125,8 @@ const struct bitcoin_tx *bitcoin_spend_ours(struct peer *peer)
 	p2sh_out = find_p2sh_out(commit, redeemscript);
 	tx->input[0].index = p2sh_out;
 	tx->input[0].sequence_number = bitcoin_nsequence(&peer->them.locktime);
+	tx->input[0].amount = tal_dup(tx->input, u64,
+				      &commit->output[p2sh_out].amount);
 
 	tx->output[0].amount = commit->output[p2sh_out].amount;
 	tx->output[0].script = scriptpubkey_p2sh(tx,