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elements: Add helpers to compute and add fee outputs.

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Signed-off-by: Christian Decker <decker.christian@gmail.com>
This commit is contained in:
Christian Decker 2019-05-06 15:24:53 +02:00 committed by Rusty Russell
parent 5e3b27c306
commit 92b8758753
2 changed files with 66 additions and 4 deletions
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57
bitcoin/tx.c
View File

@ -68,6 +68,42 @@ int bitcoin_tx_add_multi_outputs(struct bitcoin_tx *tx,
return tx->wtx->num_outputs;
}
/**
* Compute how much fee we are actually sending with this transaction.
*/
static u64 bitcoin_tx_compute_fee(const struct bitcoin_tx *tx)
{
u64 fee = 0, satoshi;
for (size_t i=0; i<tal_count(tx->input_amounts); i++)
fee += tx->input_amounts[i]->satoshis; /* Raw: fee computation */
for (size_t i=0; i<tx->wtx->num_outputs; i++) {
if (!is_elements) {
fee -= tx->wtx->outputs[i].satoshi; /* Raw: low-level helper */
} else {
beint64_t tmp;
memcpy(&tmp, &tx->wtx->outputs[i].value[1] , sizeof(tmp));
satoshi = be64_to_cpu(tmp);
fee -= satoshi;
}
}
return fee;
}
int bitcoin_tx_add_fee_output(struct bitcoin_tx *tx)
{
struct amount_sat fee;
u64 rawsats = bitcoin_tx_compute_fee(tx); /* Raw: pedantic much? */
fee.satoshis = rawsats; /* Raw: need amounts later */
/* If we aren't using elements, we don't add explicit fee outputs */
if (!is_elements || rawsats == 0)
return -1;
return bitcoin_tx_add_output(tx, NULL, fee);
}
int bitcoin_tx_add_input(struct bitcoin_tx *tx, const struct bitcoin_txid *txid,
u32 outnum, u32 sequence,
struct amount_sat amount, u8 *script)
@ -96,14 +132,21 @@ bool bitcoin_tx_check(const struct bitcoin_tx *tx)
{
u8 *newtx;
size_t written;
int flags = WALLY_TX_FLAG_USE_WITNESS;
if (wally_tx_get_length(tx->wtx, WALLY_TX_FLAG_USE_WITNESS, &written) !=
WALLY_OK)
if (wally_tx_get_length(tx->wtx, flags, &written) != WALLY_OK)
return false;
if (is_elements) {
flags |= WALLY_TX_FLAG_USE_ELEMENTS;
/* Elements transactions must have an explicit fee */
if (bitcoin_tx_compute_fee(tx) != 0)
return false;
}
newtx = tal_arr(tmpctx, u8, written);
if (wally_tx_to_bytes(tx->wtx, WALLY_TX_FLAG_USE_WITNESS, newtx,
written, &written) != WALLY_OK)
if (wally_tx_to_bytes(tx->wtx, flags, newtx, written, &written) !=
WALLY_OK)
return false;
if (written != tal_bytelen(newtx))
@ -354,6 +397,12 @@ struct bitcoin_tx *bitcoin_tx(const tal_t *ctx,
struct bitcoin_tx *tx = tal(ctx, struct bitcoin_tx);
assert(chainparams);
/* If we are constructing an elements transaction we need to
* explicitly add the fee as an extra output. So allocate one more
* than the outputs we need internally. */
if (is_elements)
output_count += 1;
wally_tx_init_alloc(WALLY_TX_VERSION_2, 0, input_count, output_count,
&tx->wtx);
tal_add_destructor(tx, bitcoin_tx_destroy);

13
bitcoin/tx.h
View File

@ -148,4 +148,17 @@ void bitcoin_tx_input_get_txid(const struct bitcoin_tx *tx, int innum,
*/
bool bitcoin_tx_check(const struct bitcoin_tx *tx);
/**
* Add an explicit fee output if necessary.
*
* An explicit fee output is only necessary if we are using an elements
* transaction, and we have a non-zero fee. This method may be called multiple
* times if input and output values do not change, however it may result in
* multiple change outputs if the fee is changed.
*
* Returns the position of the fee output, or -1 in the case of non-elements
* transactions.
*/
int bitcoin_tx_add_fee_output(struct bitcoin_tx *tx);
#endif /* LIGHTNING_BITCOIN_TX_H */