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daemon: simplify fee calculation for spends of our own commit tx.

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It's not exact, but faking a sig, measuring length, then resigning was
neither exact nor pretty.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell
2016-05-03 11:07:20 +09:30
parent 12b37d5f80
commit e18aea8d71
1 changed files with 16 additions and 25 deletions
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27
daemon/peer.c
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@@ -1174,36 +1174,27 @@ const struct bitcoin_tx *bitcoin_spend_ours(struct peer *peer)
tx->input[0].amount = tal_dup(tx->input, u64, tx->input[0].amount = tal_dup(tx->input, u64,
&commit->output[p2wsh_out].amount); &commit->output[p2wsh_out].amount);
tx->output[0].amount = commit->output[p2wsh_out].amount;
tx->output[0].script = scriptpubkey_p2sh(tx, tx->output[0].script = scriptpubkey_p2sh(tx,
bitcoin_redeem_single(tx, &peer->us.finalkey)); bitcoin_redeem_single(tx, &peer->us.finalkey));
tx->output[0].script_length = tal_count(tx->output[0].script); tx->output[0].script_length = tal_count(tx->output[0].script);
/* Use signature, until we have fee. */ /* Witness length can vary, due to DER encoding of sigs, but we
sig.stype = SIGHASH_ALL; * use 176 from an example run. */
peer_sign_spend(peer, tx, witnessscript, &sig.sig); assert(measure_tx_cost(tx) == 83 * 4);
tx->input[0].witness = bitcoin_witness_secret(tx, NULL, 0, &sig,
witnessscript);
/* FIXME: Figure out length first, then calc fee! */
/* Now, calculate the fee, given length. */
/* FIXME: Dynamic fees! */ /* FIXME: Dynamic fees! */
fee = fee_by_feerate(measure_tx_cost(tx) / 4, fee = fee_by_feerate(83 + 176 / 4,
peer->dstate->config.closing_fee_rate); peer->dstate->config.closing_fee_rate);
/* FIXME: Fail gracefully in these cases (not worth collecting) */ /* FIXME: Fail gracefully in these cases (not worth collecting) */
if (fee > tx->output[0].amount if (fee > commit->output[p2wsh_out].amount
|| is_dust_amount(tx->output[0].amount - fee)) || is_dust_amount(commit->output[p2wsh_out].amount - fee))
fatal("Amount of %"PRIu64" won't cover fee %"PRIu64, fatal("Amount of %"PRIu64" won't cover fee %"PRIu64,
tx->output[0].amount, fee); commit->output[p2wsh_out].amount, fee);
/* Re-sign with the real values. */ tx->output[0].amount = commit->output[p2wsh_out].amount - fee;
tx->input[0].witness = tal_free(tx->input[0].witness);
tx->output[0].amount -= fee;
sig.stype = SIGHASH_ALL;
peer_sign_spend(peer, tx, witnessscript, &sig.sig); peer_sign_spend(peer, tx, witnessscript, &sig.sig);
tx->input[0].witness = bitcoin_witness_secret(tx, NULL, 0, &sig, tx->input[0].witness = bitcoin_witness_secret(tx, NULL, 0, &sig,