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Calculate fee amount after output addition

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We would previously calculate the fee amount in two steps:
1. Add the weight of the empty transaction
2. Add the weight of each output

That's unnecessary: you can just use the weight of the transaction
*after* the output addition. This is clearer, but also avoids a
rare bug: if there are many outputs, adding them would cause the
"number of outputs" transaction parameter lenght to increase, and we
wouldn't notice it.
This might still happen when adding the drain output - this
commit also adds a comment as a reminder.
This commit is contained in:
Daniela Brozzoni 2022-07-11 21:52:11 +02:00
parent 00d426b885
commit ac051d7ae9
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GPG Key ID: 7DE4F1FDCED0AB87
1 changed files with 5 additions and 3 deletions
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8
src/wallet/mod.rs
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@ -736,8 +736,6 @@ where
let mut outgoing: u64 = 0;
let mut received: u64 = 0;
fee_amount += fee_rate.fee_wu(tx.weight());
let recipients = params.recipients.iter().map(|(r, v)| (r, *v));
for (index, (script_pubkey, value)) in recipients.enumerate() {
@ -753,13 +751,14 @@ where
script_pubkey: script_pubkey.clone(),
value,
};
fee_amount += fee_rate.fee_vb(serialize(&new_out).len());
tx.output.push(new_out);
outgoing += value;
}
fee_amount += fee_rate.fee_wu(tx.weight());
if params.change_policy != tx_builder::ChangeSpendPolicy::ChangeAllowed
&& self.change_descriptor.is_none()
{
@ -851,6 +850,9 @@ where
script_pubkey: drain_script,
};
// TODO: We should pay attention when adding a new output: this might increase
// the lenght of the "number of vouts" parameter by 2 bytes, potentially making
// our feerate too low
tx.output.push(drain_output);
}
};