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Support explicit change outputs
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Setting the `is_change` flag on an addressee when creating a transaction
routes all change to that output. The default behaviour is to generate a
change address for the wallet. Only one explicit change address per
asset is allowed. If an explicit change address is specified for a given
asset it follows that there will not be a generated change address for
that asset.

The tx output in the transaction json corresponding to the explicit
change output will be flagged with `is_change`, just like a generated
change address output, however the `change_index` will be set to -1.
IOW `change_index` should be interpreted as the index of the generated
(implied) change output, if there is one.
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@jkauffman1
jkauffman1 committed May 25, 2021
1 parent 5cf1614 commit 1d97818
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Showing 3 changed files with 73 additions and 25 deletions.
66 changes: 45 additions & 21 deletions src/ga_tx.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -614,15 +614,28 @@ namespace sdk {
// Add all outputs and compute the total amount of satoshi to be sent
amount required_total{ 0 };

uint32_t explicit_change_index = NO_CHANGE_INDEX;
if (num_addressees) {
size_t addressee_index = 0;
for (auto& addressee : *addressees_p) {
const auto addressee_asset_id = asset_id_from_json(net_params, addressee);
if (addressee_asset_id == asset_id) {
required_total += add_tx_addressee(session, net_params, result, tx, addressee);
const auto amount = add_tx_addressee(session, net_params, result, tx, addressee);
if (!json_get_value(addressee, "is_change", false)) {
required_total += amount;
} else {
if (explicit_change_index != NO_CHANGE_INDEX) {
set_tx_error(result, "Only one explicit change addressee allowed");
break;
}
explicit_change_index = addressee_index;
}
reordered_addressees.push_back(addressee);
}
++addressee_index;
}
}
result["change_type"][asset_id] = explicit_change_index == NO_CHANGE_INDEX ? "generated" : "explicit";

// TODO: filter per asset or assume always single asset
if (manual_selection) {
Expand Down Expand Up @@ -758,11 +771,7 @@ namespace sdk {
// so compute what we can send (everything minus the
// fee) and exit the loop
required_total = available_total - fee;
if (is_liquid) {
set_tx_output_commitment(net_params, tx, 0, asset_id, required_total.value());
} else {
tx->outputs[0].satoshi = required_total.value();
}
set_tx_output_value(net_params, tx, 0, asset_id, required_total.value());
if (num_addressees == 1u) {
addressees_p->at(0)["satoshi"] = required_total.value();
}
Expand Down Expand Up @@ -818,17 +827,30 @@ namespace sdk {
continue;
}

// We have more than the dust amount of change. Add a change
// output to collect it, then loop again in case the amount
// this increases the fee by requires more UTXOs.
add_tx_output(net_params, result, tx, result.at("change_address").at(asset_id).at("address"),
is_liquid ? 1 : 0, asset_id == "btc" ? std::string{} : asset_id);
have_change_output = true;
change_index = tx->num_outputs - 1;
if (is_liquid && include_fee) {
std::swap(tx->outputs[fee_index], tx->outputs[change_index]);
std::swap(fee_index, change_index);
// We have more than the dust amount of change. First look for an explicit change
// output in the addressees and if present send the change there
amount::value_type change_amount = (total - required_total - fee).value();

if (explicit_change_index == NO_CHANGE_INDEX) {
// No explicit change output specified, add a change output using the generated change
// address
add_tx_output(net_params, result, tx, result.at("change_address").at(asset_id).at("address"),
is_liquid ? 1 : 0, asset_id == "btc" ? std::string{} : asset_id);
have_change_output = true;
change_index = tx->num_outputs - 1;
if (is_liquid && include_fee) {
std::swap(tx->outputs[fee_index], tx->outputs[change_index]);
std::swap(fee_index, change_index);
}
} else {
// Use explicit change output
set_tx_output_value(net_params, tx, explicit_change_index, asset_id, change_amount);
auto addressees = *addressees_p;
addressees[explicit_change_index]["satoshi"] = change_amount;
change_index = explicit_change_index;
have_change_output = true;
}

result["have_change"][asset_id] = have_change_output;
result["change_index"][asset_id] = change_index;
}
Expand All @@ -850,11 +872,13 @@ namespace sdk {
} else {
auto& change_output = tx->outputs[change_index];
change_output.satoshi = change_amount;
const uint32_t new_change_index = get_uniform_uint32_t(tx->num_outputs);
// Randomize change output
if (change_index != new_change_index) {
std::swap(tx->outputs[new_change_index], change_output);
change_index = new_change_index;
if (explicit_change_index == NO_CHANGE_INDEX) {
// Randomize change output for non-explicit change
const uint32_t new_change_index = get_uniform_uint32_t(tx->num_outputs);
if (change_index != new_change_index) {
std::swap(tx->outputs[new_change_index], change_output);
change_index = new_change_index;
}
}
}
}
Expand Down
29 changes: 25 additions & 4 deletions src/transaction_utils.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -359,8 +359,10 @@ namespace sdk {
}

// Transactions with outputs below the dust threshold (except OP_RETURN)
// are not relayed by network nodes
if (!result.value("send_all", false) && satoshi.value() < session.get_dust_threshold()) {
// are not relayed by network nodes. send_all and explicit change outputs
// have amounts set to zero because they are calculated
if (!result.value("send_all", false) && !json_get_value(addressee, "is_change", false)
&& satoshi.value() < session.get_dust_threshold()) {
result["error"] = res::id_invalid_amount;
}

Expand All @@ -371,6 +373,17 @@ namespace sdk {
net_params, result, tx, address, satoshi.value(), asset_id_from_json(net_params, addressee));
}

void set_tx_output_value(const network_parameters& net_params, wally_tx_ptr& tx, uint32_t index,
const std::string& asset_id, amount::value_type satoshi)
{
const bool is_liquid = net_params.is_liquid();
if (is_liquid) {
set_tx_output_commitment(net_params, tx, index, asset_id, satoshi);
} else {
tx->outputs[index].satoshi = satoshi;
}
}

void update_tx_size_info(const wally_tx_ptr& tx, nlohmann::json& result)
{
const bool valid = tx->num_inputs != 0u && tx->num_outputs != 0u;
Expand Down Expand Up @@ -439,6 +452,8 @@ namespace sdk {
: false;
const uint32_t change_index
= have_change ? result.at("change_index").at(asset_id).get<uint32_t>() : NO_CHANGE_INDEX;
const bool explicit_change
= have_change ? result.at("change_type").at(asset_id).get<std::string>() == "explicit" : false;

amount::value_type satoshi = o.satoshi;
if (net_params.is_liquid()) {
Expand All @@ -461,10 +476,11 @@ namespace sdk {
}
};


if (is_fee) {
// Nothing to do
} else if (i == change_index) {
// Insert our change meta-data for the change output
} else if (i == change_index && !explicit_change) {
// Insert our change meta-data for the generated (non-explicit) change output
const auto& change_address = result.at("change_address").at(asset_id);
output.insert(change_address.begin(), change_address.end());
if (net_params.is_liquid()) {
Expand All @@ -477,6 +493,11 @@ namespace sdk {
if (net_params.is_liquid()) {
output["public_key"] = blinding_key_from_addr(address);
}
if (i == change_index && explicit_change) {
// This is an explicit change output (as opposed to one generated by the gdk)
// Mark with 'is_change'
output["is_change"] = true;
}
++addressee_index;
}

Expand Down
3 changes: 3 additions & 0 deletions src/transaction_utils.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -87,6 +87,9 @@ namespace sdk {
amount add_tx_addressee(ga_session& session, const network_parameters& net_params, nlohmann::json& result,
wally_tx_ptr& tx, nlohmann::json& addressee);

void set_tx_output_value(const network_parameters& net_params, wally_tx_ptr& tx, uint32_t index,
const std::string& asset_id, amount::value_type satoshi);

vbf_t generate_final_vbf(byte_span_t input_abfs, byte_span_t input_vbfs, uint64_span_t input_values,
const std::vector<abf_t>& output_abfs, const std::vector<vbf_t>& output_vbfs, uint32_t num_inputs);

Expand Down

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