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fundpsbt/utxopsbt: handle excess_as_change and all correctly.
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If you did call fundpsbt with amount 'all' and `excess_as_change`
true, you would get everything going to the change output.  That's
obviously not the intention, and we'd like to use this to add change
outputs even for "all" when have keep emergency reserves.

And change the finish_psbt() API to take an explicit change amount:
at the moment it's either all or nothing, but that will change with
emergency-sat reserves.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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@rustyrussell
rustyrussell committed Jun 29, 2023
1 parent d882a38 commit e20ceba
Showing 1 changed file with 57 additions and 34 deletions.
91 changes: 57 additions & 34 deletions wallet/reservation.c
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Original file line number Diff line number Diff line change
Expand Up @@ -349,11 +349,11 @@ static struct command_result *finish_psbt(struct command *cmd,
struct amount_sat excess,
u32 reserve,
u32 *locktime,
bool excess_as_change)
struct amount_sat change)
{
struct json_stream *response;
struct wally_psbt *psbt;
size_t change_outnum COMPILER_WANTS_INIT("gcc 9.4.0 -Og");
ssize_t change_outnum;
u32 current_height = get_block_height(cmd->ld->topology);

if (!locktime) {
Expand All @@ -365,20 +365,14 @@ static struct command_result *finish_psbt(struct command *cmd,
*locktime, BITCOIN_TX_RBF_SEQUENCE,
NULL);
assert(psbt->version == 2);
/* Should we add a change output for the excess? */
if (excess_as_change) {
struct amount_sat change;

/* Should we add a change output? (Iff it can pay for itself!) */
change = change_amount(change, feerate_per_kw, weight);
if (amount_sat_greater(change, AMOUNT_SAT(0))) {
struct pubkey pubkey;
s64 keyidx;
u8 *b32script;

/* Checks for dust, returns 0sat if below dust */
change = change_amount(excess, feerate_per_kw, weight);
if (!amount_sat_greater(change, AMOUNT_SAT(0))) {
excess_as_change = false;
goto fee_calc;
}

/* Get a change adddress */
keyidx = wallet_get_newindex(cmd->ld);
if (keyidx < 0)
Expand All @@ -392,14 +386,13 @@ static struct command_result *finish_psbt(struct command *cmd,

change_outnum = psbt->num_outputs;
psbt_append_output(psbt, b32script, change);
/* Set excess to zero */
excess = AMOUNT_SAT(0);
/* Add additional weight of output */
weight += bitcoin_tx_output_weight(
BITCOIN_SCRIPTPUBKEY_P2WPKH_LEN);
} else {
change_outnum = -1;
}

fee_calc:
/* Add a fee output if this is elements */
if (is_elements(chainparams)) {
struct amount_sat est_fee =
Expand All @@ -420,7 +413,7 @@ static struct command_result *finish_psbt(struct command *cmd,
json_add_num(response, "feerate_per_kw", feerate_per_kw);
json_add_num(response, "estimated_final_weight", weight);
json_add_amount_sat_msat(response, "excess_msat", excess);
if (excess_as_change)
if (change_outnum != -1)
json_add_num(response, "change_outnum", change_outnum);
if (reserve)
reserve_and_report(response, cmd->ld->wallet, current_height,
Expand Down Expand Up @@ -451,7 +444,7 @@ static struct command_result *json_fundpsbt(struct command *cmd,
struct utxo **utxos;
u32 *feerate_per_kw;
u32 *minconf, *weight, *min_witness_weight;
struct amount_sat *amount, input, diff;
struct amount_sat *amount, input, diff, change;
bool all, *excess_as_change, *nonwrapped;
u32 *locktime, *reserve, maxheight;

Expand Down Expand Up @@ -538,10 +531,11 @@ static struct command_result *json_fundpsbt(struct command *cmd,
}

if (all) {
/* Anything above 0 is "excess" */
/* We need to afford one non-dust output, at least. */
if (!inputs_sufficient(input, AMOUNT_SAT(0),
*feerate_per_kw, *weight,
&diff)) {
&diff)
|| amount_sat_less(diff, chainparams->dust_limit)) {
if (!topology_synced(cmd->ld->topology))
return command_fail(cmd,
FUNDING_STILL_SYNCING_BITCOIN,
Expand All @@ -551,10 +545,19 @@ static struct command_result *json_fundpsbt(struct command *cmd,
" fees",
tal_count(utxos));
}
*excess_as_change = false;
}

/* Turn excess into change. */
if (*excess_as_change) {
change = diff;
diff = AMOUNT_SAT(0);
} else {
change = AMOUNT_SAT(0);
}

return finish_psbt(cmd, utxos, *feerate_per_kw, *weight, diff, *reserve,
locktime, *excess_as_change);
locktime, change);
}

static const struct json_command fundpsbt_command = {
Expand Down Expand Up @@ -636,7 +639,7 @@ static struct command_result *json_utxopsbt(struct command *cmd,
struct utxo **utxos;
u32 *feerate_per_kw, *weight, *min_witness_weight;
bool all, *reserved_ok, *excess_as_change;
struct amount_sat *amount, input, excess;
struct amount_sat *amount, input, excess, change;
u32 current_height, *locktime, *reserve;

if (!param(cmd, buffer, params,
Expand Down Expand Up @@ -686,23 +689,43 @@ static struct command_result *json_utxopsbt(struct command *cmd,
*weight += utxo_spend_weight(utxo, *min_witness_weight);
}

/* For all, anything above 0 is "excess" */
if (!inputs_sufficient(input, all ? AMOUNT_SAT(0) : *amount,
*feerate_per_kw, *weight, &excess)) {
return command_fail(cmd, FUND_CANNOT_AFFORD,
if (all) {
/* We need to afford one non-dust output, at least. */
if (!inputs_sufficient(input, AMOUNT_SAT(0),
*feerate_per_kw, *weight,
&excess)
|| amount_sat_less(excess, chainparams->dust_limit)) {
return command_fail(cmd, FUND_CANNOT_AFFORD,
"Could not afford anything using UTXOs totalling %s with weight %u at feerate %u",
type_to_string(tmpctx,
struct amount_sat,
&input),
*weight, *feerate_per_kw);
}
*excess_as_change = false;
} else {
if (!inputs_sufficient(input, *amount,
*feerate_per_kw, *weight, &excess)) {
return command_fail(cmd, FUND_CANNOT_AFFORD,
"Could not afford %s using UTXOs totalling %s with weight %u at feerate %u",
all ? "anything" :
type_to_string(tmpctx,
struct amount_sat,
amount),
type_to_string(tmpctx,
struct amount_sat,
&input),
*weight, *feerate_per_kw);
type_to_string(tmpctx,
struct amount_sat,
amount),
type_to_string(tmpctx,
struct amount_sat,
&input),
*weight, *feerate_per_kw);
}
}
if (*excess_as_change) {
change = excess;
excess = AMOUNT_SAT(0);
} else {
change = AMOUNT_SAT(0);
}

return finish_psbt(cmd, utxos, *feerate_per_kw, *weight, excess,
*reserve, locktime, *excess_as_change);
*reserve, locktime, change);
}
static const struct json_command utxopsbt_command = {
"utxopsbt",
Expand Down

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