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feat(platform-wallet): return the exact network fee from send_payment #4095

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Merged
QuantumExplorer merged 1 commit into from
Jul 11, 2026
+23 −11
Merged

feat(platform-wallet): return the exact network fee from send_payment #4095

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9 changes: 8 additions & 1 deletion packages/rs-platform-wallet-ffi/src/dashpay.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -543,6 +543,7 @@ pub unsafe extern "C" fn platform_wallet_send_dashpay_payment(
memo: *const c_char,
core_signer_handle: *mut MnemonicResolverHandle,
out_txid: *mut [u8; 32],
out_fee_duffs: *mut u64,
) -> PlatformWalletFFIResult {
check_ptr!(core_signer_handle);
check_ptr!(out_txid);
Expand Down Expand Up @@ -592,7 +593,13 @@ pub unsafe extern "C" fn platform_wallet_send_dashpay_payment(
})
});
let result = unwrap_option_or_return!(option);
let (txid, _entry) = unwrap_result_or_return!(result);
let (txid, _entry, fee_duffs) = unwrap_result_or_return!(result);
// Exact network fee of the broadcast transaction (inputs − outputs),
// straight from the builder — nullable so callers that don't care
// can pass NULL.
if !out_fee_duffs.is_null() {
unsafe { *out_fee_duffs = fee_duffs };
}
use dashcore::hashes::Hash;
let bytes = txid.to_raw_hash().to_byte_array();
unsafe {
Expand Down
9 changes: 5 additions & 4 deletions packages/rs-platform-wallet/src/wallet/identity/network/payments.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -552,6 +552,7 @@ impl<B: TransactionBroadcaster + ?Sized> DashPayView<'_, B> {
(
dashcore::Txid,
crate::wallet::identity::types::dashpay::payment::PaymentEntry,
u64, // network fee in duffs, from the broadcast transaction
),
PlatformWalletError,
>
Expand All @@ -575,7 +576,7 @@ impl<B: TransactionBroadcaster + ?Sized> DashPayView<'_, B> {
// re-acquires that (non-reentrant) lock internally.
self.drain_pending_contact_crypto(provider).await;

let (payment_address, tx) = {
let (payment_address, tx, fee) = {
let mut wm = self.wallet_manager.write().await;

// Resolve the external account's xpub so we can derive addresses.
Expand Down Expand Up @@ -664,14 +665,14 @@ impl<B: TransactionBroadcaster + ?Sized> DashPayView<'_, B> {
// `impl<S: Signer> TransactionSigner for S`) rather than the
// resident `wallet`, so funding-input signatures are produced
// from Keychain-derived keys without a resident seed.
let (tx, _fee) = builder
let (tx, fee) = builder
.build_signed(signer, |addr| {
managed_account.address_derivation_path(&addr)
})
.await
.map_err(|e| PlatformWalletError::TransactionBuild(e.to_string()))?;

(payment_address, tx)
(payment_address, tx, fee)
Comment on lines +668 to +675

@thepastaclaw thepastaclaw Jul 11, 2026

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🔴 Blocking: Piped-through fee is a size-based recomputation, not the actual inputs-minus-outputs fee

I traced this through to TransactionBuilder::build_signed in the pinned key-wallet crate (rev 1ee1c94, key-wallet/src/wallet/managed_wallet_info/transaction_builder.rs). The value now surfaced by this PR is fee_rate.calculate_fee(encoded_size(&signed_tx)) — a fee-rate × final-serialized-size recomputation done after signing.

That is provably not the same quantity as the fee the broadcast transaction actually pays. During assemble_unsigned, the change output (if any) is sized as total_input - total_output - selection.estimated_fee, so the true on-chain fee (inputs minus outputs) always equals selection.estimated_fee — a value computed before signing from an estimated per-input size (148 bytes) and the coin-selector's dust-folding rules, not from the final signed size. The two numbers only match by coincidence:

  • Dust case: when change_amount falls below the dust threshold, accumulate_coins_with_size folds the dust into estimated_fee (total_value - target_amount) so the true paid fee is inflated by up to 546 duffs. build_signed's returned value is oblivious to this — it just does size × rate — so it undercounts the actual fee by exactly the folded-in dust amount whenever no change output is created.
  • General case: the 148-byte/input estimate used for coin selection and the real encoded size of a signed P2PKH input (variable-length DER signatures) aren't guaranteed to match, so the two fee numbers can diverge even when a change output exists.

This PR's whole purpose is to let wallets show the exact fee instead of re-estimating after the fact — but the plumbed-through value inherits an estimate that's decoupled from what the change output / dust folding actually paid. Since send_payment previously discarded this value entirely (_fee), this inaccuracy had no consumer before; this PR is what turns it into an externally-relied-upon contract, which is why it's in scope here even though the root computation lives in the external key-wallet crate.

source: ['codex']

};

// --- 3. Broadcast the transaction, releasing the build's UTXO
Expand Down Expand Up @@ -726,7 +727,7 @@ impl<B: TransactionBroadcaster + ?Sized> DashPayView<'_, B> {
})?;
}

Ok((txid, entry))
Ok((txid, entry, fee))
}
}

Expand Down
14 changes: 9 additions & 5 deletions packages/swift-sdk/Sources/SwiftDashSDK/PlatformWallet/ManagedPlatformWallet.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -2101,7 +2101,8 @@ extension ManagedPlatformWallet {

/// Send a Dash payment to an established DashPay contact.
/// `amountDuffs` is in duffs (1 DASH = 100_000_000 duffs).
/// Returns the 32-byte transaction id.
/// Returns the 32-byte transaction id plus the exact network fee
/// (duffs) of the broadcast transaction, straight from the builder.
///
/// Prerequisite: `register_external_contact_account` must have
/// run for the `(fromIdentityId, toContactIdentityId)` pair on
Expand All @@ -2113,7 +2114,7 @@ extension ManagedPlatformWallet {
toContactIdentityId: Identifier,
amountDuffs: UInt64,
memo: String? = nil
) async throws -> Data {
) async throws -> (txid: Data, feeDuffs: UInt64) {
let handle = self.handle
let fromBytes: [UInt8] = fromIdentityId.withFFIBytes { ptr in
Array(UnsafeBufferPointer(start: ptr, count: 32))
Expand All @@ -2128,7 +2129,8 @@ extension ManagedPlatformWallet {
// derived, digest signed, buffers zeroed) — the seed never becomes
// resident and no private key leaves Swift.
let coreSigner = MnemonicResolver()
return try await Task.detached(priority: .userInitiated) { () -> Data in
return try await Task.detached(priority: .userInitiated) { () -> (txid: Data, feeDuffs: UInt64) in
var feeDuffs: UInt64 = 0
var txidTuple: (
UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8,
UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8,
Expand All @@ -2153,7 +2155,8 @@ extension ManagedPlatformWallet {
amountDuffs,
memoPtr,
coreSigner.handle,
&txidTuple
&txidTuple,
&feeDuffs
)
}
if let memoCopy {
Expand All @@ -2165,7 +2168,8 @@ extension ManagedPlatformWallet {
}
}
try result.check()
return Swift.withUnsafeBytes(of: &txidTuple) { Data($0) }
let txid = Swift.withUnsafeBytes(of: &txidTuple) { Data($0) }
return (txid: txid, feeDuffs: feeDuffs)
}.value
}
}
Expand Down
2 changes: 1 addition & 1 deletion ...ges/swift-sdk/SwiftExampleApp/SwiftExampleApp/Views/DashPay/SendDashPayPaymentSheet.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -364,7 +364,7 @@ struct SendDashPayPaymentSheet: View {
// useful to pass. The Rust-side
// `PaymentEntry.memo` slot stays available for
// future local-note wiring.
let txid = try await wallet.sendDashPayPayment(
let (txid, _) = try await wallet.sendDashPayPayment(

@thepastaclaw thepastaclaw Jul 11, 2026

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💬 Nitpick: In-repo reference implementation discards the newly-exposed fee

This PR's stated motivation is that wallets could previously only re-estimate the fee after the fact. SendDashPayPaymentSheet is the in-tree DashPay send flow and the natural place to demonstrate the new capability, but it discards feeDuffs (let (txid, _) = try await wallet.sendDashPayPayment(...)) instead of surfacing it in the confirmation UI. Not a defect — the real consumer is dashwallet-ios — but the example app doesn't showcase the feature this PR adds.

source: ['claude']

fromIdentityId: senderIdentity.identityId,
toContactIdentityId: contact.identityId,
amountDuffs: duffs,
Expand Down
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