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Fix BookStep rate calculation. Extend the unit-tests.

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This commit is contained in:
Gregory Tsipenyuk
2026-04-23 13:12:49 -04:00
parent 5381133a8d
commit 00f39cdd9c
2 changed files with 101 additions and 5 deletions
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20
src/libxrpl/tx/paths/BookStep.cpp
View File

@@ -1376,12 +1376,22 @@ BookStep<TIn, TOut, TDerived>::rate(
Asset const& asset,
AccountID const& dstAccount) const
{
auto const& issuer = asset.getIssuer();
if (isXRP(issuer) || issuer == dstAccount)
return parityRate;
return asset.visit(
[&](Issue const&) { return transferRate(view, issuer); },
[&](MPTIssue const& issue) { return transferRate(view, issue.getMptID()); });
[&](Issue const& issue) -> Rate {
if (isXRP(issue.account) || issue.account == dstAccount)
return parityRate;
return transferRate(view, issue.account);
},
[&](MPTIssue const& mptIssue) -> Rate {
// For MPT, parity applies only when this asset is the final strand
// delivery AND the destination is the MPT issuer (holder → issuer,
// which is fee-free). Using strandDst_ alone is wrong because it
// incorrectly suppresses the fee when MPT is an intermediate or
// the in-side of a book that precedes the issuer's XRP receipt.
if (asset == strandDeliver_ && mptIssue.getIssuer() == dstAccount)
return parityRate;
return transferRate(view, mptIssue.getMptID());
});
};
template <class TIn, class TOut, class TDerived>

86
src/test/app/OfferMPT_test.cpp
View File

@@ -3005,6 +3005,92 @@ public:
}
};
testHelper2TokensMix(test);
// Payment trIn: MPT transfer fee must be charged when the payment
// destination is the MPT issuer and MPT crosses the DEX (1-hop).
// Bug: rate() returned parity because strandDst_ == MPT issuer.
// Fix: parity only when this asset IS the final delivered asset.
{
auto const gw = Account("gw_tr1");
auto const alice = Account("alice_tr1");
auto const bob = Account("bob_tr1");
Env env{*this, features};
env.fund(XRP(10'000), gw, alice, bob);
env.close();
MPT const USD = MPTTester(
{.env = env, .issuer = gw, .holders = {alice, bob}, .transferFee = 25'000});
// alice needs MPT(1250): MPT(1000) to bob's offer + MPT(250) transfer fee (25%)
env(pay(gw, alice, USD(1'250)));
// bob's offer: give XRP(1000), want MPT(1000)
env(offer(bob, USD(1'000), XRP(1'000)));
env.close();
// alice pays gw (MPT issuer) XRP(1000) using MPT as source
// strand: alice -> [MPT/XRP BookStep] -> gw
// strandDst_ = gw = MPT issuer, strandDeliver_ = XRP
// trIn = rate(MPT, gw): fix charges 25% (MPT != strandDeliver_)
env(pay(alice, gw, XRP(1'000)), path(~XRP), sendmax(USD(1'250)));
env.close();
// alice consumed all MPT(1250): MPT(1000) to bob + MPT(250) fee
BEAST_EXPECT(env.balance(alice, USD) == USD(0));
// bob received MPT(1000) net
BEAST_EXPECT(env.balance(bob, USD) == USD(1'000));
}
// Payment trIn (2-hop): MPT transfer fee must be charged when MPT is
// intermediate and the destination is the MPT issuer.
// BookStep2(MPT/XRP) prevStep=BookStep1 returns redeems direction
// (ownerPaysTransferFee_=false for Payment), so trIn applies.
// Bug: parity because strandDst_ == MPT issuer.
// Fix: 25% fee because MPT != strandDeliver_(XRP).
{
auto const gw = Account("gw_tr2");
auto const gw2 = Account("gw2_tr2");
auto const alice = Account("alice_tr2");
auto const bob = Account("bob_tr2");
auto const carol = Account("carol_tr2");
Env env{*this, features};
env.fund(XRP(10'000), gw, gw2, alice, bob, carol);
env.close();
MPT const MUSD = MPTTester(
{.env = env, .issuer = gw, .holders = {bob, carol}, .transferFee = 25'000});
auto const GUSD = gw2["USD"];
env(trust(alice, GUSD(10'000)));
env(trust(bob, GUSD(10'000)));
env.close();
env(pay(gw2, alice, GUSD(1'000)));
env(pay(gw, bob, MUSD(1'000)));
env.close();
// bob's offer: give MPT(1000), want GUSD(1000)
env(offer(bob, GUSD(1'000), MUSD(1'000)));
// carol's offer: give XRP(800), want MPT(800)
env(offer(carol, MUSD(800), XRP(800)));
env.close();
// Payment: alice GUSD -> [BookStep1: GUSD/MUSD] -> [BookStep2: MUSD/XRP] -> gw XRP
// strandDst_ = gw = MPT issuer, strandDeliver_ = XRP
// BookStep2 trIn: fix = 1.25 -> upstream needs MUSD(1000) for carol's MUSD(800) offer
// => alice must provide full GUSD(1000) to bob's offer; without fix alice only pays
// GUSD(800)
env(pay(alice, gw, XRP(800)), path(~MUSD), sendmax(GUSD(1'000)));
env.close();
// alice spent all GUSD(1000); bug would leave GUSD(200) unspent
BEAST_EXPECT(env.balance(alice, GUSD) == GUSD(0));
// bob gave MPT(1000) and received GUSD(1000)
BEAST_EXPECT(env.balance(bob, MUSD) == MUSD(0));
// carol received MPT(800) net (MPT(200) went to gw as fee)
BEAST_EXPECT(env.balance(carol, MUSD) == MUSD(800));
}
}
void