Fix CheckCash hard caps MPT DeliverMin at half of maxMPTokenAmount

src/libxrpl/tx/transactors/check/CheckCash.cpp

@@ -16,8 +16,9 @@
 #include <xrpl/protocol/Issue.h>
 #include <xrpl/protocol/Keylet.h>
 #include <xrpl/protocol/LedgerFormats.h>
+#include <xrpl/protocol/MPTAmount.h>
 #include <xrpl/protocol/MPTIssue.h>
-#include <xrpl/protocol/Protocol.h>
+#include <xrpl/protocol/Quality.h>
 #include <xrpl/protocol/SField.h>
 #include <xrpl/protocol/STAmount.h>
 #include <xrpl/protocol/STLedgerEntry.h>
@@ -368,18 +369,29 @@ CheckCash::doApply()
         else
         {
             // Note that for DeliverMin we don't know exactly how much
-            // currency we want flow to deliver.  We can't ask for the
-            // maximum possible currency because there might be a gateway
-            // transfer rate to account for.  Since the transfer rate cannot
-            // exceed 200%, we use 1/2 maxValue as our limit.
+            // currency we want flow to deliver.  For IOUs, use a value
+            // higher than any real delivery as the request. MPTs are
+            // bounded integral amounts, so use the maximum output the check
+            // can actually deliver without exceeding SendMax.
             auto const maxDeliverMin = [&]() {
                 return optDeliverMin->asset().visit(
                     [&](Issue const&) {
                         return STAmount(
                             optDeliverMin->asset(), STAmount::kMaxValue / 2, STAmount::kMaxOffset);
                     },
-                    [&](MPTIssue const&) {
-                        return STAmount(optDeliverMin->asset(), kMaxMpTokenAmount / 2);
+                    [&](MPTIssue const& issue) {
+                        MPTAmount maxDeliver = sendMax.mpt();
+                        auto const& issuer = issue.getIssuer();
+                        if (srcId != issuer && accountID_ != issuer)
+                        {
+                            auto const rate = transferRate(psb, issue.getMptID());
+                            // Request at most floor(SendMax / rate). The endpoint reverse pass
+                            // will quote ceil(output * rate), so this keeps the input
+                            // representable and within SendMax.
+                            maxDeliver =
+                                mulRatio(maxDeliver, QUALITY_ONE, rate.value, /*roundUp*/ false);
+                        }
+                        return STAmount(maxDeliver, issue);
                     });
             };
             STAmount const flowDeliver{

src/test/app/CheckMPT_test.cpp

@@ -31,6 +31,7 @@
 #include <xrpl/protocol/Issue.h>
 #include <xrpl/protocol/KeyType.h>
 #include <xrpl/protocol/LedgerFormats.h>
+#include <xrpl/protocol/Protocol.h>
 #include <xrpl/protocol/SField.h>
 #include <xrpl/protocol/STAmount.h>
 #include <xrpl/protocol/TER.h>
@@ -653,6 +654,32 @@ class CheckMPT_test : public beast::unit_test::Suite
             BEAST_EXPECT(ownerCount(env, alice) == 1);
             BEAST_EXPECT(ownerCount(env, bob) == 1);
         }
+
+        {
+            Env env{*this, features};
+
+            env.fund(XRP(1'000), gw, alice, bob);
+
+            // MPT DeliverMin should not be capped at half of the legal range.
+            std::uint64_t constexpr deliverMin = (kMaxMpTokenAmount / 2) + 1;
+            MPT const usd = MPTTester(
+                {.env = env, .issuer = gw, .holders = {alice, bob}, .maxAmt = kMaxMpTokenAmount});
+
+            env(pay(gw, alice, usd(deliverMin)));
+            env.close();
+
+            uint256 const chkId{getCheckIndex(alice, env.seq(alice))};
+            env(check::create(alice, bob, usd(deliverMin)));
+            env.close();
+
+            env(check::cash(bob, chkId, check::DeliverMin(usd(deliverMin))));
+            verifyDeliveredAmount(env, usd(deliverMin));
+            env.require(Balance(alice, usd(0)));
+            env.require(Balance(bob, usd(deliverMin)));
+            BEAST_EXPECT(checksOnAccount(env, alice).empty());
+            BEAST_EXPECT(checksOnAccount(env, bob).empty());
+        }
+
         {
             // Examine the effects of the asfRequireAuth flag.
             Env env(*this, features);
@@ -831,6 +858,32 @@ class CheckMPT_test : public beast::unit_test::Suite
         BEAST_EXPECT(checksOnAccount(env, alice).size() == 0);
         BEAST_EXPECT(checksOnAccount(env, bob).size() == 0);
 #endif
+
+        // With the maximum transfer fee, this is the largest output whose
+        // fee-adjusted debit is still within SendMax.
+        std::uint64_t constexpr maxDeliver = (kMaxMpTokenAmount / 3) * 2;
+        MPT const eur = MPTTester(
+            {.env = env,
+             .issuer = gw,
+             .holders = {alice, bob},
+             .transferFee = kMaxTransferFee,
+             .maxAmt = kMaxMpTokenAmount});
+
+        env(pay(gw, alice, eur(kMaxMpTokenAmount)));
+        env.close();
+
+        uint256 const chkIdMax{getCheckIndex(alice, env.seq(alice))};
+        env(check::create(alice, bob, eur(kMaxMpTokenAmount)));
+        env.close();
+
+        // The DeliverMin cap must divide SendMax by the rate before flow()
+        // computes the fee-adjusted input.
+        env(check::cash(bob, chkIdMax, check::DeliverMin(eur(maxDeliver))));
+        verifyDeliveredAmount(env, eur(maxDeliver));
+        env.require(Balance(alice, eur(1)));
+        env.require(Balance(bob, eur(maxDeliver)));
+        BEAST_EXPECT(checksOnAccount(env, alice).empty());
+        BEAST_EXPECT(checksOnAccount(env, bob).empty());
     }
 
     void