fixAMMClawbackRounding: adjust last holder's LPToken balance (#5513)

Due to rounding, the LPTokenBalance of the last LP might not match the LP's trustline balance. This was fixed for `AMMWithdraw` in `fixAMMv1_1` by adjusting the LPTokenBalance to be the same as the trustline balance. Since `AMMClawback` is also performing a withdrawal, we need to adjust LPTokenBalance as well in `AMMClawback.`

This change includes:
1. Refactored `verifyAndAdjustLPTokenBalance` function in `AMMUtils`, which both`AMMWithdraw` and `AMMClawback` call to adjust LPTokenBalance.
2. Added the unit test `testLastHolderLPTokenBalance` to test the scenario.
3. Modify the existing unit tests for `fixAMMClawbackRounding`.

include/xrpl/protocol/detail/features.macro

@@ -35,6 +35,7 @@
 // If you add an amendment here, then do not forget to increment `numFeatures`
 // in include/xrpl/protocol/Feature.h.
 
+XRPL_FIX    (AMMClawbackRounding,        Supported::no,  VoteBehavior::DefaultNo)
 XRPL_FEATURE(TokenEscrow,                Supported::yes, VoteBehavior::DefaultNo)
 XRPL_FIX    (EnforceNFTokenTrustlineV2,  Supported::yes, VoteBehavior::DefaultNo)
 XRPL_FIX    (AMMv1_3,                    Supported::yes, VoteBehavior::DefaultNo)

src/xrpld/app/misc/AMMUtils.h

@@ -125,6 +125,17 @@ isOnlyLiquidityProvider(
     Issue const& ammIssue,
     AccountID const& lpAccount);
 
+/** Due to rounding, the LPTokenBalance of the last LP might
+ * not match the LP's trustline balance. If it's within the tolerance,
+ * update LPTokenBalance to match the LP's trustline balance.
+ */
+Expected<bool, TER>
+verifyAndAdjustLPTokenBalance(
+    Sandbox& sb,
+    STAmount const& lpTokens,
+    std::shared_ptr<SLE>& ammSle,
+    AccountID const& account);
+
 }  // namespace ripple
 
 #endif  // RIPPLE_APP_MISC_AMMUTILS_H_INCLUDED

src/xrpld/app/misc/detail/AMMUtils.cpp

@@ -17,6 +17,7 @@
 */
 //==============================================================================
 
+#include <xrpld/app/misc/AMMHelpers.h>
 #include <xrpld/app/misc/AMMUtils.h>
 #include <xrpld/ledger/Sandbox.h>
 
@@ -464,4 +465,32 @@ isOnlyLiquidityProvider(
     return Unexpected<TER>(tecINTERNAL);  // LCOV_EXCL_LINE
 }
 
+Expected<bool, TER>
+verifyAndAdjustLPTokenBalance(
+    Sandbox& sb,
+    STAmount const& lpTokens,
+    std::shared_ptr<SLE>& ammSle,
+    AccountID const& account)
+{
+    if (auto const res = isOnlyLiquidityProvider(sb, lpTokens.issue(), account);
+        !res)
+        return Unexpected<TER>(res.error());
+    else if (res.value())
+    {
+        if (withinRelativeDistance(
+                lpTokens,
+                ammSle->getFieldAmount(sfLPTokenBalance),
+                Number{1, -3}))
+        {
+            ammSle->setFieldAmount(sfLPTokenBalance, lpTokens);
+            sb.update(ammSle);
+        }
+        else
+        {
+            return Unexpected<TER>(tecAMM_INVALID_TOKENS);
+        }
+    }
+    return true;
+}
+
 }  // namespace ripple

src/xrpld/app/tx/detail/AMMClawback.cpp

@@ -151,6 +151,20 @@ AMMClawback::applyGuts(Sandbox& sb)
     if (!accountSle)
         return tecINTERNAL;  // LCOV_EXCL_LINE
 
+    if (sb.rules().enabled(fixAMMClawbackRounding))
+    {
+        // retrieve LP token balance inside the amendment gate to avoid
+        // inconsistent error behavior
+        auto const lpTokenBalance = ammLPHolds(sb, *ammSle, holder, j_);
+        if (lpTokenBalance == beast::zero)
+            return tecAMM_BALANCE;
+
+        if (auto const res = verifyAndAdjustLPTokenBalance(
+                sb, lpTokenBalance, ammSle, holder);
+            !res)
+            return res.error();  // LCOV_EXCL_LINE
+    }
+
     auto const expected = ammHolds(
         sb,
         *ammSle,
@@ -248,10 +262,11 @@ AMMClawback::equalWithdrawMatchingOneAmount(
     STAmount const& amount)
 {
     auto frac = Number{amount} / amountBalance;
-    auto const amount2Withdraw = amount2Balance * frac;
+    auto amount2Withdraw = amount2Balance * frac;
 
     auto const lpTokensWithdraw =
         toSTAmount(lptAMMBalance.issue(), lptAMMBalance * frac);
+
     if (lpTokensWithdraw > holdLPtokens)
         // if lptoken balance less than what the issuer intended to clawback,
         // clawback all the tokens. Because we are doing a two-asset withdrawal,
@@ -272,6 +287,42 @@ AMMClawback::equalWithdrawMatchingOneAmount(
             mPriorBalance,
             ctx_.journal);
 
+    auto const& rules = sb.rules();
+    if (rules.enabled(fixAMMClawbackRounding))
+    {
+        auto tokensAdj =
+            getRoundedLPTokens(rules, lptAMMBalance, frac, IsDeposit::No);
+
+        // LCOV_EXCL_START
+        if (tokensAdj == beast::zero)
+            return {
+                tecAMM_INVALID_TOKENS, STAmount{}, STAmount{}, std::nullopt};
+        // LCOV_EXCL_STOP
+
+        frac = adjustFracByTokens(rules, lptAMMBalance, tokensAdj, frac);
+        auto amount2Rounded =
+            getRoundedAsset(rules, amount2Balance, frac, IsDeposit::No);
+
+        auto amountRounded =
+            getRoundedAsset(rules, amountBalance, frac, IsDeposit::No);
+
+        return AMMWithdraw::withdraw(
+            sb,
+            ammSle,
+            ammAccount,
+            holder,
+            amountBalance,
+            amountRounded,
+            amount2Rounded,
+            lptAMMBalance,
+            tokensAdj,
+            0,
+            FreezeHandling::fhIGNORE_FREEZE,
+            WithdrawAll::No,
+            mPriorBalance,
+            ctx_.journal);
+    }
+
     // Because we are doing a two-asset withdrawal,
     // tfee is actually not used, so pass tfee as 0.
     return AMMWithdraw::withdraw(

src/xrpld/app/tx/detail/AMMWithdraw.cpp

@@ -311,24 +311,9 @@ AMMWithdraw::applyGuts(Sandbox& sb)
     if (sb.rules().enabled(fixAMMv1_1))
     {
         if (auto const res =
-                isOnlyLiquidityProvider(sb, lpTokens.issue(), account_);
+                verifyAndAdjustLPTokenBalance(sb, lpTokens, ammSle, account_);
             !res)
             return {res.error(), false};
-        else if (res.value())
-        {
-            if (withinRelativeDistance(
-                    lpTokens,
-                    ammSle->getFieldAmount(sfLPTokenBalance),
-                    Number{1, -3}))
-            {
-                ammSle->setFieldAmount(sfLPTokenBalance, lpTokens);
-                sb.update(ammSle);
-            }
-            else
-            {
-                return {tecAMM_INVALID_TOKENS, false};
-            }
-        }
     }
 
     auto const tfee = getTradingFee(ctx_.view(), *ammSle, account_);