fixXChainRewardRounding: round reward shares down: (#4933)

When calculating reward shares, the amount should always be rounded
down. If the `fixUniversalNumber` amendment is not active, this works
correctly. If it is not active, then the amount is incorrectly rounded
up. This patch introduces an amendment so it will be rounded down.

Author: seelabs

src/ripple/app/tx/impl/XChainBridge.cpp

@@ -22,6 +22,7 @@
 #include <ripple/app/tx/impl/Transactor.h>
 #include <ripple/app/tx/impl/XChainBridge.h>
 #include <ripple/basics/Log.h>
+#include <ripple/basics/Number.h>
 #include <ripple/basics/XRPAmount.h>
 #include <ripple/basics/chrono.h>
 #include <ripple/beast/utility/Journal.h>
@@ -41,7 +42,6 @@
 #include <ripple/protocol/XChainAttestations.h>
 #include <ripple/protocol/digest.h>
 #include <ripple/protocol/st.h>
-
 #include <unordered_map>
 #include <unordered_set>
 
@@ -672,6 +672,12 @@ finalizeClaimHelper(
             // if the transfer failed, distribute the pool for "OnTransferFail"
             // cases (the attesters did their job)
             STAmount const share = [&] {
+                auto const round_mode =
+                    innerSb.rules().enabled(fixXChainRewardRounding)
+                    ? Number::rounding_mode::downward
+                    : Number::getround();
+                saveNumberRoundMode _{Number::setround(round_mode)};
+
                 STAmount const den{rewardAccounts.size()};
                 return divide(rewardPool, den, rewardPool.issue());
             }();

src/ripple/protocol/Feature.h

@@ -74,7 +74,7 @@ namespace detail {
 // Feature.cpp. Because it's only used to reserve storage, and determine how
 // large to make the FeatureBitset, it MAY be larger. It MUST NOT be less than
 // the actual number of amendments. A LogicError on startup will verify this.
-static constexpr std::size_t numFeatures = 69;
+static constexpr std::size_t numFeatures = 70;
 
 /** Amendments that this server supports and the default voting behavior.
    Whether they are enabled depends on the Rules defined in the validated
@@ -356,6 +356,7 @@ extern uint256 const fixNFTokenReserve;
 extern uint256 const fixInnerObjTemplate;
 extern uint256 const featurePriceOracle;
 extern uint256 const fixEmptyDID;
+extern uint256 const fixXChainRewardRounding;
 
 }  // namespace ripple
 

src/ripple/protocol/impl/Feature.cpp

@@ -463,6 +463,7 @@ REGISTER_FIX    (fixNFTokenReserve,             Supported::yes, VoteBehavior::De
 REGISTER_FIX    (fixInnerObjTemplate,           Supported::yes, VoteBehavior::DefaultNo);
 REGISTER_FEATURE(PriceOracle,                   Supported::yes, VoteBehavior::DefaultNo);
 REGISTER_FIX    (fixEmptyDID,                   Supported::yes, VoteBehavior::DefaultNo);
+REGISTER_FIX    (fixXChainRewardRounding,       Supported::yes, VoteBehavior::DefaultNo);
 
 // The following amendments are obsolete, but must remain supported
 // because they could potentially get enabled.