Enforce MaxMagnitudeUnit clamp in GetMagnitudeUnit()

Also implement 256 bit integer arithmetic in ComputeMRCFee() as an
overflow fallback.

src/gridcoin/accrual/snapshot.h

@@ -32,20 +32,6 @@ namespace {
 using namespace GRC;
 using LogFlags = BCLog::LogFlags;
 
-/*
-//!
-//! \brief Numerator of the static magnitude unit coefficient for snapshot
-//! accrual (block version 11 and greater).
-//!
-constexpr int64_t MAG_UNIT_NUMERATOR = 1;
-
-//!
-//! \brief Denominator of the static magnitude unit coefficient for snapshot
-//! accrual (block version 11 and greater).
-//!
-constexpr int64_t MAG_UNIT_DENOMINATOR = 4;
-*/
-
 //!
 //! \brief Calculates the current accrual for a CPID by adding the snapshot of
 //! accrued research rewards of the CPID's research account to rewards accrued
@@ -77,13 +63,14 @@ class SnapshotCalculator
     //!
     Allocation GetMagnitudeUnit() const
     {
-        Allocation magnitude_unit = Params().GetConsensus().DefaultMagnitudeUnit;
-
-        // Before V13 magnitude unit is 1/4.
+        // Fern+ and before V13 magnitude unit is fixed at 1/4.
         if (!IsV13Enabled(m_superblock.m_height)) {
             return Allocation(1, 4);
         }
 
+        Allocation magnitude_unit = Params().GetConsensus().DefaultMagnitudeUnit;
+        Allocation max_magnitude_unit = Params().GetConsensus().MaxMagnitudeUnit;
+
         // Find the current protocol entry value for Magnitude Weight Factor, if it exists.
         ProtocolEntryOption protocol_entry = GetProtocolRegistry().TryLastBeforeTimestamp("magnitudeunit", m_superblock.m_timestamp);
 
@@ -93,6 +80,15 @@ class SnapshotCalculator
             magnitude_unit = Fraction().FromString(protocol_entry->m_value);
         }
 
+        // Clamp to MaxMagnitudeUnit if necessary
+        if (magnitude_unit > max_magnitude_unit) {
+            magnitude_unit = max_magnitude_unit;
+            LogPrintf("WARN: %s: Magnitude Unit specified by protocol is greater than %s. Clamping to %s.",
+                      __func__,
+                      max_magnitude_unit.ToString(),
+                      max_magnitude_unit.ToString());
+        }
+
         return magnitude_unit;
     }
 
@@ -107,22 +103,32 @@ class SnapshotCalculator
     {
         CBlockIndex* sb_index = BlockFinder::FindLatestSuperblock(index);
 
-        Allocation magnitude_unit = Params().GetConsensus().DefaultMagnitudeUnit;
-
-               // Before V13 magnitude unit is 1/4.
+        // Before V13 magnitude unit is 1/4.
         if (!IsV13Enabled(sb_index->nHeight)) {
             return Allocation(1, 4);
         }
 
-               // Find the current protocol entry value for Magnitude Weight Factor, if it exists.
+        Allocation magnitude_unit = Params().GetConsensus().DefaultMagnitudeUnit;
+        Allocation max_magnitude_unit = Params().GetConsensus().MaxMagnitudeUnit;
+
+        // Find the current protocol entry value for Magnitude Weight Factor, if it exists.
         ProtocolEntryOption protocol_entry = GetProtocolRegistry().TryLastBeforeTimestamp("magnitudeunit", sb_index->GetBlockTime());
 
-               // If their is an entry prior or equal in timestamp to the superblock and it is active then set the magnitude unit
-               // to that value. If the last entry is not active (i.e. deleted), then leave at the default.
+        // If their is an entry prior or equal in timestamp to the superblock and it is active then set the magnitude unit
+        // to that value. If the last entry is not active (i.e. deleted), then leave at the default.
         if (protocol_entry != nullptr && protocol_entry->m_status == ProtocolEntryStatus::ACTIVE) {
             magnitude_unit = Fraction().FromString(protocol_entry->m_value);
         }
 
+        // Clamp to MaxMagnitudeUnit if necessary
+        if (magnitude_unit > max_magnitude_unit) {
+            magnitude_unit = max_magnitude_unit;
+            LogPrintf("WARN: %s: Magnitude Unit specified by protocol is greater than %s. Clamping to %s.",
+                      __func__,
+                      max_magnitude_unit.ToString(),
+                      max_magnitude_unit.ToString());
+        }
+
         return magnitude_unit;
     }
 

src/gridcoin/mrc.cpp

@@ -204,13 +204,27 @@ CAmount MRC::ComputeMRCFee() const
     //
     // If the MRC is exactly at the end of the zero_payout_interval, the fees are effectively
     // fee_fraction * m_research_subsidy.
-    //
-    // Overflow analysis. The accrual limit in the snapshot computer is 16384. For a zero_payout_interval of 14 days,
-    // m_research_subsidy * zero_payout_interval = 19818086400. std::numeric_limits<int64_t>::max() / 19818086400
-    // = 465401747207, which means the numerator of the fee_fraction would have to be that number or higher to cause an
-    // overflow of the below. This is not likely to happen for any reasonable choice of the fee_fraction.
-    fee = m_research_subsidy * zero_payout_interval * fee_fraction.GetNumerator()
-                             / mrc_payment_interval / fee_fraction.GetDenominator();
+
+    // To accommodate V13+ blocks which can have much larger magnitude units (up to 5), we have changed the fee calculation
+    // to switch to 256 bit integer calculations if necessary similar to what is in AccrualDelta(). The rationale for the overflow
+    // test is as follows. The m_research_subsidy converted back to GRC (which divides by 100,000,000 and therefore rounds down) + 1
+    // to get the equivalent of rounding up, then multipled by the zero_payout_interval and the fee_fraction numerator is checked
+    // against the max numeric limit of CAmount divided by 100,000,000. If it is greater than this, then 256 bit integer arithmetic
+    // is used else the original calculation is used.
+    CAmount overflow_test = (m_research_subsidy / COIN + 1) * zero_payout_interval * fee_fraction.GetNumerator();
+
+    if (overflow_test > std::numeric_limits<CAmount>::max() / COIN) {
+        arith_uint256 fee_bn = m_research_subsidy;
+        fee_bn *= zero_payout_interval;
+        fee_bn *= fee_fraction.GetNumerator();
+        fee_bn /= mrc_payment_interval;
+        fee_bn /= fee_fraction.GetDenominator();
+
+        fee = fee_bn.GetLow64();
+    } else {
+        fee = m_research_subsidy * zero_payout_interval * fee_fraction.GetNumerator()
+                                 / mrc_payment_interval / fee_fraction.GetDenominator();
+    }
 
     return fee;
 }