Improve amendment processing and activation logic:

* The amendment ballot counting code contained a minor technical
  flaw, caused by the use of integer arithmetic and rounding
  semantics, that could allow amendments to reach majority with
  slightly less than 80% support. This commit introduces an
  amendment which, if enabled, will ensure that activation
  requires at least 80% support.
* This commit also introduces a configuration option to adjust
  the amendment activation hysteresis. This option is useful on
  test networks, but should not be used on the main network as
  is a network-wide consensus parameter that should not be
  changed on a per-server basis; doing so can result in a
  hard-fork.

Fixes #3396

src/ripple/protocol/Feature.h

@@ -111,7 +111,8 @@ class FeatureCollections
         "RequireFullyCanonicalSig",
         "fix1781",  // XRPEndpointSteps should be included in the circular
                     // payment check
-        "HardenedValidations"};
+        "HardenedValidations",
+        "fixAmendmentMajorityCalc"};  // Fix Amendment majority calculation
 
     std::vector<uint256> features;
     boost::container::flat_map<uint256, std::size_t> featureToIndex;
@@ -367,6 +368,7 @@ extern uint256 const fixQualityUpperBound;
 extern uint256 const featureRequireFullyCanonicalSig;
 extern uint256 const fix1781;
 extern uint256 const featureHardenedValidations;
+extern uint256 const fixAmendmentMajorityCalc;
 
 }  // namespace ripple
 

src/ripple/protocol/SystemParameters.h

@@ -21,6 +21,7 @@
 #define RIPPLE_PROTOCOL_SYSTEMPARAMETERS_H_INCLUDED
 
 #include <ripple/basics/XRPAmount.h>
+#include <ripple/basics/chrono.h>
 #include <cstdint>
 #include <string>
 
@@ -59,6 +60,18 @@ systemCurrencyCode()
 /** The XRP ledger network's earliest allowed sequence */
 static std::uint32_t constexpr XRP_LEDGER_EARLIEST_SEQ{32570};
 
+/** The minimum amount of support an amendment should have.
+
+    @note This value is used by legacy code and will become obsolete
+          once the fixAmendmentMajorityCalc amendment activates.
+*/
+constexpr std::ratio<204, 256> preFixAmendmentMajorityCalcThreshold;
+
+constexpr std::ratio<80, 100> postFixAmendmentMajorityCalcThreshold;
+
+/** The minimum amount of time an amendment must hold a majority */
+constexpr std::chrono::seconds const defaultAmendmentMajorityTime = weeks{2};
+
 }  // namespace ripple
 
 /** Default peer port (IANA registered) */

src/ripple/protocol/impl/Feature.cpp

@@ -130,7 +130,8 @@ detail::supportedAmendments()
         "fixQualityUpperBound",
         "RequireFullyCanonicalSig",
         "fix1781",
-        "HardenedValidations"};
+        "HardenedValidations",
+        "fixAmendmentMajorityCalc"};
     return supported;
 }
 
@@ -181,7 +182,8 @@ uint256 const
     fixQualityUpperBound            = *getRegisteredFeature("fixQualityUpperBound"),
     featureRequireFullyCanonicalSig = *getRegisteredFeature("RequireFullyCanonicalSig"),
     fix1781                         = *getRegisteredFeature("fix1781"),
-    featureHardenedValidations      = *getRegisteredFeature("HardenedValidations");
+    featureHardenedValidations      = *getRegisteredFeature("HardenedValidations"),
+    fixAmendmentMajorityCalc        = *getRegisteredFeature("fixAmendmentMajorityCalc");
 
 // The following amendments have been active for at least two years. Their
 // pre-amendment code has been removed and the identifiers are deprecated.