Update weight function to fix bias in nomination

docs/stellar-core_example.cfg

@@ -634,6 +634,10 @@ EMIT_SOROBAN_TRANSACTION_META_EXT_V1=false
 # https://github.com/stellar/stellar-xdr/commit/cdc339f5e74a75e8e558fd1a853397da71f1659a
 EMIT_LEDGER_CLOSE_META_EXT_V1=false
 
+# When set to true, Core will revert to using the old, application-agnostic
+# nomination weight function for SCP leader election.
+FORCE_OLD_STYLE_LEADER_ELECTION=false
+
 # EXCLUDE_TRANSACTIONS_CONTAINING_OPERATION_TYPE (list of strings) default is empty
 # Setting this will cause the node to reject transactions that it receives if
 # they contain any operation in this list. It will not, however, stop the node

src/herder/HerderSCPDriver.cpp

@@ -25,6 +25,7 @@
 #include "xdr/Stellar-ledger.h"
 #include <Tracy.hpp>
 #include <algorithm>
+#include <cmath>
 #include <fmt/format.h>
 #include <medida/metrics_registry.h>
 #include <numeric>
@@ -1284,4 +1285,71 @@ HerderSCPDriver::TxSetValidityKeyHash::operator()(
     hashMix(res, std::get<3>(key));
     return res;
 }
+
+uint64
+HerderSCPDriver::getNodeWeight(NodeID const& nodeID, SCPQuorumSet const& qset,
+                               bool const isLocalNode) const
+{
+    Config const& cfg = mApp.getConfig();
+    bool const unsupportedProtocol = protocolVersionIsBefore(
+        mApp.getLedgerManager()
+            .getLastClosedLedgerHeader()
+            .header.ledgerVersion,
+        APPLICATION_SPECIFIC_NOMINATION_LEADER_ELECTION_PROTOCOL_VERSION);
+    if (unsupportedProtocol || !cfg.VALIDATOR_WEIGHT_CONFIG.has_value() ||
+        cfg.FORCE_OLD_STYLE_LEADER_ELECTION)
+    {
+        // Fall back on old weight algorithm if any of the following are true:
+        // 1. The network has not yet upgraded to
+        //    APPLICATION_SPECIFIC_NOMINATION_LEADER_ELECTION_PROTOCOL_VERSION,
+        // 2. The node is using manual quorum set configuration, or
+        // 3. The node has the FORCE_OLD_STYLE_LEADER_ELECTION flag
+        //    set
+        return SCPDriver::getNodeWeight(nodeID, qset, isLocalNode);
+    }
+
+    ValidatorWeightConfig const& vwc =
+        mApp.getConfig().VALIDATOR_WEIGHT_CONFIG.value();
+
+    auto entryIt = vwc.mValidatorEntries.find(nodeID);
+    if (entryIt == vwc.mValidatorEntries.end())
+    {
+        // This shouldn't be possible as the validator entries should contain
+        // all validators in the config. For this to happen, `getNodeWeight`
+        // would have to be called with a non-validator `nodeID`. Throw if
+        // building tests, and otherwise fall back on the old algorithm.
+        throw std::runtime_error(
+            fmt::format(FMT_STRING("Validator entry not found for node {}"),
+                        toShortString(nodeID)));
+    }
+
+    ValidatorEntry const& entry = entryIt->second;
+    auto homeDomainSizeIt = vwc.mHomeDomainSizes.find(entry.mHomeDomain);
+    if (homeDomainSizeIt == vwc.mHomeDomainSizes.end())
+    {
+        // This shouldn't be possible as the home domain sizes should contain
+        // all home domains in the config. For this to happen, `getNodeWeight`
+        // would have to be called with a non-validator, or the config parser
+        // would have to allow a validator without a home domain. Throw if
+        // building tests, and otherwise fall back on the old algorithm.
+        throw std::runtime_error(
+            fmt::format(FMT_STRING("Home domain size not found for domain {}"),
+                        entry.mHomeDomain));
+    }
+
+    auto qualityWeightIt = vwc.mQualityWeights.find(entry.mQuality);
+    if (qualityWeightIt == vwc.mQualityWeights.end())
+    {
+        // This shouldn't be possible as the quality weights should contain all
+        // quality levels in the config.
+        throw std::runtime_error(
+            fmt::format(FMT_STRING("Quality weight not found for quality {}"),
+                        static_cast<int>(entry.mQuality)));
+    }
+
+    // Node's weight is its quality's weight divided by the number of nodes in
+    // its home domain
+    releaseAssert(homeDomainSizeIt->second > 0);
+    return qualityWeightIt->second / homeDomainSizeIt->second;
+}
 }

src/herder/HerderSCPDriver.h

@@ -9,6 +9,7 @@
 #include "herder/TxSetUtils.h"
 #include "medida/timer.h"
 #include "scp/SCPDriver.h"
+#include "util/ProtocolVersion.h"
 #include "util/RandomEvictionCache.h"
 #include "xdr/Stellar-ledger.h"
 #include <optional>
@@ -32,6 +33,11 @@ class VirtualTimer;
 struct StellarValue;
 struct SCPEnvelope;
 
+// First protocol version supporting the application-specific weight function
+// for SCP leader election.
+ProtocolVersion constexpr APPLICATION_SPECIFIC_NOMINATION_LEADER_ELECTION_PROTOCOL_VERSION =
+    ProtocolVersion::V_22;
+
 class HerderSCPDriver : public SCPDriver
 {
   public:
@@ -129,6 +135,14 @@ class HerderSCPDriver : public SCPDriver
 
     Json::Value getQsetLagInfo(bool summary, bool fullKeys);
 
+    // Application-specific weight function. This function uses the quality
+    // levels from automatic quorum set generation to determine the weight of a
+    // validator. It is designed to ensure that:
+    // 1. Orgs of equal quality have equal chances of winning leader election.
+    // 2. Higher quality orgs win more frequently than lower quality orgs.
+    uint64 getNodeWeight(NodeID const& nodeID, SCPQuorumSet const& qset,
+                         bool isLocalNode) const override;
+
   private:
     Application& mApp;
     HerderImpl& mHerder;