Merge bitcoin#11524: [net] De-duplicate connection eviction logic

5ce7cb9 [net] De-duplicate connection eviction logic (Thomas Snider)

Pull request description:

  While reviewing the safeguards against deliberate node isolation on the network by malicious actors, I found a good de-duplication candidate.

  I think this form is much more legible (the type of `cutoffs` notwithstanding).  ReverseCompareNodeTimeConnected is not included in the list since the cutoff size is a function of the remaining number of nodes in the candidate eviction set.

Tree-SHA512: ed17999fa9250dcf8448329219324477117e4ecd2d41dedd72ad253e44630eef50b3232c420f1862ebbfb9b8c94efbba1a235b519e39ff5946865c7d69a75280

src/net.cpp

@@ -1007,6 +1007,16 @@ static bool CompareNodeTXTime(const NodeEvictionCandidate &a, const NodeEviction
     return a.nTimeConnected > b.nTimeConnected;
 }
 
+
+//! Sort an array by the specified comparator, then erase the last K elements.
+template<typename T, typename Comparator>
+static void EraseLastKElements(std::vector<T> &elements, Comparator comparator, size_t k)
+{
+    std::sort(elements.begin(), elements.end(), comparator);
+    size_t eraseSize = std::min(k, elements.size());
+    elements.erase(elements.end() - eraseSize, elements.end());
+}
+
 /** Try to find a connection to evict when the node is full.
  *  Extreme care must be taken to avoid opening the node to attacker
  *   triggered network partitioning.
@@ -1056,42 +1066,23 @@ bool CConnman::AttemptToEvictConnection()
         }
     }
 
-    if (vEvictionCandidates.empty()) return false;
-
     // Protect connections with certain characteristics
 
     // Deterministically select 4 peers to protect by netgroup.
     // An attacker cannot predict which netgroups will be protected
-    std::sort(vEvictionCandidates.begin(), vEvictionCandidates.end(), CompareNetGroupKeyed);
-    vEvictionCandidates.erase(vEvictionCandidates.end() - std::min(4, static_cast<int>(vEvictionCandidates.size())), vEvictionCandidates.end());
-
-    if (vEvictionCandidates.empty()) return false;
-
+    EraseLastKElements(vEvictionCandidates, CompareNetGroupKeyed, 4);
     // Protect the 8 nodes with the lowest minimum ping time.
     // An attacker cannot manipulate this metric without physically moving nodes closer to the target.
-    std::sort(vEvictionCandidates.begin(), vEvictionCandidates.end(), ReverseCompareNodeMinPingTime);
-    vEvictionCandidates.erase(vEvictionCandidates.end() - std::min(8, static_cast<int>(vEvictionCandidates.size())), vEvictionCandidates.end());
-
-    if (vEvictionCandidates.empty()) return false;
-
+    EraseLastKElements(vEvictionCandidates, ReverseCompareNodeMinPingTime, 8);
     // Protect 4 nodes that most recently sent us transactions.
     // An attacker cannot manipulate this metric without performing useful work.
-    std::sort(vEvictionCandidates.begin(), vEvictionCandidates.end(), CompareNodeTXTime);
-    vEvictionCandidates.erase(vEvictionCandidates.end() - std::min(4, static_cast<int>(vEvictionCandidates.size())), vEvictionCandidates.end());
-
-    if (vEvictionCandidates.empty()) return false;
-
+    EraseLastKElements(vEvictionCandidates, CompareNodeTXTime, 4);
     // Protect 4 nodes that most recently sent us blocks.
     // An attacker cannot manipulate this metric without performing useful work.
-    std::sort(vEvictionCandidates.begin(), vEvictionCandidates.end(), CompareNodeBlockTime);
-    vEvictionCandidates.erase(vEvictionCandidates.end() - std::min(4, static_cast<int>(vEvictionCandidates.size())), vEvictionCandidates.end());
-
-    if (vEvictionCandidates.empty()) return false;
-
+    EraseLastKElements(vEvictionCandidates, CompareNodeBlockTime, 4);
     // Protect the half of the remaining nodes which have been connected the longest.
     // This replicates the non-eviction implicit behavior, and precludes attacks that start later.
-    std::sort(vEvictionCandidates.begin(), vEvictionCandidates.end(), ReverseCompareNodeTimeConnected);
-    vEvictionCandidates.erase(vEvictionCandidates.end() - static_cast<int>(vEvictionCandidates.size() / 2), vEvictionCandidates.end());
+    EraseLastKElements(vEvictionCandidates, ReverseCompareNodeTimeConnected, vEvictionCandidates.size() / 2);
 
     if (vEvictionCandidates.empty()) return false;
 
@@ -1102,12 +1093,12 @@ bool CConnman::AttemptToEvictConnection()
     int64_t nMostConnectionsTime = 0;
     std::map<uint64_t, std::vector<NodeEvictionCandidate> > mapNetGroupNodes;
     for (const NodeEvictionCandidate &node : vEvictionCandidates) {
-        mapNetGroupNodes[node.nKeyedNetGroup].push_back(node);
-        int64_t grouptime = mapNetGroupNodes[node.nKeyedNetGroup][0].nTimeConnected;
-        size_t groupsize = mapNetGroupNodes[node.nKeyedNetGroup].size();
+        std::vector<NodeEvictionCandidate> &group = mapNetGroupNodes[node.nKeyedNetGroup];
+        group.push_back(node);
+        int64_t grouptime = group[0].nTimeConnected;
 
-        if (groupsize > nMostConnections || (groupsize == nMostConnections && grouptime > nMostConnectionsTime)) {
-            nMostConnections = groupsize;
+        if (group.size() > nMostConnections || (group.size() == nMostConnections && grouptime > nMostConnectionsTime)) {
+            nMostConnections = group.size();
             nMostConnectionsTime = grouptime;
             naMostConnections = node.nKeyedNetGroup;
         }