Merge #20477: net: Add unit testing of node eviction logic

fee88237e03c21bf81f21098e6b89ecfa5327cee Assert eviction at >= 29 candidates. Assert non-eviction at <= 20 candidates. (practicalswift)
685c428de0fb63ca6ec1419bb112f07d27bcdf14 test: Add unit testing of node eviction logic (practicalswift)
ed73f8cee0d7b7facbd2e8dde24a237f20c48c0c net: Move eviction node selection logic to SelectNodeToEvict(...) (practicalswift)

Pull request description:

  Add unit testing of node eviction logic.

  Closes #19966.

ACKs for top commit:
  jonatack:
    ACK fee88237e03c21bf81f21098e6b89ecfa5327cee
  MarcoFalke:
    ACK fee88237e03c21bf81f21098e6b89ecfa5327cee 🐼

Tree-SHA512: 0827c35609122ca42bfabb17feaaee35c191ab4dc2e428282af425a6c176eaeaff2789b74a4f7eb4ca6b8cb10603068e90ca800e6ef3bc3904d50e76616f7a2b

3 files changed, 210 insertions, 54 deletions

diff --git a/src/net.cpp b/src/net.cpp
index 66bf9b0315..db4b541ca7 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -16,6 +16,7 @@
 #include <net_permissions.h>
 #include <netbase.h>
 #include <node/ui_interface.h>
+#include <optional.h>
 #include <protocol.h>
 #include <random.h>
 #include <scheduler.h>
@@ -844,21 +845,6 @@ size_t CConnman::SocketSendData(CNode *pnode) const EXCLUSIVE_LOCKS_REQUIRED(pno
     return nSentSize;
 }
 
-struct NodeEvictionCandidate
-{
-    NodeId id;
-    int64_t nTimeConnected;
-    int64_t nMinPingUsecTime;
-    int64_t nLastBlockTime;
-    int64_t nLastTXTime;
-    bool fRelevantServices;
-    bool fRelayTxes;
-    bool fBloomFilter;
-    uint64_t nKeyedNetGroup;
-    bool prefer_evict;
-    bool m_is_local;
-};
-
 static bool ReverseCompareNodeMinPingTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
 {
     return a.nMinPingUsecTime > b.nMinPingUsecTime;
@@ -914,43 +900,8 @@ static void EraseLastKElements(std::vector<T> &elements, Comparator comparator,
     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.
- *  The strategy used here is to protect a small number of peers
- *   for each of several distinct characteristics which are difficult
- *   to forge.  In order to partition a node the attacker must be
- *   simultaneously better at all of them than honest peers.
- */
-bool CConnman::AttemptToEvictConnection()
+[[nodiscard]] Optional<NodeId> SelectNodeToEvict(std::vector<NodeEvictionCandidate>&& vEvictionCandidates)
 {
-    std::vector<NodeEvictionCandidate> vEvictionCandidates;
-    {
-        LOCK(cs_vNodes);
-
-        for (const CNode* node : vNodes) {
-            if (node->HasPermission(PF_NOBAN))
-                continue;
-            if (!node->IsInboundConn())
-                continue;
-            if (node->fDisconnect)
-                continue;
-            bool peer_relay_txes = false;
-            bool peer_filter_not_null = false;
-            if (node->m_tx_relay != nullptr) {
-                LOCK(node->m_tx_relay->cs_filter);
-                peer_relay_txes = node->m_tx_relay->fRelayTxes;
-                peer_filter_not_null = node->m_tx_relay->pfilter != nullptr;
-            }
-            NodeEvictionCandidate candidate = {node->GetId(), node->nTimeConnected, node->nMinPingUsecTime,
-                                               node->nLastBlockTime, node->nLastTXTime,
-                                               HasAllDesirableServiceFlags(node->nServices),
-                                               peer_relay_txes, peer_filter_not_null, node->nKeyedNetGroup,
-                                               node->m_prefer_evict, node->addr.IsLocal()};
-            vEvictionCandidates.push_back(candidate);
-        }
-    }
-
     // Protect connections with certain characteristics
 
     // Deterministically select 4 peers to protect by netgroup.
@@ -988,7 +939,7 @@ bool CConnman::AttemptToEvictConnection()
     total_protect_size -= initial_size - vEvictionCandidates.size();
     EraseLastKElements(vEvictionCandidates, ReverseCompareNodeTimeConnected, total_protect_size);
 
-    if (vEvictionCandidates.empty()) return false;
+    if (vEvictionCandidates.empty()) return nullopt;
 
     // If any remaining peers are preferred for eviction consider only them.
     // This happens after the other preferences since if a peer is really the best by other criteria (esp relaying blocks)
@@ -1020,10 +971,52 @@ bool CConnman::AttemptToEvictConnection()
     vEvictionCandidates = std::move(mapNetGroupNodes[naMostConnections]);
 
     // Disconnect from the network group with the most connections
-    NodeId evicted = vEvictionCandidates.front().id;
+    return vEvictionCandidates.front().id;
+}
+
+/** 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.
+ *  The strategy used here is to protect a small number of peers
+ *   for each of several distinct characteristics which are difficult
+ *   to forge.  In order to partition a node the attacker must be
+ *   simultaneously better at all of them than honest peers.
+ */
+bool CConnman::AttemptToEvictConnection()
+{
+    std::vector<NodeEvictionCandidate> vEvictionCandidates;
+    {
+
+        LOCK(cs_vNodes);
+        for (const CNode* node : vNodes) {
+            if (node->HasPermission(PF_NOBAN))
+                continue;
+            if (!node->IsInboundConn())
+                continue;
+            if (node->fDisconnect)
+                continue;
+            bool peer_relay_txes = false;
+            bool peer_filter_not_null = false;
+            if (node->m_tx_relay != nullptr) {
+                LOCK(node->m_tx_relay->cs_filter);
+                peer_relay_txes = node->m_tx_relay->fRelayTxes;
+                peer_filter_not_null = node->m_tx_relay->pfilter != nullptr;
+            }
+            NodeEvictionCandidate candidate = {node->GetId(), node->nTimeConnected, node->nMinPingUsecTime,
+                                               node->nLastBlockTime, node->nLastTXTime,
+                                               HasAllDesirableServiceFlags(node->nServices),
+                                               peer_relay_txes, peer_filter_not_null, node->nKeyedNetGroup,
+                                               node->m_prefer_evict, node->addr.IsLocal()};
+            vEvictionCandidates.push_back(candidate);
+        }
+    }
+    const Optional<NodeId> node_id_to_evict = SelectNodeToEvict(std::move(vEvictionCandidates));
+    if (!node_id_to_evict) {
+        return false;
+    }
     LOCK(cs_vNodes);
     for (CNode* pnode : vNodes) {
-        if (pnode->GetId() == evicted) {
+        if (pnode->GetId() == *node_id_to_evict) {
             pnode->fDisconnect = true;
             return true;
         }
diff --git a/src/net.h b/src/net.h
index 20e356562b..278e539f8d 100644
--- a/src/net.h
+++ b/src/net.h
@@ -33,6 +33,7 @@
 #include <map>
 #include <memory>
 #include <thread>
+#include <vector>
 
 class CScheduler;
 class CNode;
@@ -1239,4 +1240,21 @@ inline std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now,
     return std::chrono::microseconds{PoissonNextSend(now.count(), average_interval.count())};
 }
 
+struct NodeEvictionCandidate
+{
+    NodeId id;
+    int64_t nTimeConnected;
+    int64_t nMinPingUsecTime;
+    int64_t nLastBlockTime;
+    int64_t nLastTXTime;
+    bool fRelevantServices;
+    bool fRelayTxes;
+    bool fBloomFilter;
+    uint64_t nKeyedNetGroup;
+    bool prefer_evict;
+    bool m_is_local;
+};
+
+[[nodiscard]] Optional<NodeId> SelectNodeToEvict(std::vector<NodeEvictionCandidate>&& vEvictionCandidates);
+
 #endif // BITCOIN_NET_H
diff --git a/src/test/net_tests.cpp b/src/test/net_tests.cpp
index c86974ba5b..beac65942e 100644
--- a/src/test/net_tests.cpp
+++ b/src/test/net_tests.cpp
@@ -9,6 +9,7 @@
 #include <cstdint>
 #include <net.h>
 #include <netbase.h>
+#include <optional.h>
 #include <serialize.h>
 #include <span.h>
 #include <streams.h>
@@ -21,6 +22,7 @@
 
 #include <boost/test/unit_test.hpp>
 
+#include <algorithm>
 #include <ios>
 #include <memory>
 #include <string>
@@ -781,4 +783,147 @@ BOOST_AUTO_TEST_CASE(PoissonNextSend)
     g_mock_deterministic_tests = false;
 }
 
+std::vector<NodeEvictionCandidate> GetRandomNodeEvictionCandidates(const int n_candidates, FastRandomContext& random_context)
+{
+    std::vector<NodeEvictionCandidate> candidates;
+    for (int id = 0; id < n_candidates; ++id) {
+        candidates.push_back({
+            /* id */ id,
+            /* nTimeConnected */ static_cast<int64_t>(random_context.randrange(100)),
+            /* nMinPingUsecTime */ static_cast<int64_t>(random_context.randrange(100)),
+            /* nLastBlockTime */ static_cast<int64_t>(random_context.randrange(100)),
+            /* nLastTXTime */ static_cast<int64_t>(random_context.randrange(100)),
+            /* fRelevantServices */ random_context.randbool(),
+            /* fRelayTxes */ random_context.randbool(),
+            /* fBloomFilter */ random_context.randbool(),
+            /* nKeyedNetGroup */ random_context.randrange(100),
+            /* prefer_evict */ random_context.randbool(),
+            /* m_is_local */ random_context.randbool(),
+        });
+    }
+    return candidates;
+}
+
+// Returns true if any of the node ids in node_ids are selected for eviction.
+bool IsEvicted(std::vector<NodeEvictionCandidate> candidates, const std::vector<NodeId>& node_ids, FastRandomContext& random_context)
+{
+    Shuffle(candidates.begin(), candidates.end(), random_context);
+    const Optional<NodeId> evicted_node_id = SelectNodeToEvict(std::move(candidates));
+    if (!evicted_node_id) {
+        return false;
+    }
+    return std::find(node_ids.begin(), node_ids.end(), *evicted_node_id) != node_ids.end();
+}
+
+// Create number_of_nodes random nodes, apply setup function candidate_setup_fn,
+// apply eviction logic and then return true if any of the node ids in node_ids
+// are selected for eviction.
+bool IsEvicted(const int number_of_nodes, std::function<void(NodeEvictionCandidate&)> candidate_setup_fn, const std::vector<NodeId>& node_ids, FastRandomContext& random_context)
+{
+    std::vector<NodeEvictionCandidate> candidates = GetRandomNodeEvictionCandidates(number_of_nodes, random_context);
+    for (NodeEvictionCandidate& candidate : candidates) {
+        candidate_setup_fn(candidate);
+    }
+    return IsEvicted(candidates, node_ids, random_context);
+}
+
+namespace {
+constexpr int NODE_EVICTION_TEST_ROUNDS{10};
+constexpr int NODE_EVICTION_TEST_UP_TO_N_NODES{200};
+} // namespace
+
+BOOST_AUTO_TEST_CASE(node_eviction_test)
+{
+    FastRandomContext random_context{true};
+
+    for (int i = 0; i < NODE_EVICTION_TEST_ROUNDS; ++i) {
+        for (int number_of_nodes = 0; number_of_nodes < NODE_EVICTION_TEST_UP_TO_N_NODES; ++number_of_nodes) {
+            // Four nodes with the highest keyed netgroup values should be
+            // protected from eviction.
+            BOOST_CHECK(!IsEvicted(
+                number_of_nodes, [number_of_nodes](NodeEvictionCandidate& candidate) {
+                    candidate.nKeyedNetGroup = number_of_nodes - candidate.id;
+                },
+                {0, 1, 2, 3}, random_context));
+
+            // Eight nodes with the lowest minimum ping time should be protected
+            // from eviction.
+            BOOST_CHECK(!IsEvicted(
+                number_of_nodes, [](NodeEvictionCandidate& candidate) {
+                    candidate.nMinPingUsecTime = candidate.id;
+                },
+                {0, 1, 2, 3, 4, 5, 6, 7}, random_context));
+
+            // Four nodes that most recently sent us novel transactions accepted
+            // into our mempool should be protected from eviction.
+            BOOST_CHECK(!IsEvicted(
+                number_of_nodes, [number_of_nodes](NodeEvictionCandidate& candidate) {
+                    candidate.nLastTXTime = number_of_nodes - candidate.id;
+                },
+                {0, 1, 2, 3}, random_context));
+
+            // Up to eight non-tx-relay peers that most recently sent us novel
+            // blocks should be protected from eviction.
+            BOOST_CHECK(!IsEvicted(
+                number_of_nodes, [number_of_nodes](NodeEvictionCandidate& candidate) {
+                    candidate.nLastBlockTime = number_of_nodes - candidate.id;
+                    if (candidate.id <= 7) {
+                        candidate.fRelayTxes = false;
+                        candidate.fRelevantServices = true;
+                    }
+                },
+                {0, 1, 2, 3, 4, 5, 6, 7}, random_context));
+
+            // Four peers that most recently sent us novel blocks should be
+            // protected from eviction.
+            BOOST_CHECK(!IsEvicted(
+                number_of_nodes, [number_of_nodes](NodeEvictionCandidate& candidate) {
+                    candidate.nLastBlockTime = number_of_nodes - candidate.id;
+                },
+                {0, 1, 2, 3}, random_context));
+
+            // Combination of the previous two tests.
+            BOOST_CHECK(!IsEvicted(
+                number_of_nodes, [number_of_nodes](NodeEvictionCandidate& candidate) {
+                    candidate.nLastBlockTime = number_of_nodes - candidate.id;
+                    if (candidate.id <= 7) {
+                        candidate.fRelayTxes = false;
+                        candidate.fRelevantServices = true;
+                    }
+                },
+                {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}, random_context));
+
+            // Combination of all tests above.
+            BOOST_CHECK(!IsEvicted(
+                number_of_nodes, [number_of_nodes](NodeEvictionCandidate& candidate) {
+                    candidate.nKeyedNetGroup = number_of_nodes - candidate.id; // 4 protected
+                    candidate.nMinPingUsecTime = candidate.id;                 // 8 protected
+                    candidate.nLastTXTime = number_of_nodes - candidate.id;    // 4 protected
+                    candidate.nLastBlockTime = number_of_nodes - candidate.id; // 4 protected
+                },
+                {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19}, random_context));
+
+            // An eviction is expected given >= 29 random eviction candidates. The eviction logic protects at most
+            // four peers by net group, eight by lowest ping time, four by last time of novel tx, up to eight non-tx-relay
+            // peers by last novel block time, and four more peers by last novel block time.
+            if (number_of_nodes >= 29) {
+                BOOST_CHECK(SelectNodeToEvict(GetRandomNodeEvictionCandidates(number_of_nodes, random_context)));
+            }
+
+            // No eviction is expected given <= 20 random eviction candidates. The eviction logic protects at least
+            // four peers by net group, eight by lowest ping time, four by last time of novel tx and four peers by last
+            // novel block time.
+            if (number_of_nodes <= 20) {
+                BOOST_CHECK(!SelectNodeToEvict(GetRandomNodeEvictionCandidates(number_of_nodes, random_context)));
+            }
+
+            // Cases left to test:
+            // * "Protect the half of the remaining nodes which have been connected the longest. [...]"
+            // * "Pick out up to 1/4 peers that are localhost, sorted by longest uptime. [...]"
+            // * "If any remaining peers are preferred for eviction consider only them. [...]"
+            // * "Identify the network group with the most connections and youngest member. [...]"
+        }
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()