[net] Move eviction logic to its own file

src/node/eviction.h

@@ -0,0 +1,69 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_NODE_EVICTION_H
+#define BITCOIN_NODE_EVICTION_H
+
+#include <node/connection_types.h>
+#include <net_permissions.h>
+
+#include <chrono>
+#include <cstdint>
+#include <optional>
+#include <vector>
+
+typedef int64_t NodeId;
+
+struct NodeEvictionCandidate {
+    NodeId id;
+    std::chrono::seconds m_connected;
+    std::chrono::microseconds m_min_ping_time;
+    std::chrono::seconds m_last_block_time;
+    std::chrono::seconds m_last_tx_time;
+    bool fRelevantServices;
+    bool m_relay_txs;
+    bool fBloomFilter;
+    uint64_t nKeyedNetGroup;
+    bool prefer_evict;
+    bool m_is_local;
+    Network m_network;
+    bool m_noban;
+    ConnectionType m_conn_type;
+};
+
+/**
+ * Select an inbound peer to evict after filtering out (protecting) peers having
+ * distinct, difficult-to-forge characteristics. The protection logic picks out
+ * fixed numbers of desirable peers per various criteria, followed by (mostly)
+ * ratios of desirable or disadvantaged peers. If any eviction candidates
+ * remain, the selection logic chooses a peer to evict.
+ */
+[[nodiscard]] std::optional<NodeId> SelectNodeToEvict(std::vector<NodeEvictionCandidate>&& vEvictionCandidates);
+
+/** Protect desirable or disadvantaged inbound peers from eviction by ratio.
+ *
+ * This function protects half of the peers which have been connected the
+ * longest, to replicate the non-eviction implicit behavior and preclude attacks
+ * that start later.
+ *
+ * Half of these protected spots (1/4 of the total) are reserved for the
+ * following categories of peers, sorted by longest uptime, even if they're not
+ * longest uptime overall:
+ *
+ * - onion peers connected via our tor control service
+ *
+ * - localhost peers, as manually configured hidden services not using
+ *   `-bind=addr[:port]=onion` will not be detected as inbound onion connections
+ *
+ * - I2P peers
+ *
+ * - CJDNS peers
+ *
+ * This helps protect these privacy network peers, which tend to be otherwise
+ * disadvantaged under our eviction criteria for their higher min ping times
+ * relative to IPv4/IPv6 peers, and favorise the diversity of peer connections.
+ */
+void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& vEvictionCandidates);
+
+#endif // BITCOIN_NODE_EVICTION_H