p2p: extend inbound eviction protection by network to I2P peers

This commit extends our inbound eviction protection to I2P peers to
favorise the diversity of peer connections, as peers connected
through the I2P network are otherwise disadvantaged by our eviction
criteria for their higher latency (higher min ping times) relative
to IPv4 and IPv6 peers, as well as relative to Tor onion peers.

The `networks` array is order-dependent in the case of a tie in
candidate counts between networks (earlier array members receive
priority in the case of a tie).

Therefore, we place I2P candidates before localhost and onion ones
in terms of opportunity to recover unused remaining protected slots
from the previous iteration, guesstimating that most nodes allowing
both onion and I2P inbounds will have more onion peers, followed by
localhost, then I2P, as I2P support is only being added in the
upcoming v22.0 release.

src/net.cpp

@@ -908,16 +908,17 @@ void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& evicti
     // 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.
     // To favorise the diversity of our peer connections, reserve up to half of these protected
-    // spots for Tor/onion and localhost peers, even if they're not longest uptime overall.
+    // spots for Tor/onion, localhost and I2P peers, even if they're not longest uptime overall.
     // This helps protect these higher-latency peers that tend to be otherwise
     // disadvantaged under our eviction criteria.
     const size_t initial_size = eviction_candidates.size();
     const size_t total_protect_size{initial_size / 2};
 
-    // Disadvantaged networks to protect: localhost and Tor/onion. In case of equal counts, earlier
+    // Disadvantaged networks to protect: I2P, localhost, Tor/onion. In case of equal counts, earlier
     // array members have first opportunity to recover unused slots from the previous iteration.
     struct Net { bool is_local; Network id; size_t count; };
-    std::array<Net, 3> networks{{{/* localhost */ true, NET_MAX, 0}, {false, NET_ONION, 0}}};
+    std::array<Net, 3> networks{
+        {{false, NET_I2P, 0}, {/* localhost */ true, NET_MAX, 0}, {false, NET_ONION, 0}}};
 
     // Count and store the number of eviction candidates per network.
     for (Net& n : networks) {

src/net.h

@@ -1236,6 +1236,8 @@ struct NodeEvictionCandidate
  * - localhost peers, as manually configured hidden services not using
  *   `-bind=addr[:port]=onion` will not be detected as inbound onion connections
  *
+ * - I2P 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.

src/test/net_peer_eviction_tests.cpp

@@ -109,10 +109,10 @@ BOOST_AUTO_TEST_CASE(peer_protection_test)
         /* unprotected_peer_ids */ {0, 1, 2, 3, 4, 5},
         random_context));
 
-    // Test protection of onion and localhost peers...
+    // Test protection of onion, localhost, and I2P peers...
 
     // Expect 1/4 onion peers to be protected from eviction,
-    // if no localhost peers.
+    // if no localhost or I2P peers.
     BOOST_CHECK(IsProtected(
         num_peers, [](NodeEvictionCandidate& c) {
             c.m_is_local = false;
@@ -123,7 +123,7 @@ BOOST_AUTO_TEST_CASE(peer_protection_test)
         random_context));
 
     // Expect 1/4 onion peers and 1/4 of the other peers to be protected,
-    // sorted by longest uptime (lowest nTimeConnected), if no localhost peers.
+    // sorted by longest uptime (lowest nTimeConnected), if no localhost or I2P peers.
     BOOST_CHECK(IsProtected(
         num_peers, [](NodeEvictionCandidate& c) {
             c.nTimeConnected = c.id;
@@ -135,7 +135,7 @@ BOOST_AUTO_TEST_CASE(peer_protection_test)
         random_context));
 
     // Expect 1/4 localhost peers to be protected from eviction,
-    // if no onion peers.
+    // if no onion or I2P peers.
     BOOST_CHECK(IsProtected(
         num_peers, [](NodeEvictionCandidate& c) {
             c.m_is_local = (c.id == 1 || c.id == 9 || c.id == 11);
@@ -146,7 +146,7 @@ BOOST_AUTO_TEST_CASE(peer_protection_test)
         random_context));
 
     // Expect 1/4 localhost peers and 1/4 of the other peers to be protected,
-    // sorted by longest uptime (lowest nTimeConnected), if no onion peers.
+    // sorted by longest uptime (lowest nTimeConnected), if no onion or I2P peers.
     BOOST_CHECK(IsProtected(
         num_peers, [](NodeEvictionCandidate& c) {
             c.nTimeConnected = c.id;