Merge bitcoin/bitcoin#22284: p2p, refactor: performance improvements to ProtectEvictionCandidatesByRatio()

b1d905c225e87a4a289c0cd3593c6c21cea3fba7 p2p: earlier continuation when no remaining eviction candidates (Vasil Dimov)
c9e8d8f9b168dec2bc7b845da38449e96708cf8e p2p: process more candidates per protection iteration (Jon Atack)
02e411ec456af80d1da76085a814c68bb3aca6de p2p: iterate eviction protection only on networks having candidates (Jon Atack)
5adb06457403f8c1d874e9c6748ecbb78ef8fa2b bench: add peer eviction protection benchmarks (Jon Atack)
566357f8f7471f74729297868917aa32f6d3c390 refactor: move GetRandomNodeEvictionCandidates() to test utilities (Jon Atack)

Pull request description:

  This follow-up to #21261 improves `ProtectEvictionCandidatesByRatio()` for better performance.

  Benchmarks are added; the performance improvement is between 2x and 5x for the benchmarked cases (CPU 2.50GHz, Turbo off, performance mode, Debian Clang 11 non-debug build).

  ```
  $ ./src/bench/bench_bitcoin -filter="EvictionProtection*.*"
  ```

  The refactored code is well-covered by existing unit tests and also a fuzzer.

  - `$ ./src/test/test_bitcoin -t net_peer_eviction_tests`
  - `$ FUZZ=node_eviction ./src/test/fuzz/fuzz ../qa-assets/fuzz_seed_corpus/node_eviction`

ACKs for top commit:
  klementtan:
    Tested and code review ACK b1d905c2.
  vasild:
    ACK b1d905c225e87a4a289c0cd3593c6c21cea3fba7
  jarolrod:
    ACK b1d905c225e87a4a289c0cd3593c6c21cea3fba7

Tree-SHA512: a3a6607b9ea2fec138da9780c03f63e177b6712091c5a3ddc3804b896a7585216446310280791f5e20cc023d02d2f03a4139237e12b5c1d7f2a1fa1011610e96

1 files changed, 10 insertions, 5 deletions

diff --git a/src/net.cpp b/src/net.cpp
index 0078d71142..70ba875c4b 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -937,14 +937,17 @@ void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& evicti
     size_t num_protected{0};
 
     while (num_protected < max_protect_by_network) {
+        // Count the number of disadvantaged networks from which we have peers to protect.
+        auto num_networks = std::count_if(networks.begin(), networks.end(), [](const Net& n) { return n.count; });
+        if (num_networks == 0) {
+            break;
+        }
         const size_t disadvantaged_to_protect{max_protect_by_network - num_protected};
-        const size_t protect_per_network{
-            std::max(disadvantaged_to_protect / networks.size(), static_cast<size_t>(1))};
-
+        const size_t protect_per_network{std::max(disadvantaged_to_protect / num_networks, static_cast<size_t>(1))};
         // Early exit flag if there are no remaining candidates by disadvantaged network.
         bool protected_at_least_one{false};
 
-        for (const Net& n : networks) {
+        for (Net& n : networks) {
             if (n.count == 0) continue;
             const size_t before = eviction_candidates.size();
             EraseLastKElements(eviction_candidates, CompareNodeNetworkTime(n.is_local, n.id),
@@ -954,10 +957,12 @@ void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& evicti
             const size_t after = eviction_candidates.size();
             if (before > after) {
                 protected_at_least_one = true;
-                num_protected += before - after;
+                const size_t delta{before - after};
+                num_protected += delta;
                 if (num_protected >= max_protect_by_network) {
                     break;
                 }
+                n.count -= delta;
             }
         }
         if (!protected_at_least_one) {