feat(kad): improve automatic bootstrap

As discussed in the last maintainer call, some improvements are probably necessary for the automatic bootstrap feature (introduced by #4838). Indeed, like @drHuangMHT mentioned in #5341 and like @guillaumemichel has agreed, triggering a bootstrap every time an update happens inside the routing table consumes a lot more resources.

The idea behind the automatic bootstrap feature it that, when a peer is starting, if a routing table update happens we probably don't want to wait for the periodic bootstrap to trigger and we want to trigger it right now. However, like @guillaumemichel said, this is something we want to do at startup or when a network connectivity problem happens, we don't want to do that all the time.

This PR is a proposal to trigger automatically a bootstrap on routing table update but only when we have less that `K_VALUE` peers in it (meaning that we are starting up or something went wrong and the fact that a new peer is inserted is probably a sign that the network connectivity issue is resolved).

I have also added a new triggering condition like mentioned in the maintainer call. When discovering a new listen address and if we have no connected peers, we trigger a bootstrap. This condition is based on our own experience at Stormshield : some peers were starting before the network interfaces were up, doing so, the automatic and periodic bootstrap failed, but when the network interfaces were finally up, we were waiting X minutes for the periodic bootstrap to actually trigger a bootstrap and join the p2p network.

Pull-Request: #5474.

protocols/kad/CHANGELOG.md

@@ -23,6 +23,10 @@
   See [PR 5347](https://github.com/libp2p/rust-libp2p/pull/5347).
 - Correctly handle the `NoKnownPeers` error on automatic bootstrap.
   See [PR 5349](https://github.com/libp2p/rust-libp2p/pull/5349).
+- Improve automatic bootstrap triggering conditions:
+  trigger when the routing table is updated and we have less that `K_VALUE` peers in it,
+  trigger when a new listen address is discovered and we have no connected peers.
+  See [PR 5474](https://github.com/libp2p/rust-libp2p/pull/5474).
 
 ## 0.45.3
 

protocols/kad/src/behaviour.rs

@@ -23,7 +23,6 @@
 mod test;
 
 use crate::addresses::Addresses;
-use crate::bootstrap;
 use crate::handler::{Handler, HandlerEvent, HandlerIn, RequestId};
 use crate::kbucket::{self, Distance, KBucketsTable, NodeStatus};
 use crate::protocol::{ConnectionType, KadPeer, ProtocolConfig};
@@ -33,6 +32,7 @@ use crate::record::{
     store::{self, RecordStore},
     ProviderRecord, Record,
 };
+use crate::{bootstrap, K_VALUE};
 use crate::{jobs::*, protocol};
 use fnv::FnvHashSet;
 use libp2p_core::{ConnectedPoint, Endpoint, Multiaddr};
@@ -604,7 +604,8 @@ where
                 };
                 match entry.insert(addresses.clone(), status) {
                     kbucket::InsertResult::Inserted => {
-                        self.bootstrap_status.on_new_peer_in_routing_table();
+                        self.bootstrap_on_low_peers();
+
                         self.queued_events.push_back(ToSwarm::GenerateEvent(
                             Event::RoutingUpdated {
                                 peer: *peer,
@@ -1324,7 +1325,8 @@ where
                         let addresses = Addresses::new(a);
                         match entry.insert(addresses.clone(), new_status) {
                             kbucket::InsertResult::Inserted => {
-                                self.bootstrap_status.on_new_peer_in_routing_table();
+                                self.bootstrap_on_low_peers();
+
                                 let event = Event::RoutingUpdated {
                                     peer,
                                     is_new_peer: true,
@@ -1375,6 +1377,20 @@ where
         }
     }
 
+    /// A new peer has been inserted in the routing table but we check if the routing
+    /// table is currently small (less that `K_VALUE` peers are present) and only
+    /// trigger a bootstrap in that case
+    fn bootstrap_on_low_peers(&mut self) {
+        if self
+            .kbuckets()
+            .map(|kbucket| kbucket.num_entries())
+            .sum::<usize>()
+            < K_VALUE.get()
+        {
+            self.bootstrap_status.trigger();
+        }
+    }
+
     /// Handles a finished (i.e. successful) query.
     fn query_finished(&mut self, q: Query) -> Option<Event> {
         let query_id = q.id();
@@ -2613,6 +2629,12 @@ where
             }
             FromSwarm::DialFailure(dial_failure) => self.on_dial_failure(dial_failure),
             FromSwarm::AddressChange(address_change) => self.on_address_change(address_change),
+            FromSwarm::NewListenAddr(_) if self.connected_peers.is_empty() => {
+                // A new listen addr was just discovered and we have no connected peers,
+                // it can mean that our network interfaces were not up but they are now
+                // so it might be a good idea to trigger a bootstrap.
+                self.bootstrap_status.trigger();
+            }
             _ => {}
         }
     }

protocols/kad/src/bootstrap.rs

@@ -44,7 +44,8 @@ impl Status {
         }
     }
 
-    pub(crate) fn on_new_peer_in_routing_table(&mut self) {
+    /// Trigger a bootstrap now or after the configured `automatic_throttle` if configured.
+    pub(crate) fn trigger(&mut self) {
         // Registering `self.throttle_timer` means scheduling a bootstrap.
         // A bootstrap will be triggered when `self.throttle_timer` finishes.
         // A `throttle_timer` is useful to not trigger a batch of bootstraps when a
@@ -201,7 +202,7 @@ mod tests {
             "bootstrap to not be triggered immediately because periodic bootstrap is in ~1s"
         );
 
-        status.on_new_peer_in_routing_table(); // Connected to a new peer though!
+        status.trigger(); // Connected to a new peer though!
         assert!(
             status.next().now_or_never().is_some(),
             "bootstrap to be triggered immediately because we connected to a new peer"
@@ -226,7 +227,7 @@ mod tests {
             "bootstrap to not be triggered immediately because periodic bootstrap is in ~1s"
         );
 
-        status.on_new_peer_in_routing_table(); // Connected to a new peer though!
+        status.trigger(); // Connected to a new peer though!
         assert!(
             status.next().now_or_never().is_none(),
             "bootstrap to not be triggered immediately because throttle is 5ms"
@@ -247,7 +248,7 @@ mod tests {
         // User manually triggered a bootstrap
         do_bootstrap(&mut status);
 
-        status.on_new_peer_in_routing_table(); // Connected to a new peer though!
+        status.trigger(); // Connected to a new peer though!
 
         assert!(
             status.next().now_or_never().is_some(),
@@ -260,7 +261,7 @@ mod tests {
     ) {
         let mut status = Status::new(Some(MS_100), Some(MS_5));
 
-        status.on_new_peer_in_routing_table();
+        status.trigger();
 
         let start = Instant::now();
         await_and_do_bootstrap(&mut status).await;
@@ -280,7 +281,7 @@ mod tests {
     ) {
         let mut status = Status::new(None, Some(Duration::ZERO));
 
-        status.on_new_peer_in_routing_table();
+        status.trigger();
 
         status.next().await;
     }
@@ -304,10 +305,10 @@ mod tests {
     ) {
         let mut status = Status::new(None, Some(MS_100));
 
-        status.on_new_peer_in_routing_table();
+        status.trigger();
         for _ in 0..10 {
             Delay::new(MS_100 / 2).await;
-            status.on_new_peer_in_routing_table(); // should reset throttle_timer
+            status.trigger(); // should reset throttle_timer
         }
         assert!(
             status.next().now_or_never().is_none(),