Implement incoming ping request handling over maximum inbound limit

- If the maximum inbound connection semaphore reach its limit, `inbound_server` fn will
open a tokio task to check if the node wanted to ping us. If it is the case we respond, otherwise
drop the connection.
- Added some documentation to the `inbound_server` fn.

Author: Asurar0

p2p/p2p/src/constants.rs

@@ -3,6 +3,12 @@ use std::time::Duration;
 /// The timeout we set on handshakes.
 pub(crate) const HANDSHAKE_TIMEOUT: Duration = Duration::from_secs(20);
 
+/// The timeout we set on receiving ping requests
+pub(crate) const PING_REQUEST_TIMEOUT: Duration = Duration::from_secs(5);
+
+/// The amount of concurrency (maximum number of simultaneous tasks) we allow for handling ping requests
+pub(crate) const PING_REQUEST_CONCURRENCY: usize = 2;
+
 /// The maximum amount of connections to make to seed nodes for when we need peers.
 pub(crate) const MAX_SEED_CONNECTIONS: usize = 3;
 

p2p/p2p/src/inbound_server.rs

@@ -4,9 +4,10 @@
 //! them to the handshaker service and then adds them to the client pool.
 use std::{pin::pin, sync::Arc};
 
-use futures::StreamExt;
+use futures::{SinkExt, StreamExt};
 use tokio::{
     sync::Semaphore,
+    task::JoinSet,
     time::{sleep, timeout},
 };
 use tower::{Service, ServiceExt};
@@ -17,14 +18,22 @@ use cuprate_p2p_core::{
     services::{AddressBookRequest, AddressBookResponse},
     AddressBook, ConnectionDirection, NetworkZone,
 };
+use cuprate_wire::{
+    admin::{PingResponse, PING_OK_RESPONSE_STATUS_TEXT},
+    AdminRequestMessage, AdminResponseMessage, BucketError, Message,
+};
 
 use crate::{
     client_pool::ClientPool,
-    constants::{HANDSHAKE_TIMEOUT, INBOUND_CONNECTION_COOL_DOWN},
+    constants::{
+        HANDSHAKE_TIMEOUT, INBOUND_CONNECTION_COOL_DOWN, PING_REQUEST_CONCURRENCY,
+        PING_REQUEST_TIMEOUT,
+    },
     P2PConfig,
 };
 
-/// Starts the inbound server.
+/// Starts the inbound server. This function will listen to all incoming connections
+/// and initiate handshake if needed, after verifying the address isn't banned.
 #[instrument(level = "warn", skip_all)]
 pub async fn inbound_server<N, HS, A>(
     client_pool: Arc<ClientPool<N>>,
@@ -40,6 +49,10 @@ where
     HS::Future: Send + 'static,
     A: AddressBook<N>,
 {
+    // Copying the peer_id before borrowing for ping responses (Make us avoid a `clone()`).
+    let our_peer_id = config.basic_node_data().peer_id;
+
+    // Mandatory. Extract server config from P2PConfig
     let Some(server_config) = config.server_config else {
         tracing::warn!("No inbound server config provided, not listening for inbound connections.");
         return Ok(());
@@ -53,13 +66,18 @@ where
 
     let mut listener = pin!(listener);
 
+    // Create semaphore for limiting to maximum inbound connections.
     let semaphore = Arc::new(Semaphore::new(config.max_inbound_connections));
+    // Create ping request handling JoinSet
+    let mut ping_join_set = JoinSet::new();
 
+    // Listen to incoming connections and extract necessary information.
     while let Some(connection) = listener.next().await {
-        let Ok((addr, peer_stream, peer_sink)) = connection else {
+        let Ok((addr, mut peer_stream, mut peer_sink)) = connection else {
             continue;
         };
 
+        // If peer is banned, drop connection
         if let Some(addr) = &addr {
             let AddressBookResponse::IsPeerBanned(banned) = address_book
                 .ready()
@@ -75,11 +93,13 @@ where
             }
         }
 
+        // Create a new internal id for new peers
         let addr = match addr {
             Some(addr) => InternalPeerID::KnownAddr(addr),
             None => InternalPeerID::Unknown(rand::random()),
         };
 
+        // If we're still behind our maximum limit, Initiate handshake.
         if let Ok(permit) = semaphore.clone().try_acquire_owned() {
             tracing::debug!("Permit free for incoming connection, attempting handshake.");
 
@@ -102,8 +122,34 @@ where
                 .instrument(Span::current()),
             );
         } else {
+            // Otherwise check if the node is simply pinging us.
             tracing::debug!("No permit free for incoming connection.");
-            // TODO: listen for if the peer is just trying to ping us to see if we are reachable.
+
+            // We only handle 2 ping request conccurently. Otherwise we drop the connection immediately.
+            if ping_join_set.len() < PING_REQUEST_CONCURRENCY {
+                ping_join_set.spawn(
+                    async move {
+                        // Await first message from node. If it is a ping request we respond back, otherwise we drop the connection.
+                        let fut = timeout(PING_REQUEST_TIMEOUT, peer_stream.next());
+
+                        // Ok if timeout did not elapsed -> Some if there is a message -> Ok if it has been decoded
+                        if let Ok(Some(Ok(Message::Request(AdminRequestMessage::Ping)))) = fut.await {
+                            let response = peer_sink
+                                .send(Message::Response(AdminResponseMessage::Ping(PingResponse {
+                                        status: PING_OK_RESPONSE_STATUS_TEXT,
+                                        peer_id: our_peer_id
+                                    }
+                                )).into())
+                                .await;
+                            
+                            if let Err(err) = response {
+                                tracing::debug!("Unable to respond to ping request from peer ({addr}): {err}")    
+                            }
+                        }
+                    }
+                    .instrument(Span::current())
+                );
+            }
         }
 
         sleep(INBOUND_CONNECTION_COOL_DOWN).await;