P2PServer: ask peers about unknown blocks only once

src/p2p_server.cpp

@@ -52,6 +52,7 @@ P2PServer::P2PServer(p2pool* pool)
 	uv_mutex_init_checked(&m_blockLock);
 	uv_mutex_init_checked(&m_peerListLock);
 	uv_mutex_init_checked(&m_broadcastLock);
+	uv_mutex_init_checked(&m_missingBlockRequestsLock);
 	uv_rwlock_init_checked(&m_cachedBlocksLock);
 
 	int err = uv_async_init(&m_loop, &m_broadcastAsync, on_broadcast);
@@ -97,6 +98,7 @@ P2PServer::~P2PServer()
 	uv_mutex_destroy(&m_blockLock);
 	uv_mutex_destroy(&m_peerListLock);
 	uv_mutex_destroy(&m_broadcastLock);
+	uv_mutex_destroy(&m_missingBlockRequestsLock);
 	uv_rwlock_destroy(&m_cachedBlocksLock);
 
 	delete m_block;
@@ -617,9 +619,26 @@ void P2PServer::download_missing_blocks()
 		clients.emplace_back(client);
 	}
 
+	if (clients.empty()) {
+		return;
+	}
+
 	// Try to download each block from a random client
 	for (const hash& id : missing_blocks) {
-		send(clients[get_random64() % clients.size()],
+		P2PClient* client = clients[get_random64() % clients.size()];
+
+		{
+			MutexLock lock2(m_missingBlockRequestsLock);
+
+			const uint64_t truncated_block_id = *reinterpret_cast<const uint64_t*>(id.h);
+			if (!m_missingBlockRequests.insert({ client->m_peerId, truncated_block_id }).second) {
+				// We already asked this peer about this block
+				// Don't try to ask another peer, leave it for another timer tick
+				continue;
+			}
+		}
+
+		send(client,
 			[this, &id](void* buf)
 			{
 				uint8_t* p0 = reinterpret_cast<uint8_t*>(buf);

src/p2p_server.h

@@ -174,6 +174,9 @@ private:
 	uv_async_t m_broadcastAsync;
 	std::vector<Broadcast*> m_broadcastQueue;
 
+	uv_mutex_t m_missingBlockRequestsLock;
+	std::set<std::pair<uint64_t, uint64_t>> m_missingBlockRequests;
+
 	static void on_broadcast(uv_async_t* handle) { reinterpret_cast<P2PServer*>(handle->data)->on_broadcast(); }
 	void on_broadcast();
 };