feat: add server.maxConnections option

src/index.ts

@@ -29,6 +29,12 @@ export interface TCPOptions {
    * When closing a socket, wait this long for it to close gracefully before it is closed more forcibly
    */
   socketCloseTimeout?: number
+
+  /**
+   * Set this property to reject connections when the server's connection count gets high.
+   * https://nodejs.org/api/net.html#servermaxconnections
+   */
+  maxConnections?: number
 }
 
 /**
@@ -158,6 +164,7 @@ export class TCP implements Transport {
   createListener (options: TCPCreateListenerOptions): Listener {
     return new TCPListener({
       ...options,
+      maxConnections: this.opts.maxConnections,
       socketInactivityTimeout: this.opts.inboundSocketInactivityTimeout,
       socketCloseTimeout: this.opts.socketCloseTimeout
     })

src/listener.ts

@@ -30,6 +30,7 @@ interface Context extends TCPCreateListenerOptions {
   upgrader: Upgrader
   socketInactivityTimeout?: number
   socketCloseTimeout?: number
+  maxConnections?: number
 }
 
 type Status = {started: false} | {started: true, listeningAddr: Multiaddr, peerId: string | null }
@@ -48,6 +49,13 @@ export class TCPListener extends EventEmitter<ListenerEvents> implements Listene
 
     this.server = net.createServer(context, this.onSocket.bind(this))
 
+    // https://nodejs.org/api/net.html#servermaxconnections
+    // If set reject connections when the server's connection count gets high
+    // Useful to prevent too resource exhaustion via many open connections on high bursts of activity
+    if (context.maxConnections !== undefined) {
+      this.server.maxConnections = context.maxConnections
+    }
+
     this.server
       .on('listening', () => this.dispatchEvent(new CustomEvent('listening')))
       .on('error', err => this.dispatchEvent(new CustomEvent<Error>('error', { detail: err })))

test/max-connections.spec.ts

@@ -0,0 +1,79 @@
+import { expect } from 'aegir/chai'
+import net from 'node:net'
+import { promisify } from 'node:util'
+import { mockUpgrader } from '@libp2p/interface-mocks'
+import { multiaddr } from '@multiformats/multiaddr'
+import { TCP } from '../src/index.js'
+
+describe('maxConnections', () => {
+  const afterEachCallbacks: Array<() => Promise<any> | any> = []
+  afterEach(async () => {
+    await Promise.all(afterEachCallbacks.map(fn => fn()))
+    afterEachCallbacks.length = 0
+  })
+
+  it('reject dial of connection above maxConnections', async () => {
+    const maxConnections = 2
+    const socketCount = 4
+    const port = 9900
+
+    const seenRemoteConnections = new Set<string>()
+    const tcp = new TCP({ maxConnections })
+
+    const upgrader = mockUpgrader()
+    const listener = tcp.createListener({ upgrader })
+    // eslint-disable-next-line @typescript-eslint/promise-function-async
+    afterEachCallbacks.push(() => listener.close())
+    await listener.listen(multiaddr(`/ip4/127.0.0.1/tcp/${port}`))
+
+    listener.addEventListener('connection', (conn) => {
+      seenRemoteConnections.add(conn.detail.remoteAddr.toString())
+    })
+
+    const sockets: net.Socket[] = []
+
+    for (let i = 0; i < socketCount; i++) {
+      const socket = net.connect({ port })
+      sockets.push(socket)
+
+      // eslint-disable-next-line @typescript-eslint/promise-function-async
+      afterEachCallbacks.unshift(async () => {
+        if (!socket.destroyed) {
+          socket.destroy()
+          await new Promise((resolve) => socket.on('close', resolve))
+        }
+      })
+
+      // Wait for connection so the order of sockets is stable, sockets expected to be alive are always [0,1]
+      await new Promise<void>((resolve, reject) => {
+        socket.on('connect', () => {
+          resolve()
+        })
+        socket.on('error', (err) => {
+          reject(err)
+        })
+      })
+    }
+
+    // With server.maxConnections the TCP socket is created and the initial handshake is completed
+    // Then in the server handler NodeJS javascript code will call socket.emit('drop') if over the limit
+    // https://github.com/nodejs/node/blob/fddc701d3c0eb4520f2af570876cc987ae6b4ba2/lib/net.js#L1706
+
+    // Wait for some time for server to drop all sockets above limit
+    await promisify(setTimeout)(250)
+
+    expect(seenRemoteConnections.size).equals(maxConnections, 'wrong serverConnections')
+
+    for (let i = 0; i < socketCount; i++) {
+      const socket = sockets[i]
+
+      if (i < maxConnections) {
+        // Assert socket connected
+        expect(socket.destroyed).equals(false, `socket ${i} under limit must not be destroyed`)
+      } else {
+        // Assert socket ended
+        expect(socket.destroyed).equals(true, `socket ${i} above limit must be destroyed`)
+      }
+    }
+  })
+})