feat(pool): forward connect and disconnect events (#510)

Make the `Pool` forward the `connect` and `disconnect` events emitted by its
clients. The `Client` instance that emitted the event is passed as
argument.

README.md

@@ -580,6 +580,14 @@ Calls [`client.close(callback)`](#close) on all the clients.
 
 Calls [`client.destroy(err, callback)`](#destroy) on all the clients.
 
+#### Events
+
+* `'connect'`, emitted when a client has connected, the `Client`
+    instance is passed as argument.
+
+* `'disconnect'`, emitted when a client has disconnected, the `Client`
+    instance is passed as argument.
+
 <a name='errors'></a>
 ### `undici.errors`
 

lib/pool.js

@@ -1,5 +1,6 @@
 'use strict'
 
+const EventEmitter = require('events')
 const Client = require('./core/client')
 const {
   ClientClosedError,
@@ -14,11 +15,13 @@ const kDestroyed = Symbol('destroyed')
 const kClosedPromise = Symbol('closed promise')
 const kClosedResolve = Symbol('closed resolve')
 
-class Pool {
+class Pool extends EventEmitter {
   constructor (url, {
     connections,
     ...options
   } = {}) {
+    super()
+
     if (connections != null && (!Number.isFinite(connections) || connections <= 0)) {
       throw new InvalidArgumentError('invalid connections')
     }
@@ -50,8 +53,18 @@ class Pool {
       }
     }
 
+    function onConnect () {
+      pool.emit('connect', this)
+    }
+
+    function onDisconnect () {
+      pool.emit('disconnect', this)
+    }
+
     for (const client of this[kClients]) {
       client.on('drain', onDrain)
+      client.on('connect', onConnect)
+      client.on('disconnect', onDisconnect)
     }
   }
 

test/pool.js

@@ -3,7 +3,7 @@
 const proxyquire = require('proxyquire')
 const { test } = require('tap')
 const undici = require('..')
-const { Pool, errors } = require('..')
+const { Client, Pool, errors } = require('..')
 const { createServer } = require('http')
 const { EventEmitter } = require('events')
 const { promisify } = require('util')
@@ -12,6 +12,46 @@ const eos = require('stream').finished
 const net = require('net')
 const EE = require('events')
 
+test('connect/disconnect event(s)', (t) => {
+  const clients = 2
+
+  t.plan(clients * 3)
+
+  const server = createServer((req, res) => {
+    res.writeHead(200, {
+      Connection: 'keep-alive',
+      'Keep-Alive': 'timeout=1s'
+    })
+    res.end('ok')
+  })
+  t.tearDown(server.close.bind(server))
+
+  server.listen(0, () => {
+    const pool = new Pool(`http://localhost:${server.address().port}`, {
+      connections: clients,
+      keepAliveTimeoutThreshold: 100
+    })
+    t.tearDown(pool.close.bind(pool))
+
+    pool.on('connect', (client) => {
+      t.strictEqual(client instanceof Client, true)
+    })
+    pool.on('disconnect', (client) => {
+      t.strictEqual(client instanceof Client, true)
+    })
+
+    for (let i = 0; i < clients; i++) {
+      pool.request({
+        path: '/',
+        method: 'GET'
+      }, (err, { headers, body }) => {
+        t.error(err)
+        body.resume()
+      })
+    }
+  })
+})
+
 test('basic get', (t) => {
   t.plan(9)