net: add drop event for net server

doc/api/net.md

@@ -281,6 +281,26 @@ added: v0.1.90
 
 Emitted when the server has been bound after calling [`server.listen()`][].
 
+### Event: `'drop'`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+When the number of conenctions reached the threshold(`server.maxConnections`),
+the server will drop the connection and emit drop event. If it is a TCP server,
+the arguments is as follow.
+
+```json
+{
+  "localAddress": "::ffff:127.0.0.1",
+  "localPort": 61054,
+  "remoteAddress": "::ffff:127.0.0.1",
+  "remotePort": 61060,
+  "remoteFamily": "IPv6"
+}
+```
+
 ### `server.address()`
 
 <!-- YAML

lib/net.js

@@ -1647,6 +1647,21 @@ function onconnection(err, clientHandle) {
   }
 
   if (self.maxConnections && self._connections >= self.maxConnections) {
+    const data = {};
+    if (clientHandle.getsockname) {
+      const localInfo = {};
+      clientHandle.getsockname(localInfo);
+      data.localAddress = localInfo.address;
+      data.localPort = localInfo.port;
+    }
+    if (clientHandle.getpeername) {
+      const remoteInfo = {};
+      clientHandle.getpeername(remoteInfo);
+      data.remoteAddress = remoteInfo.address;
+      data.remotePort = remoteInfo.port;
+      data.remoteFamily = remoteInfo.family;
+    }
+    self.emit('drop', data);
     clientHandle.close();
     return;
   }

test/parallel/test-net-server-drop-connections.js

@@ -0,0 +1,26 @@
+'use strict';
+const common = require('../common');
+const net = require('net');
+const assert = require('assert');
+
+let firstSocket;
+const server = net.createServer(common.mustCall((socket) => {
+  firstSocket = socket;
+}, 1));
+
+server.maxConnections = 1;
+
+server.on('drop', common.mustCall((data) => {
+  assert.strictEqual(!!data.localAddress, true);
+  assert.strictEqual(!!data.localPort, true);
+  assert.strictEqual(!!data.remoteAddress, true);
+  assert.strictEqual(!!data.remotePort, true);
+  assert.strictEqual(!!data.remoteFamily, true);
+  firstSocket.destroy();
+  server.close();
+}, 1));
+
+server.listen(0, () => {
+  net.createConnection(server.address().port);
+  net.createConnection(server.address().port);
+});