net: add drop event for net server

Author: theanarkh

doc/api/net.md

@@ -281,6 +281,23 @@ added: v0.1.90
 
 Emitted when the server has been bound after calling [`server.listen()`][].
 
+### Event: `'drop'`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+When the number of connections reaches the threshold of `server.maxConnections`,
+the server will drop new connections and emit `'drop'` event instead. If it is a
+TCP server, the argument is as follows, otherwise the argument is `undefined`.
+
+* `data` {Object} The argument passed to event listener.
+  * `localAddress` {string}  Local address.
+  * `localPort` {number} Local port.
+  * `remoteAddress` {string} Remote address.
+  * `remotePort` {number} Remote port.
+  * `remoteFamily` {string} Remote IP family. `'IPv4'` or `'IPv6'`.
+
 ### `server.address()`
 
 <!-- YAML

lib/net.js

@@ -1647,6 +1647,25 @@ function onconnection(err, clientHandle) {
   }
 
   if (self.maxConnections && self._connections >= self.maxConnections) {
+    if (clientHandle.getsockname || clientHandle.getpeername) {
+      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);
+    } else {
+      self.emit('drop');
+    }
     clientHandle.close();
     return;
   }

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

@@ -0,0 +1,26 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+
+let firstSocket;
+const server = net.createServer(common.mustCall((socket) => {
+  firstSocket = socket;
+}));
+
+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();
+}));
+
+server.listen(0, () => {
+  net.createConnection(server.address().port);
+  net.createConnection(server.address().port);
+});