child_process: fix memory leak in .fork()

Entries in the `net.Server#_slaves` array that is used to track handles
sent from the master to workers were not deleted when a worker exited,
resulting in a slow but inexorable memory leak.

PR-URL: #15679
Backport-PR-URL: #16586
Fixes: #15651
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/internal/socket_list.js

@@ -8,6 +8,7 @@ class SocketListSend extends EventEmitter {
     super();
     this.key = key;
     this.child = child;
+    child.once('exit', () => this.emit('exit', this));
   }
 
   _request(msg, cmd, callback) {

lib/net.js

@@ -1664,6 +1664,10 @@ Server.prototype.listenFD = internalUtil.deprecate(function(fd, type) {
 Server.prototype._setupSlave = function(socketList) {
   this._usingSlaves = true;
   this._slaves.push(socketList);
+  socketList.once('exit', (socketList) => {
+    const index = this._slaves.indexOf(socketList);
+    this._slaves.splice(index, 1);
+  });
 };
 
 Server.prototype.ref = function() {

test/parallel/test-child-process-fork-net2.js

@@ -156,6 +156,7 @@ if (process.argv[2] === 'child') {
   }
 
   process.on('exit', function() {
+    assert.strictEqual(server._slaves.length, 0);
     assert.strictEqual(disconnected, count);
     assert.strictEqual(connected, count);
   });