cluster: correctly handle relative unix socket paths

Relative unix sockets paths were previously interpreted relative
to the master's CWD, which was inconsistent with non-cluster behavior.
A test case has been added as well.

Fixes: nodejs#16387

Author: laino

lib/internal/cluster/child.js

@@ -1,6 +1,7 @@
 'use strict';
 const assert = require('assert');
 const util = require('util');
+const path = require('path');
 const EventEmitter = require('events');
 const Worker = require('internal/cluster/worker');
 const { internal, sendHelper } = require('internal/cluster/utils');
@@ -48,7 +49,14 @@ cluster._setupWorker = function() {
 
 // obj is a net#Server or a dgram#Socket object.
 cluster._getServer = function(obj, options, cb) {
-  const indexesKey = [options.address,
+  let address = options.address;
+
+  // Resolve unix socket paths to absolute paths
+  if (options.port < 0 && typeof address === 'string' &&
+      process.platform !== 'win32')
+    address = path.resolve(address);
+
+  const indexesKey = [address,
                       options.port,
                       options.addressType,
                       options.fd ].join(':');
@@ -64,6 +72,8 @@ cluster._getServer = function(obj, options, cb) {
     data: null
   }, options);
 
+  message.address = address;
+
   // Set custom data on handle (i.e. tls tickets key)
   if (obj._getServerData)
     message.data = obj._getServerData();

lib/internal/cluster/master.js

@@ -2,6 +2,7 @@
 const assert = require('assert');
 const { fork } = require('child_process');
 const util = require('util');
+const path = require('path');
 const EventEmitter = require('events');
 const RoundRobinHandle = require('internal/cluster/round_robin_handle');
 const SharedHandle = require('internal/cluster/shared_handle');
@@ -275,6 +276,18 @@ function queryServer(worker, message) {
   var handle = handles[key];
 
   if (handle === undefined) {
+    let address = message.address;
+
+    // Find shortest path for unix sockets because of the ~100 byte limit
+    if (message.port < 0 && typeof address === 'string' &&
+        process.platform !== 'win32') {
+
+      address = path.relative(process.cwd(), address);
+
+      if (message.address.length < address.length)
+        address = message.address;
+    }
+
     var constructor = RoundRobinHandle;
     // UDP is exempt from round-robin connection balancing for what should
     // be obvious reasons: it's connectionless. There is nothing to send to
@@ -286,7 +299,7 @@ function queryServer(worker, message) {
     }
 
     handles[key] = handle = new constructor(key,
-                                            message.address,
+                                            address,
                                             message.port,
                                             message.addressType,
                                             message.fd,

test/parallel/test-cluster-net-listen-relative-path.js

@@ -0,0 +1,48 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const cluster = require('cluster');
+const net = require('net');
+const path = require('path');
+const fs = require('fs');
+
+// Ref:
+// https://github.com/nodejs/node/issues/16387
+// https://github.com/nodejs/node/pull/16749
+
+if (common.isWindows)
+  common.skip('On Windows named pipes live in their own ' +
+              'filesystem and don\'t have a ~100 byte limit');
+
+// Choose a socket name such that the absolute path would exceed 100 bytes.
+const socketDir = './unix-socket-dir';
+const socketName = 'A'.repeat(100 - socketDir.length - 1);
+
+// Make sure we're not in a weird environment
+assert.strictEqual(path.resolve(socketDir, socketName).length > 100, true,
+                   'absolute socket path should be longer than 100 bytes');
+
+if (cluster.isMaster) {
+  // ensure that the worker exits peacefully
+  process.chdir(common.tmpDir);
+  fs.mkdirSync(socketDir);
+  cluster.fork().on('exit', common.mustCall(function(statusCode) {
+    assert.strictEqual(statusCode, 0);
+
+    assert.strictEqual(
+      fs.existsSync(path.join(socketDir, socketName)), false,
+      'Socket should be removed when the worker exits');
+  }));
+} else {
+  process.chdir(socketDir);
+
+  const server = net.createServer(common.mustNotCall());
+
+  server.listen(socketName, common.mustCall(function() {
+    assert.strictEqual(
+      fs.existsSync(socketName), true,
+      'Socket created in CWD');
+
+    process.disconnect();
+  }));
+}