child_process: add public API for IPC channel

This commit adds a public channel property to ChildProcess. The
existing _channel property is aliased to the new property, with
the intention of deprecating and removing it in the future.

Fixes: https://github.com/nodejs/node/issues/9313
PR-URL: https://github.com/nodejs/node/pull/9322
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

doc/api/child_process.md

@@ -776,6 +776,16 @@ added: v0.5.9
 The `'message'` event is triggered when a child process uses [`process.send()`][]
 to send messages.
 
+### child.channel
+<!-- YAML
+added: REPLACEME
+-->
+
+* {Object} A pipe representing the IPC channel to the child process.
+
+The `child.channel` property is a reference to the child's IPC channel. If no
+IPC channel currently exists, this property is `undefined`.
+
 ### child.connected
 <!-- YAML
 added: v0.7.2
@@ -1145,6 +1155,7 @@ console.log('中文测试');
 [`'error'`]: #child_process_event_error
 [`'exit'`]: #child_process_event_exit
 [`'message'`]: #child_process_event_message
+[`child.channel`]: #child_process_child_channel
 [`child.connected`]: #child_process_child_connected
 [`child.disconnect()`]: #child_process_child_disconnect
 [`child.kill()`]: #child_process_child_kill_signal

doc/api/process.md

@@ -508,6 +508,16 @@ $ bash -c 'exec -a customArgv0 ./node'
 'customArgv0'
 ```
 
+## process.channel
+<!-- YAML
+added: REPLACEME
+-->
+
+If the Node.js process was spawned with an IPC channel (see the
+[Child Process][] documentation), the `process.channel`
+property is a reference to the IPC channel. If no IPC channel exists, this
+property is `undefined`.
+
 ## process.chdir(directory)
 <!-- YAML
 added: v0.1.17

lib/internal/child_process.js

@@ -69,7 +69,7 @@ const handleConversion = {
         // the slave should keep track of the socket
         message.key = socket.server._connectionKey;
 
-        var firstTime = !this._channel.sockets.send[message.key];
+        var firstTime = !this.channel.sockets.send[message.key];
         var socketList = getSocketList('send', this, message.key);
 
         // the server should no longer expose a .connection property
@@ -409,7 +409,15 @@ ChildProcess.prototype.unref = function() {
 
 
 function setupChannel(target, channel) {
-  target._channel = channel;
+  target.channel = channel;
+
+  // _channel can be deprecated in version 8
+  Object.defineProperty(target, '_channel', {
+    get() { return target.channel; },
+    set(val) { target.channel = val; },
+    enumerable: true
+  });
+
   target._handleQueue = null;
   target._pendingHandle = null;
 
@@ -465,7 +473,7 @@ function setupChannel(target, channel) {
       target.disconnect();
       channel.onread = nop;
       channel.close();
-      target._channel = null;
+      target.channel = null;
       maybeClose(target);
     }
   };
@@ -491,7 +499,7 @@ function setupChannel(target, channel) {
       });
 
       // Process a pending disconnect (if any).
-      if (!target.connected && target._channel && !target._handleQueue)
+      if (!target.connected && target.channel && !target._handleQueue)
         target._disconnect();
 
       return;
@@ -547,7 +555,7 @@ function setupChannel(target, channel) {
   };
 
   target._send = function(message, handle, options, callback) {
-    assert(this.connected || this._channel);
+    assert(this.connected || this.channel);
 
     if (message === undefined)
       throw new TypeError('"message" argument cannot be undefined');
@@ -667,11 +675,11 @@ function setupChannel(target, channel) {
   // connected will be set to false immediately when a disconnect() is
   // requested, even though the channel might still be alive internally to
   // process queued messages. The three states are distinguished as follows:
-  // - disconnect() never requested: _channel is not null and connected
+  // - disconnect() never requested: channel is not null and connected
   //   is true
-  // - disconnect() requested, messages in the queue: _channel is not null
+  // - disconnect() requested, messages in the queue: channel is not null
   //   and connected is false
-  // - disconnect() requested, channel actually disconnected: _channel is
+  // - disconnect() requested, channel actually disconnected: channel is
   //   null and connected is false
   target.connected = true;
 
@@ -692,10 +700,10 @@ function setupChannel(target, channel) {
   };
 
   target._disconnect = function() {
-    assert(this._channel);
+    assert(this.channel);
 
     // This marks the fact that the channel is actually disconnected.
-    this._channel = null;
+    this.channel = null;
 
     if (this._pendingHandle) {
       this._pendingHandle.close();
@@ -729,7 +737,7 @@ function setupChannel(target, channel) {
 
 const INTERNAL_PREFIX = 'NODE_';
 function handleMessage(target, message, handle) {
-  if (!target._channel)
+  if (!target.channel)
     return;
 
   var eventName = 'message';
@@ -860,7 +868,7 @@ function _validateStdio(stdio, sync) {
 
 
 function getSocketList(type, slave, key) {
-  var sockets = slave._channel.sockets[type];
+  var sockets = slave.channel.sockets[type];
   var socketList = sockets[key];
   if (!socketList) {
     var Construct = type === 'send' ? SocketListSend : SocketListReceive;

lib/internal/process/stdio.js

@@ -60,9 +60,9 @@ function setupStdio() {
         // sitting on fd=0, in such case the pipe for this fd is already
         // present and creating a new one will lead to the assertion failure
         // in libuv.
-        if (process._channel && process._channel.fd === fd) {
+        if (process.channel && process.channel.fd === fd) {
           stdin = new net.Socket({
-            handle: process._channel,
+            handle: process.channel,
             readable: true,
             writable: false
           });

test/parallel/test-child-process-fork-regr-gh-2847.js

@@ -44,8 +44,8 @@ var server = net.createServer(function(s) {
   }
 
   worker.process.once('close', common.mustCall(function() {
-    // Otherwise the crash on `_channel.fd` access may happen
-    assert.strictEqual(worker.process._channel, null);
+    // Otherwise the crash on `channel.fd` access may happen
+    assert.strictEqual(worker.process.channel, null);
     server.close();
   }));
 

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

@@ -5,6 +5,8 @@ var fork = require('child_process').fork;
 var args = ['foo', 'bar'];
 
 var n = fork(common.fixturesDir + '/child-process-spawn-node.js', args);
+
+assert.strictEqual(n.channel, n._channel);
 assert.deepStrictEqual(args, ['foo', 'bar']);
 
 n.on('message', function(m) {

test/parallel/test-child-process-recv-handle.js

@@ -36,12 +36,12 @@ function master() {
 }
 
 function worker() {
-  process._channel.readStop();  // Make messages batch up.
+  process.channel.readStop();  // Make messages batch up.
   process.stdout.ref();
   process.stdout.write('ok\r\n');
   process.stdin.once('data', common.mustCall((data) => {
     assert.strictEqual(data.toString(), 'ok\r\n');
-    process._channel.readStart();
+    process.channel.readStart();
   }));
   let n = 0;
   process.on('message', common.mustCall((msg, handle) => {

test/parallel/test-child-process-silent.js

@@ -21,8 +21,8 @@ if (process.argv[2] === 'pipe') {
   const child = childProcess.fork(process.argv[1], ['pipe'], {silent: true});
 
   // Allow child process to self terminate
-  child._channel.close();
-  child._channel = null;
+  child.channel.close();
+  child.channel = null;
 
   child.on('exit', function() {
     process.exit(0);