From 9d112bd598dce0e9b2930dc5af3c738f63b9763f Mon Sep 17 00:00:00 2001
From: Anna Henningsen <anna@addaleax.net>
Date: Fri, 15 Mar 2019 18:21:06 +0100
Subject: [PATCH] worker: allow synchronously draining message ports

In combination with Atomics, this makes it possible to implement
generic synchronous functionality, e.g. `importScript()`, in Workers
purely by communicating with other threads.
---
 doc/api/worker_threads.md                     | 20 +++++++++++++++++++
 lib/internal/worker.js                        |  1 +
 lib/worker_threads.js                         |  2 ++
 .../test-worker-message-port-drain-manual.js  | 19 ++++++++++++++++++
 4 files changed, 42 insertions(+)
 create mode 100644 test/parallel/test-worker-message-port-drain-manual.js

diff --git a/doc/api/worker_threads.md b/doc/api/worker_threads.md
index 16787723c6e57e..a545febf3d2565 100644
--- a/doc/api/worker_threads.md
+++ b/doc/api/worker_threads.md
@@ -49,6 +49,26 @@ The above example spawns a Worker thread for each `parse()` call. In actual
 practice, use a pool of Workers instead for these kinds of tasks. Otherwise, the
 overhead of creating Workers would likely exceed their benefit.
 
+## worker.drainMessagePort(port)
+<!-- YAML
+added: REPLACEME
+-->
+
+* `port` {MessagePort}
+
+Make `port` emit all currently pending messages synchronously.
+
+```js
+const { MessageChannel, drainMessagePort } = require('worker_threads');
+const { port1, port2 } = new MessageChannel();
+
+port1.postMessage({ hello: 'world' });
+port2.once('message', (message) => console.log(message));
+
+drainMessagePort(port2);
+console.log('done emitting messages');  // This is printed *after* the message.
+```
+
 ## worker.isMainThread
 <!-- YAML
 added: v10.5.0
diff --git a/lib/internal/worker.js b/lib/internal/worker.js
index 02f6661b548e5b..cd4f13e2db3e30 100644
--- a/lib/internal/worker.js
+++ b/lib/internal/worker.js
@@ -251,6 +251,7 @@ function pipeWithoutWarning(source, dest) {
 }
 
 module.exports = {
+  drainMessagePort,
   ownsProcessState,
   isMainThread,
   threadId,
diff --git a/lib/worker_threads.js b/lib/worker_threads.js
index 722e47caf1092f..ae7543012bc983 100644
--- a/lib/worker_threads.js
+++ b/lib/worker_threads.js
@@ -1,6 +1,7 @@
 'use strict';
 
 const {
+  drainMessagePort,
   isMainThread,
   threadId,
   Worker
@@ -13,6 +14,7 @@ const {
 } = require('internal/worker/io');
 
 module.exports = {
+  drainMessagePort,
   isMainThread,
   MessagePort,
   MessageChannel,
diff --git a/test/parallel/test-worker-message-port-drain-manual.js b/test/parallel/test-worker-message-port-drain-manual.js
new file mode 100644
index 00000000000000..d2bf588b2337d9
--- /dev/null
+++ b/test/parallel/test-worker-message-port-drain-manual.js
@@ -0,0 +1,19 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const { MessageChannel, drainMessagePort } = require('worker_threads');
+
+const { port1, port2 } = new MessageChannel();
+const messages = [];
+port2.on('message', (message) => messages.push(message));
+
+const message = { hello: 'world' };
+
+assert.deepStrictEqual(messages, []);
+port1.postMessage(message);
+port1.postMessage(message);
+assert.deepStrictEqual(messages, []);
+drainMessagePort(port2);
+assert.deepStrictEqual(messages, [ message, message ]);
+
+port1.close();