worker: add support for platformData

The `worker.platformData` and `worker.setPlatformData()` APIs allow
an arbitrary, cloneable JavaScript value to be set and passed to all
new Worker instances spawned from the current context. It is similar
to `workerData` except that `platformData` is set independently of
the `new Worker()` constructor, and the the value is passed
automatically to all new Workers.

This is a *partial* fix of nodejs#30992
but does not implement a complete fix.

Signed-off-by: James M Snell <jasnell@gmail.com>

doc/api/worker_threads.md

@@ -176,6 +176,32 @@ if (isMainThread) {
 }
 ```
 
+## `worker.platformData`
+<!-- YAML
+added: REPLACEME
+-->
+
+An arbitrary JavaScript value that contains a clone of the data passed
+to the spawning threads `worker.setPlatformData()` function. The
+`worker.platformData` is similar to `worker.workerData` except that
+every new `Worker` receives it's own copy of `platformData` automatically.
+
+```js
+const {
+  Worker,
+  isMainThread,
+  setPlatformData,
+  platformData
+} = require('worker_threads');
+
+if (isMainThread) {
+  setPlatformData('Hello World!');
+  const worker = new Worker(__filename);
+} else {
+  console.log(platformData);  // Prints 'Hello, world!'.
+}
+```
+
 ## `worker.receiveMessageOnPort(port)`
 <!-- YAML
 added: v12.3.0
@@ -246,6 +272,20 @@ new Worker('process.env.SET_IN_WORKER = "foo"', { eval: true, env: SHARE_ENV })
   });
 ```
 
+## `worker.setPlatformData(value)`
+<!--YAML
+added: REPLACEME
+-->
+
+* `value` {any} Any arbitrary, cloneable JavaScript value that will be cloned
+  and passed automatically to all new `Worker` instances.
+
+The `worker.setPlatformData()` API sets the value of the `worker.platformData`
+in all new `Worker` instances spawned from the current context.
+
+Calling `worker.setPlatformData()` will have no impact on the value of
+`worker.platformData` on existing threads.
+
 ## `worker.threadId`
 <!-- YAML
 added: v10.5.0

lib/internal/main/worker_thread.js

@@ -105,6 +105,7 @@ port.on('message', (message) => {
       filename,
       doEval,
       workerData,
+      platformData,
       publicPort,
       manifestSrc,
       manifestURL,
@@ -126,6 +127,7 @@ port.on('message', (message) => {
     }
     publicWorker.parentPort = publicPort;
     publicWorker.workerData = workerData;
+    publicWorker.platformData = platformData;
 
     // The counter is only passed to the workers created by the main thread, not
     // to workers created by other workers.

lib/internal/worker.js

@@ -90,6 +90,8 @@ let debug = require('internal/util/debuglog').debuglog('worker', (fn) => {
 
 let cwdCounter;
 
+let platformData;
+
 if (isMainThread) {
   cwdCounter = new Uint32Array(new SharedArrayBuffer(4));
   const originalChdir = process.chdir;
@@ -99,6 +101,9 @@ if (isMainThread) {
   };
 }
 
+function setPlatformData(value) {
+  platformData = value;
+}
 class Worker extends EventEmitter {
   constructor(filename, options = {}) {
     super();
@@ -228,6 +233,7 @@ class Worker extends EventEmitter {
       doEval,
       cwdCounter: cwdCounter || workerIo.sharedCwdCounter,
       workerData: options.workerData,
+      platformData,
       publicPort: port2,
       manifestURL: getOptionValue('--experimental-policy') ?
         require('internal/process/policy').url :
@@ -493,6 +499,7 @@ module.exports = {
   SHARE_ENV,
   resourceLimits:
     !isMainThread ? makeResourceLimits(resourceLimitsRaw) : {},
+  setPlatformData,
   threadId,
   Worker,
 };

lib/worker_threads.js

@@ -4,6 +4,7 @@ const {
   isMainThread,
   SHARE_ENV,
   resourceLimits,
+  setPlatformData,
   threadId,
   Worker
 } = require('internal/worker');
@@ -33,5 +34,7 @@ module.exports = {
   Worker,
   parentPort: null,
   workerData: null,
+  platformData: null,
   BroadcastChannel,
+  setPlatformData,
 };

test/parallel/test-worker-platform-data.js

@@ -0,0 +1,19 @@
+'use strict';
+
+require('../common');
+const {
+  Worker,
+  isMainThread,
+  platformData,
+  setPlatformData,
+} = require('worker_threads');
+
+const { strictEqual } = require('assert');
+
+if (isMainThread) {
+  setPlatformData('foo');
+  strictEqual(platformData, null);
+  new Worker(__filename);
+} else {
+  strictEqual(platformData, 'foo');
+}