worker: Add experimental SynchronousWorker implementation

Adds SynchronousWorker from the standalone synchronous-worker module
directly into Node.js (as Experimental).

The synchronous-worker module was originally written by Anna
Henningsen and is incorporated here at Matteo Collina's request and
Anna's permission.

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

LICENSE

@@ -984,8 +984,7 @@ The externally maintained libraries used by Node.js are:
       - Strongtalk assembler, the basis of the files assembler-arm-inl.h,
         assembler-arm.cc, assembler-arm.h, assembler-ia32-inl.h,
         assembler-ia32.cc, assembler-ia32.h, assembler-x64-inl.h,
-        assembler-x64.cc, assembler-x64.h, assembler-mips-inl.h,
-        assembler-mips.cc, assembler-mips.h, assembler.cc and assembler.h.
+        assembler-x64.cc, assembler-x64.h, assembler.cc and assembler.h.
         This code is copyrighted by Sun Microsystems Inc. and released
         under a 3-clause BSD license.
 
@@ -1871,3 +1870,28 @@ The externally maintained libraries used by Node.js are:
     NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
     SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   """
+
+- synchronous-worker, located at lib/worker_threads.js, is licensed as follows:
+  """
+    The MIT License (MIT)
+
+    Copyright (c) 2020 Anna Henningsen
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in all
+    copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+    SOFTWARE.
+  """

doc/api/worker_threads.md

@@ -1329,6 +1329,228 @@ from the running process and will preload the same preload scripts as the main
 thread. If the preload script unconditionally launches a worker thread, every
 thread spawned will spawn another until the application crashes.
 
+## Synchronous Workers
+
+> Stability: 1 - Experimental
+
+### Class: `SynchronousWorker`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* Extends: {EventEmitter}
+
+A `SynchronousWorker` is effectively a Node.js environment that runs within the
+same thread.
+
+```cjs
+const { SynchronousWorker } = require('node:worker_threads');
+const w = new SynchronousWorker();
+const myAsyncFunction = w.createRequire(__filename)('my-module');
+const response = w.runLoopUntilPromiseResolved(myAsyncFunction('http://example.org'));
+const text = w.runLoopUntilPromiseResolved(response.text());
+console.log(text);
+```
+
+#### `new SynchronousWorker([options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `options` {Object}
+  * `sharedEventLoop` {boolean} When `true`, use the same event loop as the
+    outer Node.js instance. If this is passed, the `runLoop()` and
+    `runLoopUntilPromiseResolved()` methods become unavailable.
+    **Default:** `false`.
+  * `sharedMicrotaskQueue` {boolean} When true, use the same microtask queue as
+    the outer Node.js instance. This is used for resolving promises created in
+    the inner context, including those implicitly generated by `async/await`.
+    If this is passed, the `runLoopUntilPromiseResolved()` method becomes
+    unavailable. **Default:** `false`.
+
+While setting `sharedEventLoop` to `false` and `sharedMicrotaskQueue` to `true`
+is accepted, they typically do not make sense together.
+
+#### `synchronousWorker.runLoop([mode])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `mode` {string} One of either `'default'`, `'once'`, or `'nowait'`.
+
+Spin the event loop of the inner Node.js instance. `mode` can be either
+`default`, `once` or `nowait`. See the [libuv documentation for `uv_run()`][]
+for details on these modes.
+
+#### `synchronousWorker.runLoopUntilPromiseResolved(promise)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `promise` {Promise}
+
+Spin the event loop of the innsert Node.js instance until a specific `Promise`
+is resolved.
+
+#### `synchronousWorker.runInWorkerScope(fn)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `fn` {Function}
+
+Wrap `fn` and run it as if it were run on the event loop of the inner Node.js
+instance. In particular, this ensures that Promises created by the function
+itself are resolved correctly. You should generally use this to run any code
+inside the innert Node.js instance that performs asynchronous activity and that
+is not already running in an asynchronous context (you can compare this to
+the code that runs synchronously from the main file of a Node.js application).
+
+#### `synchronousWorker.loopAlive`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* Type: {boolean}
+
+This is a read-only boolean property indicating whether there are currently any
+items on the event loop of the inner Node.js instance.
+
+#### `synchronousWorker.stop()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+Interrupt any execution of code inside the inner Node.js instance, i.e.
+return directly from a `.runLoop()`, `.runLoopUntilPromiseResolved()` or
+`.runInWorkerScope()` call. This will render the Node.js instance unusable
+and is generally comparable to running `process.exit()`.
+
+This method returns a `Promise` that will be resolved when all resources
+associated with this Node.js instance are released. This `Promise` resolves on
+the event loop of the _outer_ Node.js instance.
+
+#### `synchronousWorker.createRequire(filename)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `filename` {string}
+
+Create a `require()` function that can be used for loading code inside the
+inner Node.js instance.
+
+#### `synchronousWorker.globalThis`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* Type: {Object}
+
+Returns a reference to the global object of the inner Node.js instance.
+
+#### `synchronousWorker.process`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* Type: {Object}
+
+Returns a reference to the `process` object of the inner Node.js instance.
+
+### FAQ
+
+#### What does a SynchronousWorker do?
+
+Creates a new Node.js instance, using the same thread and the same JS heap.
+You can create Node.js API objects, like network sockets, inside the new
+Node.js instance, and spin the underlying event loop manually.
+
+#### Where did SynchronousWorker come from?
+
+`SynchronousWorker` was originally developer by Node.js core contributor
+Anna Henningsen and published as a separate module [`synchronous-worker`][] on
+npm under the MIT license. It was integrated into Node.js core with Anna's
+permission. The majority of the code, documentation, and tests were adopted
+almost verbatim from the original module.
+
+#### Why would I use a SynchronousWorker?
+
+The most common use case is probably running asynchronous code synchronously,
+in situations where doing so cannot be avoided (even though one should try
+really hard to avoid it). Another popular npm package that does this is
+[`deasync`][], but `deasync`
+
+* solves this problem by starting the event loop while it is already running
+  (which is explicitly _not_ supported by libuv and may lead to crashes)
+* doesn’t allow specifying _which_ resources or callbacks should be waited for,
+  and instead allows everything inside the current thread to progress.
+
+#### How can I avoid using SynchronousWorker?
+
+If you do not need to directly interact with the objects inside the inner
+Node.js instance, a lot of the time Worker threads together with
+[`Atomics.wait()`][] will give you what you need.
+
+#### My async functions/Promises/… don’t work
+
+If you run a `SynchronousWorker` with its own microtask queue (i.e. in default
+mode), code like this will not work as expected:
+
+```cjs
+const { SynchronousWorker } = require('node:worker_threads');
+const w = new SynchronousWorker();
+let promise;
+w.runInWorkerScope(() => {
+  promise = (async () => {
+    return w.createRequire(__filename)('node-fetch')('...');
+  })();
+});
+w.runLoopUntilPromiseResolved(promise);
+```
+
+The reason for this is that `async` functions (and Promise `.then()` handlers)
+add their microtasks to the microtask queue for the Context in which the
+async function (or `.then()` callback) was defined, and not the Context in which
+the original `Promise` was created. Put in other words, it is possible for a
+`Promise` chain to be run on different microtask queues.
+
+While this behavior may be counterintuitive, it is what the V8 engine does,
+and is not under the control of Node.js.
+
+What this means is that you will need to make sure that the functions are
+compiled in the Context in which they are supposed to be run; the two main
+ways to achieve that are to:
+
+* Put them in a separate file that is loaded through `w.createRequire()`
+* Use `w.createRequire(__filename)('vm').runInThisContext()` to manually compile
+  the code for the function in the Context of the target Node.js instance.
+
+For example:
+
+```cjs
+const { SynchronousWorker } = require('node:worker_threads');
+const w = new SynchronousWorker();
+const req = w.createRequire(__filename);
+let promise;
+w.runInWorkerScope(() => {
+  promise = req('vm').runInThisContext(`(async(req) => {
+    return await req('node-fetch')('...');
+  })`)(req);
+});
+w.runLoopUntilPromiseResolved(promise);
+```
+
 [Addons worker support]: addons.md#worker-support
 [ECMAScript module loader]: esm.md#data-imports
 [HTML structured clone algorithm]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm
@@ -1341,6 +1563,7 @@ thread spawned will spawn another until the application crashes.
 [`--max-semi-space-size`]: cli.md#--max-semi-space-sizesize-in-megabytes
 [`ArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer
 [`AsyncResource`]: async_hooks.md#class-asyncresource
+[`Atomics.wait()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics/wait
 [`Buffer.allocUnsafe()`]: buffer.md#static-method-bufferallocunsafesize
 [`Buffer`]: buffer.md
 [`ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST`]: errors.md#err_missing_message_port_in_transfer_list
@@ -1354,6 +1577,7 @@ thread spawned will spawn another until the application crashes.
 [`Worker constructor options`]: #new-workerfilename-options
 [`Worker`]: #class-worker
 [`data:` URL]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs
+[`deasync`]: https://www.npmjs.com/package/deasync
 [`fs.close()`]: fs.md#fsclosefd-callback
 [`fs.open()`]: fs.md#fsopenpath-flags-mode-callback
 [`markAsUntransferable()`]: #workermarkasuntransferableobject
@@ -1378,6 +1602,7 @@ thread spawned will spawn another until the application crashes.
 [`require('node:worker_threads').parentPort`]: #workerparentport
 [`require('node:worker_threads').threadId`]: #workerthreadid
 [`require('node:worker_threads').workerData`]: #workerworkerdata
+[`synchronous-worker`]: https://github.com/addaleax/synchronous-worker
 [`trace_events`]: tracing.md
 [`v8.getHeapSnapshot()`]: v8.md#v8getheapsnapshot
 [`vm`]: vm.md
@@ -1390,4 +1615,5 @@ thread spawned will spawn another until the application crashes.
 [browser `MessagePort`]: https://developer.mozilla.org/en-US/docs/Web/API/MessagePort
 [child processes]: child_process.md
 [contextified]: vm.md#what-does-it-mean-to-contextify-an-object
+[libuv documentation for `uv_run()`]: http://docs.libuv.org/en/v1.x/loop.html#c.uv_run
 [v8.serdes]: v8.md#serialization-api