lib: make AbortSignal cloneable/transferable

Allows for using `AbortSignal` across worker threads and contexts.

```js
const ac = new AbortController();
const mc = new MessageChannel();
mc.port1.onmessage = ({ data }) => {
  data.addEventListener('abort', () => {
    console.log('aborted!');
  });
};
mc.port2.postMessage(ac.signal, [ac.signal]);
```

Signed-off-by: James M Snell <[email protected]>

PR-URL: #41050
Refs: whatwg/dom#948
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Robert Nagy <[email protected]>

lib/internal/abort_controller.js

@@ -47,13 +47,37 @@ const {
   setTimeout,
 } = require('timers');
 
-const kAborted = Symbol('kAborted');
-const kReason = Symbol('kReason');
-const kTimeout = Symbol('kTimeout');
+const {
+  messaging_deserialize_symbol: kDeserialize,
+  messaging_transfer_symbol: kTransfer,
+  messaging_transfer_list_symbol: kTransferList
+} = internalBinding('symbols');
 
-const timeOutSignals = new SafeSet();
+let _MessageChannel;
+let makeTransferable;
+
+// Loading the MessageChannel and makeTransferable have to be done lazily
+// because otherwise we'll end up with a require cycle that ends up with
+// an incomplete initialization of abort_controller.
+
+function lazyMessageChannel() {
+  _MessageChannel ??= require('internal/worker/io').MessageChannel;
+  return new _MessageChannel();
+}
+
+function lazyMakeTransferable(obj) {
+  makeTransferable ??=
+    require('internal/worker/js_transferable').makeTransferable;
+  return makeTransferable(obj);
+}
 
 const clearTimeoutRegistry = new SafeFinalizationRegistry(clearTimeout);
+const timeOutSignals = new SafeSet();
+
+const kAborted = Symbol('kAborted');
+const kReason = Symbol('kReason');
+const kCloneData = Symbol('kCloneData');
+const kTimeout = Symbol('kTimeout');
 
 function customInspect(self, obj, depth, options) {
   if (depth < 0)
@@ -165,7 +189,68 @@ class AbortSignal extends EventTarget {
       timeOutSignals.delete(this);
     }
   }
+
+  [kTransfer]() {
+    validateAbortSignal(this);
+    const aborted = this.aborted;
+    if (aborted) {
+      const reason = this.reason;
+      return {
+        data: { aborted, reason },
+        deserializeInfo: 'internal/abort_controller:ClonedAbortSignal',
+      };
+    }
+
+    const { port1, port2 } = this[kCloneData];
+    this[kCloneData] = undefined;
+
+    this.addEventListener('abort', () => {
+      port1.postMessage(this.reason);
+      port1.close();
+    }, { once: true });
+
+    return {
+      data: { port: port2 },
+      deserializeInfo: 'internal/abort_controller:ClonedAbortSignal',
+    };
+  }
+
+  [kTransferList]() {
+    if (!this.aborted) {
+      const { port1, port2 } = lazyMessageChannel();
+      port1.unref();
+      port2.unref();
+      this[kCloneData] = {
+        port1,
+        port2,
+      };
+      return [port2];
+    }
+    return [];
+  }
+
+  [kDeserialize]({ aborted, reason, port }) {
+    if (aborted) {
+      this[kAborted] = aborted;
+      this[kReason] = reason;
+      return;
+    }
+
+    port.onmessage = ({ data }) => {
+      abortSignal(this, data);
+      port.close();
+      port.onmessage = undefined;
+    };
+    // The receiving port, by itself, should never keep the event loop open.
+    // The unref() has to be called *after* setting the onmessage handler.
+    port.unref();
+  }
+}
+
+function ClonedAbortSignal() {
+  return createAbortSignal();
 }
+ClonedAbortSignal.prototype[kDeserialize] = () => {};
 
 ObjectDefineProperties(AbortSignal.prototype, {
   aborted: { enumerable: true }
@@ -185,7 +270,7 @@ function createAbortSignal(aborted = false, reason = undefined) {
   ObjectSetPrototypeOf(signal, AbortSignal.prototype);
   signal[kAborted] = aborted;
   signal[kReason] = reason;
-  return signal;
+  return lazyMakeTransferable(signal);
 }
 
 function abortSignal(signal, reason) {
@@ -252,4 +337,5 @@ module.exports = {
   kAborted,
   AbortController,
   AbortSignal,
+  ClonedAbortSignal,
 };

test/parallel/test-abortsignal-cloneable.js

@@ -0,0 +1,78 @@
+'use strict';
+
+const common = require('../common');
+const { ok, strictEqual } = require('assert');
+const { setImmediate: pause } = require('timers/promises');
+
+function deferred() {
+  let res;
+  const promise = new Promise((resolve) => res = resolve);
+  return { res, promise };
+}
+
+(async () => {
+  const ac = new AbortController();
+  const mc = new MessageChannel();
+
+  const deferred1 = deferred();
+  const deferred2 = deferred();
+  const resolvers = [deferred1, deferred2];
+
+  mc.port1.onmessage = common.mustCall(({ data }) => {
+    data.addEventListener('abort', common.mustCall(() => {
+      strictEqual(data.reason, 'boom');
+    }));
+    resolvers.shift().res();
+  }, 2);
+
+  mc.port2.postMessage(ac.signal, [ac.signal]);
+
+  // Can be cloned/transferd multiple times and they all still work
+  mc.port2.postMessage(ac.signal, [ac.signal]);
+
+  mc.port2.close();
+
+  // Although we're using transfer semantics, the local AbortSignal
+  // is still usable locally.
+  ac.signal.addEventListener('abort', common.mustCall(() => {
+    strictEqual(ac.signal.reason, 'boom');
+  }));
+
+  await Promise.all([ deferred1.promise, deferred2.promise ]);
+
+  ac.abort('boom');
+
+  // Because the postMessage used by the underlying AbortSignal
+  // takes at least one turn of the event loop to be processed,
+  // and because it is unref'd, it won't, by itself, keep the
+  // event loop open long enough for the test to complete, so
+  // we schedule two back to back turns of the event to ensure
+  // the loop runs long enough for the test to complete.
+  await pause();
+  await pause();
+
+})().then(common.mustCall());
+
+{
+  const signal = AbortSignal.abort('boom');
+  ok(signal.aborted);
+  strictEqual(signal.reason, 'boom');
+  const mc = new MessageChannel();
+  mc.port1.onmessage = common.mustCall(({ data }) => {
+    ok(data instanceof AbortSignal);
+    ok(data.aborted);
+    strictEqual(data.reason, 'boom');
+    mc.port1.close();
+  });
+  mc.port2.postMessage(signal, [signal]);
+}
+
+{
+  // The cloned AbortSignal does not keep the event loop open
+  // waiting for the abort to be triggered.
+  const ac = new AbortController();
+  const mc = new MessageChannel();
+  mc.port1.onmessage = common.mustCall();
+  mc.port2.postMessage(ac.signal, [ac.signal]);
+  mc.port2.close();
+}