async_hooks: add AsyncResource.bind utility

Creates an internal AsyncResource and binds a function to it,
ensuring that the function is invoked within execution context
in which bind was called.

Author: jasnell

doc/api/async_hooks.md

@@ -729,6 +729,26 @@ class DBQuery extends AsyncResource {
 }
 ```
 
+#### `static AsyncResource.bind(fn[, type])`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `fn` {Function} The function to bind to the current execution context.
+* `type` {string} An optional name to associate with the underlying
+  `AsyncResource`.
+
+Binds the given function to the current execution context.
+
+#### `asyncResource.bind(fn)`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `fn` {Function} The function to bind to the current `AsyncResource`.
+
+Binds the given function to execute to this `AsyncResource`'s scope.
+
 #### `asyncResource.runInAsyncScope(fn[, thisArg, ...args])`
 <!-- YAML
 added: v9.6.0
@@ -900,12 +920,12 @@ const { createServer } = require('http');
 const { AsyncResource, executionAsyncId } = require('async_hooks');
 
 const server = createServer((req, res) => {
-  const asyncResource = new AsyncResource('request');
-  // The listener will always run in the execution context of `asyncResource`.
-  req.on('close', asyncResource.runInAsyncScope.bind(asyncResource, () => {
-    // Prints: true
-    console.log(asyncResource.asyncId() === executionAsyncId());
+  req.on('close', AsyncResource.bind(() => {
+    // Execution context is bound to the current outer scope.
   }));
+  req.on('close', () => {
+    // Execution context is bound to the scope that caused 'close' to emit.
+  });
   res.end();
 }).listen(3000);
 ```

lib/async_hooks.js

@@ -211,6 +211,15 @@ class AsyncResource {
   triggerAsyncId() {
     return this[trigger_async_id_symbol];
   }
+
+  bind(fn) {
+    return this.runInAsyncScope.bind(this, fn);
+  }
+
+  static bind(fn, type) {
+    type = type || fn.name;
+    return (new AsyncResource(type || 'bound-anonymous-fn')).bind(fn);
+  }
 }
 
 const storageList = [];

test/parallel/test-asyncresource-bind.js

@@ -0,0 +1,26 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { AsyncResource, executionAsyncId } = require('async_hooks');
+
+const fn = common.mustCall(AsyncResource.bind(() => {
+  return executionAsyncId();
+}));
+
+setImmediate(() => {
+  const asyncId = executionAsyncId();
+  assert.notStrictEqual(asyncId, fn());
+});
+
+const asyncResource = new AsyncResource('test');
+
+const fn2 = asyncResource.bind(() => {
+  return executionAsyncId();
+});
+
+setImmediate(() => {
+  const asyncId = executionAsyncId();
+  assert.strictEqual(asyncResource.asyncId(), fn2());
+  assert.notStrictEqual(asyncId, fn2());
+});