lib: AsyncLocalStorage.bind() and AsyncLocalStorage.snapshot()

Author: flakey5

doc/api/async_context.md

@@ -147,6 +147,49 @@ this time will print the stack trace and exit. See
 Creating an async resource within the `onPropagate` callback will result in
 a recursive call to `onPropagate`.
 
+### Static method: `AsyncLocalStorage.bind(fn)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `fn` {Function} The function to bind to the current execution context.
+
+Binds the given function to the current execution context.
+
+The returned function will have an `asyncResource` property referencing
+the `AsyncResource` to which the function is bound.
+
+### Static method: `AsyncLocalStorage.snapshot()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+Returns a callback that captures the current async context and invokes a
+callback passed into it within the captured async context.
+
+```js
+const asyncLocalStorage = new AsyncLocalStorage();
+const runInAsyncScope = asyncLocalStorage.run(123, () => asyncLocalStorage.snapshot());
+const result = asyncLocalStorage.run(321, () => runInAsyncScope(() => asyncLocalStorage.getStore()));
+console.log(result);  // returns 123
+```
+
+AsyncLocalStorage.snapshot() can replace the use of AsyncResource for simple
+async context tracking purposes, for example:
+
+```js
+class Foo {
+  #runInAsyncScope = AsyncLocalStorage.snapshot();
+
+  get() { return this.#runInAsyncScope(() => asyncLocalStorage.getStore()); }
+}
+
+const foo = asyncLocalStorage.run(123, () => new Foo());
+console.log(asyncLocalStorage.run(321, () => foo.get())); // returns 123
+```
+
 ### `asyncLocalStorage.disable()`
 
 <!-- YAML

lib/async_hooks.js

@@ -288,6 +288,14 @@ class AsyncLocalStorage {
     this._onPropagate = onPropagate;
   }
 
+  static bind(fn) {
+    return new AsyncResource('bound-anonymous-fn').bind(fn);
+  }
+
+  static snapshot() {
+    return AsyncLocalStorage.bind((cb, ...args) => cb(...args));
+  }
+
   disable() {
     if (this.enabled) {
       this.enabled = false;

test/parallel/test-async-local-storage-bind.js

@@ -0,0 +1,17 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { AsyncLocalStorage } = require('async_hooks');
+
+[1, false, '', {}, []].forEach((i) => {
+    assert.throws(() => AsyncLocalStorage.bind(i), {
+        code: 'ERR_INVALID_ARG_TYPE'
+    });
+});
+
+const fn = common.mustCall(AsyncLocalStorage.bind(() => 123));
+assert.strictEqual(fn(), 123);
+
+const fn2 = AsyncLocalStorage.bind(common.mustCall(arg => assert.strictEqual(arg, 'test')));
+fn2('test');

test/parallel/test-async-local-storage-snapshot.js

@@ -0,0 +1,10 @@
+'use strict';
+
+require('../common');
+const { strictEqual } = require('assert');
+const { AsyncLocalStorage } = require('async_hooks');
+
+const asyncLocalStorage = new AsyncLocalStorage();
+const runInAsyncScope = asyncLocalStorage.run(123, () => AsyncLocalStorage.snapshot());
+const result = asyncLocalStorage.run(321, () => runInAsyncScope(() => asyncLocalStorage.getStore()));
+strictEqual(result, 123);