async_hooks: add AsyncLocal class

Introduces new AsyncLocal API to provide capabilities
for building continuation local storage on top of it.

The implementation is based on async hooks.

Public API is inspired by ThreadLocal class in Java.

benchmark/async_hooks/async-resource-vs-destroy.js

@@ -8,7 +8,8 @@ const common = require('../common.js');
 const {
   createHook,
   executionAsyncResource,
-  executionAsyncId
+  executionAsyncId,
+  AsyncLocal
 } = require('async_hooks');
 const { createServer } = require('http');
 
@@ -18,7 +19,7 @@ const connections = 500;
 const path = '/';
 
 const bench = common.createBenchmark(main, {
-  type: ['async-resource', 'destroy'],
+  type: ['async-resource', 'destroy', 'async-local'],
   asyncMethod: ['callbacks', 'async'],
   n: [1e6]
 });
@@ -102,6 +103,29 @@ function buildDestroy(getServe) {
   }
 }
 
+function buildAsyncLocal(getServe) {
+  const server = createServer(getServe(getCLS, setCLS));
+  const asyncLocal = new AsyncLocal();
+
+  return {
+    server,
+    close
+  };
+
+  function getCLS() {
+    return asyncLocal.get();
+  }
+
+  function setCLS(state) {
+    asyncLocal.set(state);
+  }
+
+  function close() {
+    asyncLocal.remove();
+    server.close();
+  }
+}
+
 function getServeAwait(getCLS, setCLS) {
   return async function serve(req, res) {
     setCLS(Math.random());
@@ -126,7 +150,8 @@ function getServeCallbacks(getCLS, setCLS) {
 
 const types = {
   'async-resource': buildCurrentResource,
-  'destroy': buildDestroy
+  'destroy': buildDestroy,
+  'async-local': buildAsyncLocal,
 };
 
 const asyncMethods = {

doc/api/async_hooks.md

@@ -579,6 +579,110 @@ const server = net.createServer((conn) => {
 Promise contexts may not get valid `triggerAsyncId`s by default. See
 the section on [promise execution tracking][].
 
+### Class: `AsyncLocal`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+This class can be used to store a value which follows asynchronous execution
+flow. Any value set on an `AsyncLocal` instance is propagated to any callback
+or promise executed within the flow. Because of that, a continuation local
+storage can be build with an `AsyncLocal` instance. This API is similar to
+thread local storage in other runtimes and languages.
+
+The implementation relies on async hooks to follow the execution flow.
+So, if an application or a library does not play nicely with async hooks,
+the same problems will be seen with the `AsyncLocal` API. In order to fix
+such issues the `AsyncResource` API should be used.
+
+The following example shows how to use `AsyncLocal` to build a simple logger
+that assignes ids to HTTP requests and includes them into messages logged
+within each request.
+
+```js
+const http = require('http');
+const { AsyncLocal } = require('async_hooks');
+
+const asyncLocal = new AsyncLocal();
+
+function print(msg) {
+  const id = asyncLocal.get();
+  console.log(`${id !== undefined ? id : '-'}:`, msg);
+}
+
+let idSeq = 0;
+http.createServer((req, res) => {
+  asyncLocal.set(idSeq++);
+  print('start');
+  setImmediate(() => {
+    print('finish');
+    res.end();
+  });
+}).listen(8080);
+
+http.get('http://localhost:8080');
+http.get('http://localhost:8080');
+// Prints:
+//   0: start
+//   1: start
+//   0: finish
+//   1: finish
+```
+
+#### `new AsyncLocal()`
+
+Creates a new instance of `AsyncLocal`.
+
+### `asyncLocal.get()`
+
+* Returns: {any}
+
+Returns the value of the `AsyncLocal` in current execution context,
+or `undefined` if the value is not set or the `AsyncLocal` was removed.
+
+### `asyncLocal.set(value)`
+
+* `value` {any}
+
+Sets the value for the `AsyncLocal` within current execution context.
+
+Once set, the value will be kept through the subsequent asynchronous calls,
+unless overridden by calling `asyncLocal.set(value)`:
+
+```js
+const asyncLocal = new AsyncLocal();
+
+setImmediate(() => {
+  asyncLocal.set('A');
+
+  setImmediate(() => {
+    console.log(asyncLocal.get());
+    // Prints: A
+
+    asyncLocal.set('B');
+    console.log(asyncLocal.get());
+    // Prints: B
+  });
+
+  console.log(asyncLocal.get());
+  // Prints: A
+});
+```
+
+If the `AsyncLocal` was removed before this call is made,
+[`ERR_ASYNC_LOCAL_CANNOT_SET_VALUE`][] is thrown.
+
+### `asyncLocal.remove()`
+
+When called, removes all values stored in the `AsyncLocal` and disables
+callbacks for the internal `AsyncHook` instance. Calling `asyncLocal.remove()`
+multiple times will have no effect.
+
+Any subsequent `asyncLocal.get()` calls will return `undefined`.
+Any subsequent `asyncLocal.set(value)` calls will throw
+[`ERR_ASYNC_LOCAL_CANNOT_SET_VALUE`][].
+
 ## Promise execution tracking
 
 By default, promise executions are not assigned `asyncId`s due to the relatively
@@ -868,3 +972,4 @@ for (let i = 0; i < 10; i++) {
 [PromiseHooks]: https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit
 [`Worker`]: worker_threads.html#worker_threads_class_worker
 [promise execution tracking]: #async_hooks_promise_execution_tracking
+[`ERR_ASYNC_LOCAL_CANNOT_SET_VALUE`]: errors.html#ERR_ASYNC_LOCAL_CANNOT_SET_VALUE

doc/api/errors.md

@@ -643,6 +643,11 @@ by the `assert` module.
 An attempt was made to register something that is not a function as an
 `AsyncHooks` callback.
 
+<a id="ERR_ASYNC_LOCAL_CANNOT_SET_VALUE"></a>
+### `ERR_ASYNC_LOCAL_CANNOT_SET_VALUE`
+
+An attempt was made to set value for a `AsyncLocal` after it was removed.
+
 <a id="ERR_ASYNC_TYPE"></a>
 ### `ERR_ASYNC_TYPE`
 

lib/async_hooks.js

@@ -4,12 +4,14 @@ const {
   NumberIsSafeInteger,
   ReflectApply,
   Symbol,
+  WeakMap,
 } = primordials;
 
 const {
   ERR_ASYNC_CALLBACK,
   ERR_ASYNC_TYPE,
-  ERR_INVALID_ASYNC_ID
+  ERR_INVALID_ASYNC_ID,
+  ERR_ASYNC_LOCAL_CANNOT_SET_VALUE,
 } = require('internal/errors').codes;
 const { validateString } = require('internal/validators');
 const internal_async_hooks = require('internal/async_hooks');
@@ -132,6 +134,63 @@ function createHook(fns) {
   return new AsyncHook(fns);
 }
 
+// AsyncLocal API //
+
+const locals = [];
+const localsHook = createHook({
+  init(asyncId, type, triggerAsyncId, resource) {
+    const execRes = executionAsyncResource();
+    // Using var here instead of let because "for (var ...)" is faster than let.
+    // Refs: https://github.com/nodejs/node/pull/30380#issuecomment-552948364
+    for (var i = 0; i < locals.length; i++) {
+      locals[i][kPropagateSymbol](execRes, resource);
+    }
+  }
+});
+
+const kResToValSymbol = Symbol('resToVal');
+const kPropagateSymbol = Symbol('propagate');
+
+class AsyncLocal {
+  constructor() {
+    this[kResToValSymbol] = new WeakMap();
+    locals.push(this);
+    localsHook.enable();
+  }
+
+  [kPropagateSymbol](execRes, initRes) {
+    const value = this[kResToValSymbol].get(execRes);
+    // Always overwrite value to prevent issues with reused resources.
+    this[kResToValSymbol].set(initRes, value);
+  }
+
+  get() {
+    if (this[kResToValSymbol]) {
+      return this[kResToValSymbol].get(executionAsyncResource());
+    }
+    return undefined;
+  }
+
+  set(value) {
+    if (!this[kResToValSymbol]) {
+      throw new ERR_ASYNC_LOCAL_CANNOT_SET_VALUE();
+    }
+    this[kResToValSymbol].set(executionAsyncResource(), value);
+  }
+
+  remove() {
+    const index = locals.indexOf(this);
+    if (index === -1)
+      return;
+
+    delete this[kResToValSymbol];
+    locals.splice(index, 1);
+    if (locals.size === 0) {
+      localsHook.disable();
+    }
+  }
+}
+
 
 // Embedder API //
 
@@ -213,6 +272,7 @@ module.exports = {
   executionAsyncId,
   triggerAsyncId,
   executionAsyncResource,
+  AsyncLocal,
   // Embedder API
   AsyncResource,
 };

lib/internal/errors.js

@@ -730,6 +730,8 @@ E('ERR_AMBIGUOUS_ARGUMENT', 'The "%s" argument is ambiguous. %s', TypeError);
 E('ERR_ARG_NOT_ITERABLE', '%s must be iterable', TypeError);
 E('ERR_ASSERTION', '%s', Error);
 E('ERR_ASYNC_CALLBACK', '%s must be a function', TypeError);
+E('ERR_ASYNC_LOCAL_CANNOT_SET_VALUE', 'Cannot set value for removed AsyncLocal',
+  Error);
 E('ERR_ASYNC_TYPE', 'Invalid name for async "type": %s', TypeError);
 E('ERR_BROTLI_INVALID_PARAM', '%s is not a valid Brotli parameter', RangeError);
 E('ERR_BUFFER_OUT_OF_BOUNDS',

test/async-hooks/test-async-local-isolation.js

@@ -0,0 +1,29 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const async_hooks = require('async_hooks');
+const { AsyncLocal } = async_hooks;
+
+// This test ensures isolation of `AsyncLocal`s
+// from each other in terms of stored values
+
+const asyncLocalOne = new AsyncLocal();
+const asyncLocalTwo = new AsyncLocal();
+
+setTimeout(() => {
+  assert.strictEqual(asyncLocalOne.get(), undefined);
+  assert.strictEqual(asyncLocalTwo.get(), undefined);
+
+  asyncLocalOne.set('foo');
+  asyncLocalTwo.set('bar');
+  assert.strictEqual(asyncLocalOne.get(), 'foo');
+  assert.strictEqual(asyncLocalTwo.get(), 'bar');
+
+  asyncLocalOne.set('baz');
+  asyncLocalTwo.set(42);
+  setTimeout(() => {
+    assert.strictEqual(asyncLocalOne.get(), 'baz');
+    assert.strictEqual(asyncLocalTwo.get(), 42);
+  }, 0);
+}, 0);

test/async-hooks/test-async-local-propagation.js

@@ -0,0 +1,30 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const async_hooks = require('async_hooks');
+const { AsyncLocal } = async_hooks;
+
+// This test ensures correct work of the global hook
+// that serves for propagation of all `AsyncLocal`s
+// in the context of `.get()`/`.set(value)` calls
+
+const asyncLocal = new AsyncLocal();
+
+setTimeout(() => {
+  assert.strictEqual(asyncLocal.get(), undefined);
+
+  asyncLocal.set('A');
+  setTimeout(() => {
+    assert.strictEqual(asyncLocal.get(), 'A');
+
+    asyncLocal.set('B');
+    setTimeout(() => {
+      assert.strictEqual(asyncLocal.get(), 'B');
+    }, 0);
+
+    assert.strictEqual(asyncLocal.get(), 'B');
+  }, 0);
+
+  assert.strictEqual(asyncLocal.get(), 'A');
+}, 0);

test/async-hooks/test-async-local-removal.js

@@ -0,0 +1,30 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const async_hooks = require('async_hooks');
+const { AsyncLocal } = async_hooks;
+
+// This test ensures correct work of the global hook
+// that serves for propagation of all `AsyncLocal`s
+// in the context of `.remove()` call
+
+const asyncLocalOne = new AsyncLocal();
+asyncLocalOne.set(1);
+const asyncLocalTwo = new AsyncLocal();
+asyncLocalTwo.set(2);
+
+setImmediate(() => {
+  // Removal of one local should not affect others
+  asyncLocalTwo.remove();
+  assert.strictEqual(asyncLocalOne.get(), 1);
+
+  // Removal of the last active local should not
+  // prevent propagation of locals created later
+  asyncLocalOne.remove();
+  const asyncLocalThree = new AsyncLocal();
+  asyncLocalThree.set(3);
+  setImmediate(() => {
+    assert.strictEqual(asyncLocalThree.get(), 3);
+  });
+});