doc: expand on promises and async_hooks

doc/api/async_hooks.md

@@ -509,6 +509,9 @@ const server = net.createServer(function onConnection(conn) {
 });
 ```
 
+Note that promise contexts may not get precise executionAsyncIds by default.
+See the section on [promise execution tracking][].
+
 #### `async_hooks.triggerAsyncId()`
 
 * Returns: {number} The ID of the resource responsible for calling the callback
@@ -531,6 +534,57 @@ const server = net.createServer((conn) => {
 });
 ```
 
+Note that promise contexts may not get valid triggerAsyncIds by default. See
+the section on [promise execution tracking][].
+
+## Promise execution tracking
+
+By default, promise executions are not assigned asyncIds due to the relatively
+expensive nature of the [promise introspection API][PromiseHooks] provided by
+V8. This means that programs using promises or `async`/`await` will not get
+correct execution and trigger ids for promise callback contexts by default.
+
+Here's an example:
+
+```js
+const ah = require('async_hooks');
+Promise.resolve(1729).then(() => {
+  console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);
+});
+// produces:
+// eid 1 tid 0
+```
+
+Observe that the `then` callback claims to have executed in the context of the
+outer scope even though there was an asynchronous hop involved. Also note that
+the triggerAsyncId value is 0, which means that we are missing context about the
+resource that caused (triggered) the `then` callback to be executed.
+
+Installing async hooks via `async_hooks.createHook` enables promise execution
+tracking. Example:
+
+```js
+const ah = require('async_hooks');
+ah.createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
+Promise.resolve(1729).then(() => {
+  console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);
+});
+// produces:
+// eid 7 tid 6
+```
+
+In this example, adding any actual hook function enabled the tracking of
+promises. There are two promises in the example above; the promise created by
+`Promise.resolve()` and the promise returned by the call to `then`. In the
+example above, the first promise got the asyncId 6 and the latter got asyncId 7.
+During the execution of the `then` callback, we are executing in the context of
+promise with asyncId 7. This promise was triggered by async resource 6.
+
+Another subtlety with promises is that `before` and `after` callbacks are run
+only on chained promises. That means promises not created by `then`/`catch` will
+not have the `before` and `after` callbacks fired on them. For more details see
+the details of the V8 [PromiseHooks][] API.
+
 ## JavaScript Embedder API
 
 Library developers that handle their own asynchronous resources performing tasks
@@ -655,3 +709,5 @@ constructor.
 [`destroy` callback]: #async_hooks_destroy_asyncid
 [`init` callback]: #async_hooks_init_asyncid_type_triggerasyncid_resource
 [Hook Callbacks]: #async_hooks_hook_callbacks
+[PromiseHooks]: https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk
+[promise execution tracking]: #async_hooks_promise_execution_tracking