inspector: enable async stack traces

Implement a special async_hooks listener that forwards information
about async tasks to V8Inspector asyncTask* API, thus enabling
DevTools feature "async stack traces".

The feature is enabled only on 64bit platforms due to a technical
limitation of V8 Inspector: inspector uses a pointer as a task id,
while async_hooks use 64bit numbers as ids.

To avoid performance penalty of async_hooks when not debugging,
the new listener is enabled only when the process enters a debug mode:

 - When the process is started with `--inspect` or `--inspect-brk`,
   the listener is enabled immediately and async stack traces
   lead all the way to the first tick of the event loop.

 - When the debug mode is enabled via SIGUSR1 or `_debugProcess()`,
   the listener is enabled together with the debugger. As a result,
   only async operations started after the signal was received
   will be correctly observed and reported to V8 Inspector. For example,
   a `setInterval()` called in the first tick of the event will not be
   shown in the async stack trace when the callback is invoked. This
   behaviour is consistent with Chrome DevTools.

Last but not least, this commit fixes handling of InspectorAgent's
internal property `enabled_` to ensure it's set back to `false`
after the debugger is deactivated (typically via `process._debugEnd()`).

Fixes: https://github.com/nodejs/node/issues/11370
PR-URL: https://github.com/nodejs/node/pull/13870
Reviewed-by: Timothy Gu <timothygu99@gmail.com>
Reviewed-by: Anna Henningsen <anna@addaleax.net>

1 files changed, 64 insertions, 0 deletions

diff --git a/lib/internal/inspector_async_hook.js b/lib/internal/inspector_async_hook.js
new file mode 100644
index 0000000000..e32a026cd6
--- /dev/null
+++ b/lib/internal/inspector_async_hook.js
@@ -0,0 +1,64 @@
+'use strict';
+
+const { createHook } = require('async_hooks');
+const inspector = process.binding('inspector');
+const config = process.binding('config');
+
+if (!inspector || !inspector.asyncTaskScheduled) {
+  exports.setup = function() {};
+  return;
+}
+
+const hook = createHook({
+  init(asyncId, type, triggerAsyncId, resource) {
+    // It's difficult to tell which tasks will be recurring and which won't,
+    // therefore we mark all tasks as recurring. Based on the discussion
+    // in https://github.com/nodejs/node/pull/13870#discussion_r124515293,
+    // this should be fine as long as we call asyncTaskCanceled() too.
+    const recurring = true;
+    inspector.asyncTaskScheduled(type, asyncId, recurring);
+  },
+
+  before(asyncId) {
+    inspector.asyncTaskStarted(asyncId);
+  },
+
+  after(asyncId) {
+    inspector.asyncTaskFinished(asyncId);
+  },
+
+  destroy(asyncId) {
+    inspector.asyncTaskCanceled(asyncId);
+  },
+});
+
+function enable() {
+  if (config.bits < 64) {
+    // V8 Inspector stores task ids as (void*) pointers.
+    // async_hooks store ids as 64bit numbers.
+    // As a result, we cannot reliably translate async_hook ids to V8 async_task
+    // ids on 32bit platforms.
+    process.emitWarning(
+      'Warning: Async stack traces in debugger are not available ' +
+      `on ${config.bits}bit platforms. The feature is disabled.`,
+      {
+        code: 'INSPECTOR_ASYNC_STACK_TRACES_NOT_AVAILABLE',
+      });
+  } else {
+    hook.enable();
+  }
+}
+
+function disable() {
+  hook.disable();
+}
+
+exports.setup = function() {
+  inspector.registerAsyncHook(enable, disable);
+
+  if (inspector.isEnabled()) {
+    // If the inspector was already enabled via --inspect or --inspect-brk,
+    // the we need to enable the async hook immediately at startup.
+    enable();
+  }
+};