lib: lazy loaded

This makes sure the async hooks and more is only loaded in case it
is actually necessary.

PR-URL: #20567
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/internal/bootstrap/node.js

@@ -63,7 +63,17 @@
     if (global.__coverage__)
       NativeModule.require('internal/process/write-coverage').setup();
 
-    NativeModule.require('internal/trace_events_async_hooks').setup();
+
+    {
+      const traceEvents = process.binding('trace_events');
+      const traceEventCategory = 'node,node.async_hooks';
+
+      if (traceEvents.categoryGroupEnabled(traceEventCategory)) {
+        NativeModule.require('internal/trace_events_async_hooks')
+          .setup(traceEvents, traceEventCategory);
+      }
+    }
+
 
     if (process.config.variables.v8_enable_inspector) {
       NativeModule.require('internal/inspector_async_hook').setup();

lib/internal/trace_events_async_hooks.js

@@ -1,71 +1,63 @@
 'use strict';
 
-const trace_events = process.binding('trace_events');
-const async_wrap = process.binding('async_wrap');
-const async_hooks = require('async_hooks');
-
-// Use small letters such that chrome://tracing groups by the name.
-// The behavior is not only useful but the same as the events emitted using
-// the specific C++ macros.
-const BEFORE_EVENT = 'b'.charCodeAt(0);
-const END_EVENT = 'e'.charCodeAt(0);
-
-// In trace_events it is not only the id but also the name that needs to be
-// repeated. Since async_hooks doesn't expose the provider type in the
-// non-init events, use a map to manually map the asyncId to the type name.
-const typeMemory = new Map();
-
-const trace_event_category = 'node,node.async_hooks';
-
-// It is faster to emit trace_events directly from C++. Thus, this happens in
-// async_wrap.cc. However, events emitted from the JavaScript API or the
-// Embedder C++ API can't be emitted from async_wrap.cc. Thus they are
-// emitted using the JavaScript API. To prevent emitting the same event
-// twice the async_wrap.Providers list is used to filter the events.
-const nativeProviders = new Set(Object.keys(async_wrap.Providers));
-
-const hook = async_hooks.createHook({
-  init(asyncId, type, triggerAsyncId, resource) {
-    if (nativeProviders.has(type)) return;
-
-    typeMemory.set(asyncId, type);
-    trace_events.emit(BEFORE_EVENT, trace_event_category,
-                      type, asyncId,
-                      'triggerAsyncId', triggerAsyncId,
-                      'executionAsyncId', async_hooks.executionAsyncId());
-  },
-
-  before(asyncId) {
-    const type = typeMemory.get(asyncId);
-    if (type === undefined) return;
-
-    trace_events.emit(BEFORE_EVENT, trace_event_category,
-                      type + '_CALLBACK', asyncId);
-  },
-
-  after(asyncId) {
-    const type = typeMemory.get(asyncId);
-    if (type === undefined) return;
-
-    trace_events.emit(END_EVENT, trace_event_category,
-                      type + '_CALLBACK', asyncId);
-  },
-
-  destroy(asyncId) {
-    const type = typeMemory.get(asyncId);
-    if (type === undefined) return;
-
-    trace_events.emit(END_EVENT, trace_event_category,
-                      type, asyncId);
-
-    // cleanup asyncId to type map
-    typeMemory.delete(asyncId);
-  }
-});
-
-
-exports.setup = function() {
-  if (trace_events.categoryGroupEnabled(trace_event_category)) {
-    hook.enable();
-  }
+exports.setup = function(traceEvents, traceEventCategory) {
+  const async_wrap = process.binding('async_wrap');
+  const async_hooks = require('async_hooks');
+
+  // Use small letters such that chrome://tracing groups by the name.
+  // The behavior is not only useful but the same as the events emitted using
+  // the specific C++ macros.
+  const BEFORE_EVENT = 'b'.charCodeAt(0);
+  const END_EVENT = 'e'.charCodeAt(0);
+
+  // In traceEvents it is not only the id but also the name that needs to be
+  // repeated. Since async_hooks doesn't expose the provider type in the
+  // non-init events, use a map to manually map the asyncId to the type name.
+  const typeMemory = new Map();
+
+  // It is faster to emit traceEvents directly from C++. Thus, this happens
+  // in async_wrap.cc. However, events emitted from the JavaScript API or the
+  // Embedder C++ API can't be emitted from async_wrap.cc. Thus they are
+  // emitted using the JavaScript API. To prevent emitting the same event
+  // twice the async_wrap.Providers list is used to filter the events.
+  const nativeProviders = new Set(Object.keys(async_wrap.Providers));
+
+  async_hooks.createHook({
+    init(asyncId, type, triggerAsyncId, resource) {
+      if (nativeProviders.has(type)) return;
+
+      typeMemory.set(asyncId, type);
+      traceEvents.emit(BEFORE_EVENT, traceEventCategory,
+                       type, asyncId,
+                       'triggerAsyncId', triggerAsyncId,
+                       'executionAsyncId', async_hooks.executionAsyncId());
+    },
+
+    before(asyncId) {
+      const type = typeMemory.get(asyncId);
+      if (type === undefined) return;
+
+      traceEvents.emit(BEFORE_EVENT, traceEventCategory,
+                       type + '_CALLBACK', asyncId);
+    },
+
+    after(asyncId) {
+      const type = typeMemory.get(asyncId);
+      if (type === undefined) return;
+
+      traceEvents.emit(END_EVENT, traceEventCategory,
+                       type + '_CALLBACK', asyncId);
+    },
+
+    destroy(asyncId) {
+      const type = typeMemory.get(asyncId);
+      if (type === undefined) return;
+
+      traceEvents.emit(END_EVENT, traceEventCategory,
+                       type, asyncId);
+
+      // cleanup asyncId to type map
+      typeMemory.delete(asyncId);
+    }
+  }).enable();
 };