node/lib/internal/trace_events_async_hooks.js

'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://traceing 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();

// 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, 'node.async_hooks',
                      type, asyncId,
                      'triggerAsyncId', triggerAsyncId,
                      'executionAsyncId', async_hooks.executionAsyncId());
  },

  before(asyncId) {
    const type = typeMemory.get(asyncId);
    if (type === undefined) return;

    trace_events.emit(BEFORE_EVENT, 'node.async_hooks',
                      type + '_CALLBACK', asyncId);
  },

  after(asyncId) {
    const type = typeMemory.get(asyncId);
    if (type === undefined) return;

    trace_events.emit(END_EVENT, 'node.async_hooks',
                      type + '_CALLBACK', asyncId);
  },

  destroy(asyncId) {
    const type = typeMemory.get(asyncId);
    if (type === undefined) return;

    trace_events.emit(END_EVENT, 'node.async_hooks',
                      type, asyncId);

    // cleanup asyncId to type map
    typeMemory.delete(asyncId);
  }
});


exports.setup = function() {
  if (trace_events.categoryGroupEnabled('node.async_hooks')) {
    hook.enable();
  }
};