nodejs 14.0.0源码分析之setImmediate

function setImmediate(callback, arg1, arg2, arg3) {
  if (typeof callback !== 'function') {
    throw new ERR_INVALID_CALLBACK(callback);
  }

  let i, args;
  switch (arguments.length) {
    case 1:
      break;
    case 2:
      args = [arg1];
      break;
    case 3:
      args = [arg1, arg2];
      break;
    default:
      args = [arg1, arg2, arg3];
      for (i = 4; i < arguments.length; i++) {
        args[i - 1] = arguments[i];
      }
      break;
  }

  return new Immediate(callback, args);
}

const Immediate = class Immediate {
  constructor(callback, args) {
    this._idleNext = null;
    this._idlePrev = null;
    this._onImmediate = callback;
    this._argv = args;
    this._destroyed = false;
    this[kRefed] = false;

    initAsyncResource(this, 'Immediate');

    this.ref();
    // Immediate链表的节点个数，包括ref和unref状态
    immediateInfo[kCount]++;
    // 加入链表中
    immediateQueue.append(this);
  }
  // 打上ref标记，往libuv的idle链表插入一个节点，如果还没有的话
  ref() {
    if (this[kRefed] === false) {
      this[kRefed] = true;
      if (immediateInfo[kRefCount]++ === 0)
        toggleImmediateRef(true);
    }
    return this;
  }
  // 和上面相反
  unref() {
    if (this[kRefed] === true) {
      this[kRefed] = false;
      if (--immediateInfo[kRefCount] === 0)
        toggleImmediateRef(false);
    }
    return this;
  }

  hasRef() {
    return !!this[kRefed];
  }
};

const immediateQueue = new ImmediateList();

// 双向非循环的链表
function ImmediateList() {
  this.head = null;
  this.tail = null;
}

// Appends an item to the end of the linked list, adjusting the current tail's
// previous and next pointers where applicable
ImmediateList.prototype.append = function(item) {
  // 尾指针非空，说明链表非空，直接追加在尾节点后面
  if (this.tail !== null) {
    this.tail._idleNext = item;
    item._idlePrev = this.tail;
  } else {
    // 尾指针是空说明链表是空的，头尾指针都指向item
    this.head = item;
  }
  this.tail = item;
};

// Removes an item from the linked list, adjusting the pointers of adjacent
// items and the linked list's head or tail pointers as necessary
ImmediateList.prototype.remove = function(item) {
  // 如果item在中间则自己全身而退，前后两个节点连上
  if (item._idleNext !== null) {
    item._idleNext._idlePrev = item._idlePrev;
  }

  if (item._idlePrev !== null) {
    item._idlePrev._idleNext = item._idleNext;
  }
  // 是头指针，则需要更新头指针指向item的下一个，因为item被删除了，尾指针同理
  if (item === this.head)
    this.head = item._idleNext;
  if (item === this.tail)
    this.tail = item._idlePrev;
  // 重置前后指针
  item._idleNext = null;
  item._idlePrev = null;
};

void ToggleImmediateRef(const FunctionCallbackInfo<Value>& args) {
  Environment::GetCurrent(args)->ToggleImmediateRef(args[0]->IsTrue());
}

void Environment::ToggleImmediateRef(bool ref) {
  if (started_cleanup_) return;
  // 往idle链表插入/删除一个节点，插入节点是防止在poll io阶段阻塞
  if (ref) {
    // Idle handle is needed only to stop the event loop from blocking in poll.
    uv_idle_start(immediate_idle_handle(), [](uv_idle_t*){ });
  } else {
    uv_idle_stop(immediate_idle_handle());
  }
}

  uv_check_start(immediate_check_handle(), CheckImmediate);

void Environment::CheckImmediate(uv_check_t* handle) {

  // 省略部分代码
  // 没有Immediate节点需要处理
  if (env->immediate_info()->count() == 0 || !env->can_call_into_js())
    return;

  do {
      // 执行js层回调immediate_callback_function
    MakeCallback(env->isolate(),
                 env->process_object(),
                 env->immediate_callback_function(),
                 0,
                 nullptr,
                 {0, 0}).ToLocalChecked();
  } while (env->immediate_info()->has_outstanding() && env->can_call_into_js());
  // 所有的immediate节点都处理完了，删除idle链表的那个节点，即允许poll io阶段阻塞
  if (env->immediate_info()->ref_count() == 0)
    env->ToggleImmediateRef(false);
}

const { setupTimers } = internalBinding('timers')
setupTimers(processImmediate, processTimers);
// js层setupTimers =》 c++层 SetupTimers
void SetupTimers(const FunctionCallbackInfo<Value>& args) {
  CHECK(args[0]->IsFunction());
  CHECK(args[1]->IsFunction());
  auto env = Environment::GetCurrent(args);

  env->set_immediate_callback_function(args[0].As<Function>());
  env->set_timers_callback_function(args[1].As<Function>());
}

const immediateQueue = new ImmediateList();
const outstandingQueue = new ImmediateList()

function processImmediate() {
    // 上次执行processImmediate的时候有遗留的节点则执行outstandingQueue队列，这时候immediateQueue队列是空的
    const queue = outstandingQueue.head !== null ?
      outstandingQueue : immediateQueue;
    let immediate = queue.head;

    /*
      在执行immediateQueue队列的话，先置空队列，避免执行回调的时候一直往队列加节点，死循环。
      所以新加的接口会插入新的队列，不会在本次被执行。
      并打一个标记,全部immediateQueue节点都被执行则清空，否则会再执行processImmediate一次，见Environment::CheckImmediate
    */
    if (queue !== outstandingQueue) {
      queue.head = queue.tail = null;
      immediateInfo[kHasOutstanding] = 1;
    }

    let prevImmediate;
    let ranAtLeastOneImmediate = false;
    while (immediate !== null) {
      // 执行宏任务
      if (ranAtLeastOneImmediate)
        runNextTicks();
      else
        ranAtLeastOneImmediate = true;

      // 宏任务把该节点删除了，则不需要指向他的回调了，继续下一个
      if (immediate._destroyed) {
        outstandingQueue.head = immediate = prevImmediate._idleNext;
        continue;
      }

      immediate._destroyed = true;
      // 执行完要修改个数
      immediateInfo[kCount]--;
      if (immediate[kRefed])
        immediateInfo[kRefCount]--;
      immediate[kRefed] = null;
      // 见上面if (immediate._destroyed)的注释
      prevImmediate = immediate;

      // 执行回调，指向下一个节点
      try {
        const argv = immediate._argv;
        if (!argv)
          immediate._onImmediate();
        else
          immediate._onImmediate(...argv);
      } finally {
        immediate._onImmediate = null;

        if (destroyHooksExist())
          emitDestroy(asyncId);

        outstandingQueue.head = immediate = immediate._idleNext;
      }
    }
    // 当前执行的是outstandingQueue的话则把他清空
    if (queue === outstandingQueue)
      outstandingQueue.head = null;
    // 全部节点执行完
    immediateInfo[kHasOutstanding] = 0;
  }