Node.jsStreamondata触发时机与顺序的探索-创新互联

上次写Stream pipe细节时，在源码中发现一段无用逻辑，由此引发了对Stream data事件触发时机与顺序的探索。

无用逻辑

当时研究pipe细节是基于Node.js v8.11.1的源码，其中针对上游的ondata事件处理有如下一段代码：

// If the user pushes more data while we're writing to dest then we'll end up
// in ondata again. However, we only want to increase awaitDrain once because
// dest will only emit one 'drain' event for the multiple writes.
// => Introduce a guard on increasing awaitDrain.
var increasedAwaitDrain = false;
src.on('data', ondata);
function ondata(chunk) {
  debug('ondata');
  increasedAwaitDrain = false;
  var ret = dest.write(chunk);
  if (false === ret && !increasedAwaitDrain) {
    if (((state.pipesCount === 1 && state.pipes === dest) ||
        (state.pipesCount > 1 && state.pipes.indexOf(dest) !== -1)) &&
      !cleanedUp) {
      debug('false write response, pause', src._readableState.awaitDrain);
      src._readableState.awaitDrain++;
      increasedAwaitDrain = true;
    }
    src.pause();
  }
}