node/test/parallel/test-stream-pipe-await-drain-push-while-write.js

'use strict';
const common = require('../common');
const stream = require('stream');
const assert = require('assert');

const awaitDrainStates = [
  1, // after first chunk before callback
  1, // after second chunk before callback
  0 // resolving chunk pushed after first chunk, awaitDrain is decreased
];

// A writable stream which pushes data onto the stream which pipes into it,
// but only the first time it's written to. Since it's not paused at this time,
// a second write will occur. If the pipe increases awaitDrain twice, we'll
// never get subsequent chunks because 'drain' is only emitted once.
const writable = new stream.Writable({
  write: common.mustCall(function(chunk, encoding, cb) {
    if (chunk.length === 32 * 1024) { // first chunk
      const beforePush = readable._readableState.awaitDrain;
      readable.push(new Buffer(34 * 1024)); // above hwm
      // We should check if awaitDrain counter is increased.
      const afterPush = readable._readableState.awaitDrain;
      assert.strictEqual(afterPush - beforePush, 1,
                         'Counter is not increased for awaitDrain');
    }

    assert.strictEqual(
      awaitDrainStates.shift(),
      readable._readableState.awaitDrain,
      'State variable awaitDrain is not correct.'
    );
    cb();
  }, 3)
});

// A readable stream which produces two buffers.
const bufs = [new Buffer(32 * 1024), new Buffer(33 * 1024)]; // above hwm
const readable = new stream.Readable({
  read: function() {
    while (bufs.length > 0) {
      this.push(bufs.shift());
    }
  }
});

readable.pipe(writable);