p.js

var assert = require('assert');
var stream = require('stream');
var r = new stream.Readable({ encoding: 'ascii' });

// at the first read, no data is available yet.
var s = [['','',''],['a','b','c'],['d','e','f'],['g','h','i']];
r._read = function(n, cb) {
  console.error('_READ', s[0] ? s[0].join('') : 'EOF');
  var l = s.shift();
  if (l) {
    r.push(l[0]);
    r.push(l[1]);
    r.push(l[2]);
  } else {
    cb(null, null);
  }
};

var ended = false;
r.on('end', function() {
  ended = true;
});

assert.equal(r.read(), null);
assert.equal(r.read(), 'abc');
r.push('1');
r.push('2');
r.push('3');
assert.equal(r.read(), '123def');
assert.equal(r.read(), 'ghi');
assert.equal(r.read(), null);
process.on('exit', function() {
  assert(ended);
  console.log('ok');
});