a.js

// this one passes with https://github.com/isaacs/node/commit/6377c814c65bc41dcf401bb97c57a4d15ba0e68d
if (process.argv[2] === 'child')
  child();
else
  parent();

function parent() {
  var spawn = require('child_process').spawn;
  var node = process.execPath;
  var child = spawn(node, [__filename, 'child']) //, {stdio:'inherit'});
  console.log('pid=%d', child.pid);
  child.stdout.on('data', function(c) {
    console.log("stdout: " + (''+c).trim().split(/\n/).join('\nstdout: '));
  });
  child.stderr.on('data', function(c) {
    console.log("stderr: " + (''+c).trim().split(/\n/).join('\nstderr: '));
  });
  child.on('exit', function() {
    console.log('child exit');
  });
}

function child() {
  console.log('hello');
  console.log(process.stdout);
}

b.js

// this one fails
var spawn = require('child_process').spawn
var env = { NODE_DEBUG: 'net' }

var c1 = spawn('node',['-e','console.log(1)']);
c1.stdout.pipe(process.stdout);
c1.stderr.pipe(process.stderr);
console.error('c1 pid =', c1.pid);

// var c2 = spawn('node',['-e','console.log(2)'], {env: env, stdio:'inherit'})
// console.error('c2 pid = %d', c2.pid);