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flagged-respawn

A tool for respawning node binaries when special flags are present.

Say you wrote a command line tool that runs arbitrary javascript (e.g. task runner, test framework, etc). For the sake of discussion, let's pretend it's a testing harness you've named testify.

Everything is going splendidly until one day you decide to test some code that relies on a feature behind a v8 flag in node (--harmony, for example). Without much thought, you run testify --harmony spec tests.js.

It doesn't work. After digging around for a bit, you realize this produces a process.argv of:

['node', '/usr/local/bin/test', '--harmony', 'spec', 'tests.js']

Crap. The --harmony flag is in the wrong place! It should be applied to the node command, not our binary. What we actually wanted was this:

['node', '--harmony', '/usr/local/bin/test', 'spec', 'tests.js']

Flagged-respawn fixes this problem and handles all the edge cases respawning creates, such as: - Providing a method to determine if a respawn is needed. - Piping stderr/stdout from the child into the parent. - Making the parent process exit with the same code as the child. - If the child is killed, making the parent exit with the same signal.

To see it in action, clone this repository and run npm install / npm run respawn / npm run nospawn.

#!/usr/bin/env node

const flaggedRespawn = require('flagged-respawn');

// get a list of all possible v8 flags for the running version of node
const v8flags = require('v8flags').fetch();

flaggedRespawn(v8flags, process.argv, function (ready, child) {
  if (ready) {
    console.log('Running!');
    // your cli code here
  } else {
    console.log('Special flags found, respawning.');
  }
  if (process.pid !== child.pid) {
    console.log('Respawned to PID:', child.pid);
  }
});