flagged-respawn 

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


What is it?

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](http://nodejs.org/docs/latest/api/process.html#process_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.

Sample Usage

js
#!/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);
  }
});

API

<u>flaggedRespawn(flags, argv, [ forcedFlags, ] callback) : Void</u>

Respawns the script itself when *argv* has special flag contained in *flags* and/or *forcedFlags* is not empty. Because members of *flags* and *forcedFlags* are passed to node command, each of them needs to be a node flag or a V8 flag.

Forbid respawning

If --no-respawning flag is given in *argv*, this function does not respawned even if *argv* contains members of flags or *forcedFlags* is not empty. (This flag is also used internally to prevent from respawning more than once).

Parameter:

• <u><i>callback</i>(ready, proc, argv) : Void</u>
  
  *callback* function is called both when respawned or not, and it can be distinguished by callback's argument: *ready*. (*ready* indicates whether a process spawned its child process (false) or not (true), but it does not indicate whether a process is a spawned child process or not. *ready* for a spawned child process is true.)
  
  *argv* is an array of command line arguments which is respawned (when *ready* is false) or is passed current process except flags within *flags* and --no-respawning (when *ready* is true).
  
  Parameter:
  
  | Parameter | Type | Description | |:----------|:-------:|:--------------------------| | *ready* | boolean | True, if not respawning and is ready to execute main function. | | *proc* | object | Child process object if respawned, otherwise current process object. | | *argv* | Array | An array of command line arguments. |

Release History

• 2017-12-16 - v1.0.0 - Force/Forbid respawn, Improved API & testing
• 2016-03-22 - v0.3.2 - fix issue with v8 flags values being dropped
• 2014-09-12 - v0.3.1 - use { stdio: 'inherit' } for spawn to maintain colors
• 2014-09-11 - v0.3.0 - for real this time
• 2014-09-11 - v0.2.0 - cleanup
• 2014-09-04 - v0.1.1 - initial release