flagged-respawn

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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 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

$3

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:

| Parameter | Type | Description |
| :------------ | :-------------: | :--------------------------------------------------------------------------------------- |
| _flags_ | Array | An array of node flags and V8 flags which are available when present in _argv_. |
| _argv_ | Array | Command line arguments to respawn. |
| _forcedFlags_ | Array or String | An array of node flags or a string of a single flag and V8 flags for respawning forcely. |
| _callback_ | function | A called function when not respawning or after respawned. |

- callback(ready, proc, argv) : Void

_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. |

License

MIT

[downloads-image]: https://img.shields.io/npm/dm/flagged-respawn.svg?style=flat-square
[npm-url]: https://www.npmjs.com/package/flagged-respawn
[npm-image]: https://img.shields.io/npm/v/flagged-respawn.svg?style=flat-square

[ci-url]: https://github.com/gulpjs/flagged-respawn/actions?query=workflow:dev
[ci-image]: https://img.shields.io/github/workflow/status/gulpjs/flagged-respawn/dev?style=flat-square

[coveralls-url]: https://coveralls.io/r/gulpjs/flagged-respawn
[coveralls-image]: https://img.shields.io/coveralls/gulpjs/flagged-respawn/master.svg?style=flat-square