evanlucas / spawn-wrap

Wrap all spawned Node.js child processes by adding environs and arguments ahead of the main JavaScript file argument.

spawn-wrap

Wrap all spawned Node.js child processes by adding environs and arguments ahead of the main JavaScript file argument.

Any child processes launched by that child process will also be wrapped in a similar fashion.

This is a bit of a brutal hack, designed primarily to support code coverage reporting in cases where tests or the system under test are loaded via child processes rather than via require().

It can also be handy if you want to run your own mock executable instead of some other thing when child procs call into it.

USAGE

var wrap = require('spawn-wrap')

// wrap(wrapperArgs, environs)
var unwrap = wrap(['/path/to/my/main.js', 'foo=bar'], { FOO: 1 })

// later to undo the wrapping, you can call the returned function
unwrap()

In this example, the /path/to/my/main.js file will be used as the "main" module, whenever any Node or io.js child process is started, whether via a call to spawn or exec, whether node is invoked directly as the command or as the result of a shebang #! lookup.

In /path/to/my/main.js, you can do whatever instrumentation or environment manipulation you like. When you're done, and ready to run the "real" main.js file (ie, the one that was spawned in the first place), you can do this:

// /path/to/my/main.js
// process.argv[1] === 'foo=bar'
// and process.env.FOO === '1'

// my wrapping manipulations
setupInstrumentationOrCoverageOrWhatever()
process.on('exit', function (code) {
  storeCoverageInfoSynchronously()
})

// now run the instrumented and covered or whatever codes
require('spawn-wrap').runMain()

CAVEATS

The initial wrap call uses synchronous I/O. Probably you should not be using this script in any production environments anyway.

Also, this will slow down child process execution by a lot, since we're adding a few layers of indirection.

I would not be very surprised to find out that it works on Windows. No way to tell for certain, though.