Add support for proper handling of file protocol (in windows)

[kriszyp]

Trying to debug node with DevTools with transpiled code on Windows is a mess. First, this is not the fault of node-source-map-support at all. It works great on windows on its own. But the hacks necessary to get this to work do break node-source-map-support.

The basic problem is this: Node uses OS file paths, and DevTools doesn't understand OS file paths, it only understands URIs (including file:// URIs). So to actually get DevTools to comprehend source maps (so that you can set breakpoints and step through source code), you need to map your node modules to file:// URIs, so that DevTools can then do the relative path lookups to find your source maps/code. This means your modules that registered in Node need to have filenames like "file:///C:/dev/my-module.js". And this is where the most critical issue in node-source-map-support comes in: supportRelativeURL will break that up into "file://" and "/C:/dev/my-module.js" which path.resolve incorrectly computes to something like "C:/C:/dev/module.js" (plus relative path). And because supportRelativeURL can't be replaced by options, I think this actually needs to be fixed in node-source-map-support.

I figured while I was fixing this, I would also fix the default retrieveFileHandler to be able to properly handle file:// support (since existsSync/readFileSync doesn't on its own). Although technically you can provide a custom file handler to override this, as a user.

If you are wondering how I map node modules to file:// URLs, in our application I have been monkey patching the vm.runInThisContext function:

  vm.runInThisContext = function(wrapper, options) {
    if (options && options.filename && options.filename.match(/\.js$/) && options.filename.indexOf('node_modules') == -1) {
      options.filename = 'file:///' + options.filename.replace(/\\/g, '/')
    }
    return runInThisContext.apply(this, arguments)
  }

I suppose bonus support would be to actually add an option that does this for the user.

With this all in place, I now have the complete zen harmony of perfectly source-mapped modules in my stack traces and dev tools. I can set breakpoints in source code, I can click on stack traces, and go to the right source in dev tools. It all works beautifully.

I haven't done anything with tests. I wasn't sure how much of this would be willing to be accepted. Let me know if you want anything else for this PR.

[LinusU]

Seems like a good approach 👍

Any idea why the tests are failing on 0.x?

[LinusU]

I don't see how this could break anything that is working right now, so lets ship it!

Thank you for the PR

[LinusU]

Published as 0.5.1

[kriszyp]

Wow, that was fast, thank you!