Support win32 paths

https://github.com/webpack/webpack-dev-middleware/pull/366#issuecomment-462072465

memfs does not handle win32 paths starting with C:\ (unless you are actually on a windows machine). Probably not on your roadmap but it’s a blocker for a personal pipe dream of replacing memory-fs with memfs.

const {vol} = require('memfs')
// treated as relative
vol.mkdirpSync("C:\\test")  // <-- creates a folder in my current working directory.
console.log(vol.toJSON())
// { '/Users/username/my-test/C:\\test': null }

const {vol} = require('memfs')
// treated as relative (and throws errors as expected)
// https://github.com/streamich/memfs/blob/master/docs/relative-paths.md
vol.mkdirSync("C:\\test"); // <-- throws Error: ENOENT: no such file or directory
vol.mkdirSync("C:\\"); // <-- throws Error: ENOENT: no such file or directory

By comparison, memory-fs treats paths starting with a / as probably posix and starting with [A-Z]: as probably win32. In the examples above, the presence of C: would be enough to convince memory-fs that it was an absolute win32 path.

memory-fs has no support for relative paths and throws if it encounters one. The relative path compromise that memfs makes is probably preferred as it’s more similar to what fs does. But in the case of a win32 path prefix, memory-fs makes the more flexible choice.

FWIW, on my machine (a Mac), the following works exactly as memfs does. I don’t know what it does on a Windows machine.

const fs = require('fs')
// treated as relative
fs.mkdirSync("C:\\test") // <-- creates a folder in my current working directory.

[streamich]

To write tests for Windows, my first try would be:

1. Create a separate test suite for Windows.
2. At the very beginning, before importing anything, set process.platform = 'win32'.
3. memfs should pick that up and treat all paths as Windows paths.

[heygrady]

I looked deeper. Seems like you handle win32 correctly but only when process.platform === 'win32'.

Is there a way to temporarily enable win32 mode? Would it be possible to inspect paths to see if there are absolute win32 paths? Or, can you recommend a way to write tests for win32 mode?

// it seems like this would _always_ truthy.
// on my mac, `pathModule === pathModule.posix` and `pathModule !== pathModule.win32`
// however, `!!pathModule.win32 === true`
if (pathModule.posix) {
  const { posix } = pathModule;
  sep = posix.sep;
  relative = posix.relative;
} else {
  // would this even happen? is `path.posix` simply not available on windows?
  sep = pathModule.sep;
  relative = pathModule.relative;
}

const isWin = process.platform === 'win32';

// apparently you can check if the path in question is an absolute path by platform
pathModule.win32.isAbsolute('C:\\') // --> true
pathModule.posix.isAbsolute('C:\\') // --> false