URL(filePath:) should resolve Windows drive-relative paths

[jrflat]

When a URL is initialized with a Windows drive-relative path like URL(filePath: "S:relative/path"), we need to check the current working directory of the S: drive (not necessarily the cwd of the process) and use that as the base URL. Then, we should strip the S: from the relative path so that it resolves correctly against the base path.

E.g. if the current working directory of the S: drive is /current/directory/, we would get

let url = URL(filePath: "S:relative/path")
print(url.relativePath) // "relative/path"
print(url.baseURL?.path) // "S:/current/directory"
print(url.path) // "S:/current/directory/relative/path"

[jrflat]

@swift-ci please test

[compnerd]

I wonder if this is better written as:

let bIsAbsolute: Bool = path.withNTPathRepresentation { !PathIsRelativeW($0) }

[jrflat]

Yeah, that sounds like a much nicer/robust replacement for the isAbsolute code, I'll do that, thanks!

I think we would still need to differentiate between C:relative\path and relative\path if we want to strip the C: before storing the relative path in URL, so we'll still need to do the alpha/colon/slash check here.

[jrflat]

Hm, I'm seeing PathIsRelativeW return FALSE for C:relative\path (probably because it's resolved against the absolute cwd for C:), so it might not work for this use case.

[jrflat]

We could rework the URL(filePath:) initializer to immediately standardize C:relative\path to its absolute representation, and not use the relative path + base URL distinction. This would essentially be:

bIsAbsolute = !PathIsRelativeW(path)
if bIsAbsolute {
  path = GetFullPathNameW(path)
  baseURL = nil
} else {
  // path is relative
  baseURL = URL.currentDirectory()
}

PathIsRelative("/slash/path") does return TRUE, though, so we'd need to make sure we strip the leading slash in other code when merging with the base path.