[SR-6527] Linux: Empty `URLComponents` does not generate valid URL

[mbrandonw]

Previous ID	SR-6527
Radar	None
Original Reporter	@mbrandonw
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug, Linux
Assignee	None
Priority	Medium

md5: 22fd63926de4684cc07b905a8d223d78

Issue Description:

This code:

    let baseUrl = URL(string: "https://www.pointfree.co")
    let urlComponents = URLComponents()
    let url = urlComponents.url(relativeTo: baseUrl)

generates a valid `URL` on Darwin, but generates `nil` on Linux.

I am using Xcode 9.2 beta 9C34b, Swift 4.0.3

[pushkarnk]

This is interesting.

`urlComponents.url(relativeTo: baseUrl)` uses `urlComponents.string` which evaluates to an empty string (on both Linux and Darwin) and passes it to `URL(string:relativeTo)` which is expected to return nil if `string` is empty.

So, the `URLComponents.url(relativeTo)` implementation is a composition of `urlComponents.string` and `URL(string:relativeTo)` both of which are behaving as expected. May be it needs an alternate implementation?

[pushkarnk]

https://developer.apple.com/documentation/foundation/urlcomponents/1780048-url does not discuss the case of URLComponents value where nothing is defined. May be we should simply add a special case to make the behaviour same as Darwin.

[ShikiSuen]

I wonder whether there's anyone still looking at this issue.
This brings platform inconsistencies between macOS and other OSes.

[ShikiSuen]

For anyone who wants platform consistencies with this or similar API, I suggest using this one instead:

      #if canImport(Darwin)
      let filename = url.pathComponents.last
      #else
      let filename = url.pathComponents?.last
      #endif

The last is always nullable whatever the OS is.

[ShikiSuen]

Update: I found that the urls(forResourcesWithExtension returns NSURL in lieu of URL on non-Darwin platforms.

NSURL on non-Darwin platforms have API inconsitencies comparing to URL, hence manual conversion required:

let urls = nsURLs.compactMap { URL(string: $0.absoluteString) }