[5.5] Do not crash on relative file URLs in a dependency

[neonichu]

5.5 cherry-pick of #3603

[neonichu]

@swift-ci please test

[karwa]

I just happened to spot this by accident, but it doesn’t look correct - in the URL file://../best, the .. is in the host position, and is not part of the path.

Personally, I don’t agree that this syntax should be supported. Relative file URLs are not a thing. Even RFC-8089 says:

The path component represents the absolute path to the file in the
file system.

The reason is that, with the proper 3 slashes (file:///../best), the path would be /../best, which is an absolute path identical to /best on a POSIX system. Removing the leasing slash in the path doesn’t make it relative - it just breaks the URL by putting data in the wrong section.

[neonichu]

Thanks, @karwa, looks like you're right and Foundation.URL also behaves exactly like that:

import Foundation

let url = URL(string: "file://../best")!
print("host: \(url.host ?? "<empty>")")
print("path: \(url.path)")

prints

host: ..
path: /best

Based on that, it seems appropriate to change this to checking whether there's a host component and rejecting the URL if that is the case.

[neonichu]

Amended with cherry-pick of #3625

[neonichu]

@swift-ci please test

[neonichu]

:0: error: module file '/home/buildnode/jenkins/workspace/swift-package-manager-PR-Linux/build/buildbot_linux/foundation_static-linux-x86_64/module-cache/1O5EHM9AO5UP2/SwiftShims-5INC0PO62ITL.pcm' is out of date and needs to be rebuilt: signature mismatch

[neonichu]

@swift-ci please test linux

[neonichu]

Let's see how #3630 shakes out before merging this.

[neonichu]

Actually, let's get this merged. We are getting closer to the 5.5 release and this diagnostic is already a big improvement over crashing. If there's time, we can make another cherry pick of a refined diagnostic.