FileManager: repair homeDirectoryForCurrentUser on Windows

[compnerd]

The homeDirectory(forUser:) is currently not functioning properly for users other than the current user. Switch to directly using the CoreFoundation API which simplifies the logic and makes the functionality work on Windows. This uses the forced unwrap for the return value, which was previously the case and resulted in a trap on Windows.

[compnerd]

CC: @parkera

[compnerd]

@swift-ci please test

[etcwilde]

I'd like to understand more about what is failing in homeDirectory(forUser:). It looks like it's going through the same steps as your fix here, with the exception of checking that the username isn't an empty string, and using a guard on the user directory URL (any safety gained by that is immediately lost by the force-unwrap at the upper layer, so I don't consider that to be all that different). Is NSUserName() not working right for the current user?

https://github.com/apple/swift-corelibs-foundation/blob/ff83ef2d90e48d9bc3465b13d524ab40cd5f2356/Sources/Foundation/FileManager.swift#L1150-L1154

[compnerd]

It isn't homeDirectory(forUser:) that is failing, it is that NetUserGetInfo will not provide a home directory for the user.

https://github.com/apple/swift-corelibs-foundation/blob/main/CoreFoundation/Base.subproj/CFPlatform.c#L594

By passing in the explicit nil for the username, we directly invoke CFCopyHomeDirectoryURL which will give you the home directory for the current user.

[etcwilde]

We should probably fix CFCopyHomeDirectoryURLForUser so that it can properly handle the current username. homeDirectory(forUser:) and homeDirectoryForCurrentUser should return the same thing when the function is passed the current username (assuming the current user isn't a system account, in which case I expect the computed property to explode and the function to return nil).

For performance/inlining/fast-pathing, I'm fine with the direct nil-passing in the computed property and a special-case for userName == GetCurrentUser() for the function.

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test

[etcwilde]

I'll accept it. It doesn't cause joy, but at least I can read it.