[WIP] Use the directory package to create new-install symlinks

[YellPika]

Running cabal new-install on Windows currently gives the error "Symlinking feature not available on Windows". Symlinks have been supported on Windows since Vista (with administrator privileges), and symlinks can currently be created without administrator privileges on Windows 10 when in developer mode. This change lets cabal new-install create symlinks on Windows.

I've been using this patch on my own Windows system for ~4 months (ever since haskell/directory#83 was fixed) with no issues.

One caveat is that directory-1.3.3.0 or newer is required for in order to create symlinks on Windows 10 without administrator privileges.

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[23Skidoo]

Sounds like a good idea.

[hvr]

That's a fairly recent directory version (it only started getting bundled w/ GHC 8.6); can we make this conditional on if os(windows)?

[YellPika]

I don't understand the problem. Are you worried about the stability of recent versions of directory?

This change uses the createFileLink function, which was added in directory-1.3.1.0, so the lower bound will need to be raised anyways. Is 1.3.1.0 an acceptable lower bound? One potential downside is that the behaviour of new-install on Windows will differ depending on which version of directory was used to compile cabal-install.

[Mistuke]

I think the concern is mostly support, does directory-1.3.1.0 support the same versions of GHC that cabal supports, if not it can't be unconditionally used.

This change uses the createFileLink function, which was added in directory-1.3.1.0, so the lower bound will need to be raised anyways. Is 1.3.1.0 an acceptable lower bound? One potential downside is that the behaviour of new-install on Windows will differ depending on which version of directory was used to compile cabal-install

This is quite normal, and just means the distro needs to be built with an up-to-date compiler/cabal. Long filenames support is another thing that already depends on which version of GHC is used.

[hvr]

Well, we have all those *.Compat.* modules for a reason... ;-)

See

• https://github.com/haskell/cabal/tree/master/Cabal/Distribution/Compat
• https://github.com/haskell/cabal/tree/master/cabal-install/Distribution/Client/Compat

So I'd strongly suggest to try to follow the prior-art from those compat modules to avoid requiring such a bleeding edge directory version

[YellPika]

Travis is reporting that the bootstrap builds are failing. I tried having bootstrap.sh download the appropriate directory version, but this caused dependency conflicts. Any advice on how to resolve this issue?

[Mistuke]

Implementation wise no objections from me, just a question, have you tested this on older versions of Windows or windows 10 with developer mode off? Just want to make sure that the errors propagated by the directory function are properly handled.

I assume since the old implementation threw an error as well, but just want to check that what is thrown now makes sense.

[YellPika]

Before, any attempt to use new-install would report:

Symlinking feature not available on Windows

Running without developer mode now gives a failure message like the following:

C:\Users\Anthony\AppData\Roaming\local\bin\ghcid.exe: createFileLink:CreateSymbolicLink:
permission denied (A required privilege is not held by the client. Creating symbolic links
usually requires administrative rights.)

I don't have any machines with older versions of Windows, so unfortunately I can't test there.

[Mistuke]

Yeah that error looks fine to me. No objections from my part. I'll leave it to @hvr about the directory bounds

[23Skidoo]

An alternative solution/fallback would be to copy and record the checksum of the installed exe somewhere, and after that overwrite only if the checksum of the existing file matches the recorded one (or the user forced an overwrite explicitly).

[veqryn]

I'm brand new to haskell and trying to setup my dev environment on windows. Immediately ran into the error message that Symlinking feature not available on Windows, despite that being false.
Any chance this PR can get traction again?

[phadej]

Symlinking on windows is done in #6519, but it looks it doesn't made it into 3.2. Will be in the next major cabal-install