Distribution.Client.InstallSymlink

A record needed to either check if a symlink is possible or to create a symlink. Also used if copying instead of symlinking.

We would like by default to install binaries into some location that is on the user's PATH. For per-user installations on Unix systems that basically means the ~bin directory. On the majority of platforms the ~bin directory will be on the user's PATH. However some people are a bit nervous about letting a package manager install programs into ~bin.

A compromise solution is that instead of installing binaries directly into ~bin, we could install them in a private location under ~.cabalbin and then create symlinks in ~bin. We can be careful when setting up the symlinks that we do not overwrite any binary that the user installed. We can check if it was a symlink we made because it would point to the private dir where we install our binaries. This means we can install normally without worrying and in a later phase set up symlinks, and if that fails then we report it to the user, but even in this case the package is still in an OK installed state.

This is an optional feature that users can choose to use or not. It is controlled from the config file. Of course it only works on POSIX systems with symlinks so is not available to Windows users.

Symlink binary.

The paths are take in pieces, so we can make relative link when possible. True if creating the symlink was successful. False if there was another file there already that we did not own. Other errors like permission errors just propagate as exceptions.

Can we symlink a binary?

True if creating the symlink would be succeed, being optimistic that the user will agree if prompted to overwrite.

Try to make a symlink in a temporary directory.

If this works, we can try to symlink: even on Windows.