Store "stack-path" files in directories that have been shortened with SHAs

[mgsloan]

The resolution for #1145 / #1027 is to replace some long paths with 8 character SHAs on windows, due to the MAX_PATH limitation. It would be good to store a file, maybe called SHA/stack-path, with the path that would have been used in place of the SHA. This is useful for a variety circumstances:

• If a user is curious what's in the directory.
• If garbage collection gets implemented at some point. (stack rm command #133)
• It will allow SHAs to also be used for snapshot dependencies. The contents of the snapshots folder is scanned and parsed as part of stack init - https://github.com/fpco/stack/blob/master/src/Stack/Init.hs#L229

It gets a little bit tricky to ensure that every possible execution involving these paths will also create the path file, though. These paths are used in quite a few places, and often they get created be external tools.

/me shakes fist menacingly at MAX_PATH

[DemiMarie]

Another solution is to do what CoreCLR is doing – have GHC implicitly use extended paths.

[mgsloan]

It gets a little bit tricky to ensure that every possible execution involving these paths will also create the path file, though. These paths are used in quite a few places, and often they get created be external tools.

I'm thinking that the solution here is to have an in-memory cache of whether a particular path.yaml file has been written - something like Set (Path Abs Dir). Then we can just check this cache each time the paths are used. While this is slightly less efficient than statically knowing when to initialize these path.yaml files, it also seems less error-prone.

Creating these path.yaml files also means creating the folders. This means that functions like installationRootLocal, installationRootDeps, and distRelativeDir will have the side effect of creating the folders, when on windows. This seems relatively benign to me, but I think it makes sense to also create the folders on linux, for the sake of consistency.

[snoyberg]

Moved to wishlist: https://github.com/commercialhaskell/stack/wiki/Wishlist