Cache gems into ~/.bundle/cache [WIP]

[stefanlance]

We probably also want to test that this works when separate local bundles are installed on the same machine, i.e., that they don't overwrite the .gems in the cache, although I'm unsure how to do this just yet.

[segiddins]

cache_globally?

[segiddins]

Only gems with no extensions can be safely cached globally, I think.

[indirect]

@segiddins this cache is for the .gem files, not for the installed gems. so all gems can be cached here

[indirect]

~ doesn't work on Windows, so well need to use whatever Bundler currently uses to find the global settings file.

[indirect]

This implementation looks good to me, but as I read it I realized that I think we can't do global caching this way. Because load_global_cache reads every cached .gem file, ungzipping and untarring it to extract the .gemspec file and then evaluating the gemspec, it's going to get slower and slower as the global cache grows.

Instead of loading gemspecs from the global cache, I think we will have to use it as a pure download cache instead—when the RubyGems source is asked to download a .gem file, it checks the global cache and provides the cached .gem if present, and downloads the gem into the global cache if it is not. Make sense?

[indirect]

we need to remove this change--the global cache should be for downloads only. It will be too slow to use the global cache for gem metadata, because getting the metadata out of .gem files means decompressing them and evaluating lots of ruby code.

[stefanlance]

See #3983