u3d/install: allow to specify packages download directory using an environment variable

[niezbop]

This adds a new option to u3d install so that the user can specify the target of its destination.
u3d install <SOME STUFF> --download_path <SOME PATH> will download if not already present at specified path and install it from this file.
u3d install <SOME STUFF> --no-install --download_path <SOME PATH> will download packages at some location, and make sure that they don't already exist there.

This works for both absolute and relative paths.

[lacostej]

I was thinking of doing this today, but with the use of a simple environment variable instead. That way the change had a much smaller footprint.

If we keep it as is, an alternative would be to use the u3d_core/globals to store the value instead of passing it to all methods down the call chain.

Together with adding a class method at downloader level

def self.download_dir
  # today's implementation
  # File.join(DOWNLOAD_PATH, DOWNLOAD_DIRECTORY
  # new implementation something like
  # to be discussed
  U3dCore::Globals.download_dir || ENV['U3D_DOWNLOAD_DIR'] || File.join(DOWNLOAD_PATH, DOWNLOAD_DIRECTORY)
end

the change becomes smaller. WDYT?

[niezbop]

I agree that it is better as my change was introducing lots of API modifications that I wasn't peculiarly happy about. I believe that using environment variables makes sense for this feature but we should be careful not too introduce too many of those. But once again, it makes sense in this case, so I will do the modifications asap (not on my computer right now).

[lacostej]

U3D_DOWNLOAD_PATH

[lacostej]

should we fail instead? Should we fail the whole setup if the value is already set? (It might be useful for self-testing environment though).

[lacostej]

or should we call the test with

with_env_values('U3D_DOWNLOAD_PATH' => nil) do

instead?

[lacostej]

Fine!