Enable `auto_clean_without_path` by default

[deivid-rodriguez]

auto_clean_without_path is a flag to enable cleaning up of unnecessary gems after bundle install, even when Bundler is configured to install to GEM_HOME (Bundler 2 default). Needs investigation of why it was introduced and whether we still want it.

[hsbt]

This flag seems reasonable with #8653 feature. But we should avoid to use with global path like GEM_HOME.

[deivid-rodriguez]

I read the code and the flag seems to work exactly as you expect, @hsbt. Basically, the new Bundler 4 default will be to not install to system gems. This flag controls, assuming the new default is in place, whether to cleanup unnecessary gems or not. So, it never affects situations where we install to system gems.

I think we have a bit of a mess of settings & feature flags, because we also have a clean setting to explicitly control whether or not to cleanup unnecessary gems after install, and these additional flag is only considered if clean is not set.

So, I think questions here are:

• Do we want to cleanup by default once Bundler switches to install to a local folder by default?

It seems answer is YES.

• How do we want this behavior controlled?

I think what we do now is fine: use the existing clean setting, and if clean is not set, default to true if Bundler is installing to a local folder, and to false if Bundler is installing to system gems. That seems enough flexibility to me, so we can maybe get rid of this setting altogether?

[deivid-rodriguez]

I checked how this setting was originally introduced and it makes sense. See rubygems/bundler#5961.

The reason for the setting is that, if path is explicitly configured, it may actually be a shared location which we don't want to clean up because removing gems for an app may affect others. The only way to be sure is to check that path is not explicitly configured and we're using the new default of installing gems to a local folder.

However, I think it's unnecessary to have a new setting for this and we can reuse the default_install_uses_path one.

[hsbt]

use the existing clean setting, and if clean is not set, default to true if Bundler is installing to a local folder, and to false if Bundler is installing to system gems.

I'm +1 to the above behavior.

[deivid-rodriguez]

Did you read rubygems/bundler#5961? I initially thought we could simplify this to the behavior you agree with, but after reading that issue I understood why we have the current implementation.

You could have configured bundle config path /usr/local/bundle for example, and be using that as a global home for many apps. Cleaning that by default in one app could affect other apps. Hence the current slightly more complex behavior of:

Use clean setting if set. If clean not set, default to true only if Bundler is not installing to system gems AND path is not explicitly configured.