Motivation / Background

When an engine is mounted at "/" and SCRIPT_NAME is being used, the URL helpers incorrectly drop the script name from the generated URLs.

Detail

The issue is in the private method ActionDispatch::Routing::RoutesProxy#merge_script_names which doesn't handle the edge case of new_script_name being a sole slash "/" with no segment following it.

This bug does not manifest for non-root paths with trailing slashes (like "/bar/") because the paths are cleaned up by Mapper::Mapping#normalize_path in Mapper::Resources.add_route.

The fix I've adopted here is simply to remove any trailing "/" from the new_script_name variable. We could also just return previous_script_name if new_script_name == "/", however I thought running the names through the existing logic would be preferable to a special-case guard clause.

Additional information

I think this issue has been present since the changes introduced in #29898 in Rails v5.2.0. cc @deivid-rodriguez in case he has feedback! 🙏

I think this might fix #34023. cc @jeremy in case he remembers what was going on there.

Checklist

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• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
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• Tests are added or updated if you fix a bug or add a feature.
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