Yeah, for now I only wanted to get pghero to work. The best solution I think would be to allow easier repacking of the .gem before installation, so we could add the tarball in the gem.
https://github.com/lfittl/pg_query/blob/master/ext/pg_query/extconf.rb#L13 does a check whether the tarball exists already, and I've seen this pattern increasingly in native extensions, but right now I can't find a good way to take advantage of it.

What do you think of passing a custom gemConfig to pghero in that case?

On the other hand it's not a new problem, we have the same issue with libv8 and other gems, ... so it's unfair of me to block you on that issue.

When putting the whishful thinker hat on, ideally I would like the ruby gems to include what external files they want to download in their gemspec metadata, and standardize where those would be downloaded on disk. If that were the case, bundix could do the prefetch-url dance and then place it in the right place for us at runtime.

Thank you for your contributions.
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Closing because the proposed solution is far from ideal.