[ruby-core:112082] [Ruby master Bug#18567] Depending on default gems in stdlib gems when not needed considered harmful

Issue #18567 has been updated by Eregon (Benoit Daloze).


hsbt (Hiroshi SHIBATA) wrote in #note-37:
> I can work it with reasonable effort, not a requirement.

I am fine with that, and would appreciate that very much.

I guess there won't be many new default gems anyway, it seems almost every stdlib is a default gem nowadays.

> Because I'm not truffleruby developer and it's not my primary work.

I understand, but unfortunately the process of making new default gems actually breaks things and it seems best to avoid breaking as much as possible.
The main issue so far is we learn too late about it, when it's already broken, vs spend a bit of time to fix it before people depend on the gem.

> You don't know, I already worked it. In my experience, `jruby` and `truffleruby` jobs sometimes failed with CRuby changes. This fails are abandoned usually.

I didn't know, thank you.
In case it doesn't work, I would love if you can just mention it to me for TruffleRuby/`@headius` for JRuby.
That's the ideal time to fix it, when creating the new default gem.

> You should work truffleruby support for default gems repo before your proposal.

I am working on that, see e.g.:
* https://github.com/oracle/truffleruby/issues/2644
* https://github.com/ruby/bigdecimal/pull/247
* https://github.com/eregon/truffleruby-gem-tracker/blob/fcf6bc0057f72275fdadc7ee2e385da26f2b640d/gems.yml#L3-L42

----------------------------------------
Bug #18567: Depending on default gems in stdlib gems when not needed considered harmful
https://bugs.ruby-lang.org/issues/18567#change-101516

* Author: Eregon (Benoit Daloze)
* Status: Closed
* Priority: Normal
* Assignee: hsbt (Hiroshi SHIBATA)
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
CRuby over time moves more and more code to default gems, to bundled gems and or even stops shipping some gems which used to be stdlibs (#5481).
I believe the main motivation for that is being to fix security issues without needing to make a CRuby release, and that makes sense.

There are however multiple unwanted side effects of this:

1. Removing gems from stdlib (e.g., #17873) is a breaking change, which makes upgrading to a new Ruby version more difficult.
   I think this should only be done if there is a clear gain.
   Being a default gem is already enough to fix a security issue without a CRuby release.
2. When any gem depends on a default gem, it tends to break on all Ruby implementations except CRuby, and for older Ruby versions.

Let's focus on this second point.
There can be good reasons to depend on a specific version (or ~>/>=) of a default gem, for instance to ensure a given security issue is addressed.
In other cases, I think there is no value to depend explicitly on a default gem, it would work without an explicit dependency since it is still "in stdlib".

And it is actually harmful to depend on default gems for JRuby, TruffleRuby and older Ruby versions, because the default gem does typically not work there yet, but the stdlib works just fine!
The reason for that is simple, those gems used to be stdlib and so were implemented directly in the Ruby implementation.

Also depending on default gem will typically be redundant with what's in stdlib, which is then a waste of network, time and disk.

For larger default gems (e.g., openssl), I believe the solution is those gems to support JRuby, TruffleRuby, etc.
This is useful so the behavior for a given version of the gem is compatible between Ruby implementations, has the same security fixes, etc.
It is however a large effort and overhead to do this, and it only makes sense if people are going to need to depend on such a gem explicitly (either for security or new features in a given version), otherwise the version in stdlib is good enough and much simpler.

Here are I think some clear cases of default gems which are clearly more overhead than what they gain:
* io-wait: just a few methods very tight to IO internals, should really be core
* io-nonblock just a few methods very tight to IO internals, should really be core
* digest: has a public header and so versioning it doesn't work. Also it makes sense to reuse e.g. MessageDigest on JVM for better performance.
* strscan: this accesses a lot of Regexp internal, it would fit better in each implementation repo as a non-gem stdlib.

These are all small, they are all fairly stable, and it's unclear why they are even default gems in the first place.
They also seem fairly unlikely to have security issues.

So this is what I propose:
* Do not depend on default gems in stdlib gems unless necessary (for security or feature), or unless we know the next version of Ruby will no longer ship that gem. An example is `net-protocol` depending needlessly on `io-wait`, I'll make a PR for that.
* I think those gems listed just above should no longer be default gems in the future to clarify the situation. They should either be core or regular non-gem stdlib.



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