[ruby-core:122039] [Ruby Bug#21324] Namespace loads RubyGems in root Namespace but it should not

Issue #21324 has been updated by jas (Jasveen Sandral).


I've identified the issue and prepared a fix.

The problem is that RubyGems is being loaded into the root namespace during Ruby's boot process, and then all user namespaces inherit this same Gem constant through the Copy-on-Write mechanism. This causes all namespaces to share the same RubyGems environment and state, leading to version conflicts.

The fix involves:

1. In `rb_initialize_main_namespace()`:
   - Detecting if the Gem constant exists in Object
   - Removing it explicitly with rb_const_remove()
   - Loading a fresh RubyGems environment

2. In `namespace_initialize()` (for Namespace.new):
   - Getting the namespace's own Object constant
   - Checking and removing any inherited Gem constant
   - Loading a clean RubyGems environment

This ensures each namespace (main and optional) gets its own isolated RubyGems state, preventing version conflicts when loading gems across namespaces.

I've created a PR with these changes: https://github.com/ruby/ruby/pull/13313

----------------------------------------
Bug #21324: Namespace loads RubyGems in root Namespace but it should not
https://bugs.ruby-lang.org/issues/21324#change-113188

* Author: Eregon (Benoit Daloze)
* Status: Open
* ruby -v: ruby 3.5.0dev (2025-05-10T07:50:29Z namespace-on-read-.. bd4f57f96b) +PRISM [x86_64-linux]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
RubyGems has tons of mutable state, isn't core library and isn't "builtin classes" either, so it should not be in root Namespace, but it is currently:
```
$ RUBY_NAMESPACE=1 ruby -ve 'ns = Namespace.new; p ns::Gem.equal?(Gem)'
ruby 3.5.0dev (2025-05-10T07:50:29Z namespace-on-read-.. bd4f57f96b) +PRISM [x86_64-linux]
ruby: warning: Namespace is experimental, and the behavior may change in the future!
See doc/namespace.md for know issues, etc.
true
```

A concrete example of what breaks most likely due to this:
```
$ gem i delegate:0.3.1
$ RUBY_NAMESPACE=1 ruby -ve 'require "delegate"; p Delegator::VERSION; ns = Namespace.new; File.write "ns.rb", "gem %{delegate}, %{0.3.1}; require :delegate.to_s; p Delegator::VERSION"; ns.require "./ns"'
ruby 3.5.0dev (2025-05-10T07:50:29Z namespace-on-read-.. bd4f57f96b) +PRISM [x86_64-linux]
ruby: warning: Namespace is experimental, and the behavior may change in the future!
See doc/namespace.md for know issues, etc.
"0.4.0"
/home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/specification.rb:2232:in 'Gem::Specification#check_version_conflict': can't activate delegate-0.3.1, already activated delegate-0.4.0 (Gem::LoadError)
	from /home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/specification.rb:1383:in 'Gem::Specification#activate'
	from /home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_gem.rb:62:in 'block in Kernel#gem'
	from /home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_gem.rb:62:in 'Thread::Mutex#synchronize'
	from /home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_gem.rb:62:in 'Kernel#gem'
	from /home/eregon/ns.rb:1:in '<top (required)>'
	from -e:1:in 'Namespace#require'
	from -e:1:in '<main>'
```
i.e. `ns` sees delegate-0.4.0 was loaded in main namespace but it shouldn't.

Previously mentioned in https://bugs.ruby-lang.org/issues/21311#note-36



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