Memory leak when Mock aliases a method about mspec HOT 5 CLOSED

And since that symbol is used as method, even if the method gets erased, the symbol remains.

Isn't the cause of the memory leak a bug/limitation of CRuby then?

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I'm not sure how to qualify it.

It's a documented behavior and you can even see how many immortal symbols you have.

3.2.3 :003 > require 'objspace'
3.2.3 :003 > ObjectSpace.count_symbols
 => {:mortal_dynamic_symbol=>7, :immortal_dynamic_symbol=>66, :immortal_static_symbol=>7799, :immortal_symbol=>7865}
3.2.3 :004 > 1000000.times { |i| a = Object.new; a.singleton_class.alias_method(:"tic_#{i}", :to_s) }
 => 1000000
3.2.3 :005 > ObjectSpace.count_symbols
 => {:mortal_dynamic_symbol=>7, :immortal_dynamic_symbol=>1000066, :immortal_static_symbol=>7803, :immortal_symbol=>1007869

A memory leak in C++ is normally not considered a bug in C++ itself, it's the programmer or library not freeing memory. This behavior here is a programmer doing something that is documented to not release the memory without any attempt at reusing the same memory each time.

In any case, I think it makes sense to limit the triggering of that behavior in a low-level tool such as mspec.

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CRuby is holding onto a Symbol the user is no longer holding onto, so IMO it's a CRuby bug and worth reporting.
e.g. maybe method names should be :mortal_dynamic_symbol.
I would think there is no such issue on TruffleRuby for instance.
This also means CRuby cannot properly GC code and would be subject to DoS if loading unlimited amounts of dynamically-generated code.

This behavior here is a programmer doing something that is documented to not release the memory

Is it documented as in part of some documentation?

I'm not against working around this in mspec though, so PR welcome as long as it keeps things simple.

Also do you know how big of a memory leak does it cause in practice?
I have never noticed this being an issue when running ruby/spec for example.

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Here is some talking about it for ruby 2.7: https://ruby-doc.org/core-2.7.1/doc/security_rdoc.html#label-Symbols. It's kind of hard to find any doc about up to date ruby at all... I don't know why.

In practice, you won't notice. Unless you are me who is trying to run ruby/spec specs many many time to try to notice actual memory leaks :). Then mocks means an unusable test for that purpose.

I'll look into a PR. Thanks!

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Thanks, that's clearly documented, I didn't know.
Not sure why such Symbols are not GC'd but it probably simplifies the global method cache or so.
FWIW the link to the master version of that: https://docs.ruby-lang.org/en/master/security_rdoc.html#label-Symbols

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