Register C global variables to Ruby GC to avoid problems with GC.compact

[casperisfine]

Since Ruby 2.7, C global variables holding references to Ruby objects must be declared to the GC even if they are also held from the Ruby side, otherwise a call to GC.compact might move them to another location.

For instance here's an exception we've seen caused by this:

bundler/gems/mysql2-d3c815c68f22/lib/mysql2/client.rb:131:in `_query': NoMethodError: undefined method `new_with_args' for "2":String (ActiveRecord::StatementInvalid)
    from bundler/gems/mysql2-d3c815c68f22/lib/mysql2/client.rb:131:in `block in query'
    from bundler/gems/mysql2-d3c815c68f22/lib/mysql2/client.rb:130:in `handle_interrupt'
    from bundler/gems/mysql2-d3c815c68f22/lib/mysql2/client.rb:130:in `query'

In this instance Mysql2::Error was moved to another location, and cMysql2Error was now pointing to an instance of "2".

We had a similar issue in Shopify/liquid-c#55, however here I tried to reproduce it in the test suite by repeatedly calling GC.compact, but without any success. Maybe the mysql2 test suite is not producing enough GC objects to demonstrate the bug.

cc @csfrancis @XrXr @rafaelfranca

[sodabrew]

Thank you!

[casperisfine]

Welcome. Let me know if there's anything needed to get this merged.

I tried some more tricks to reproduce the bug (generate a lot of garbage before loding the extension & then compact), but still no luck.

I wish there was a way to force the GC to move a particular object, that would be very useful to test this kind of stuff.

[sodabrew]

Do all of the static variables need to be flagged against movement as well? For example

  intern_usec = rb_intern("usec");

[casperisfine]

@sodabrew I'll let @XrXr correct me if I say something wrong.

But I didn't mark these symbols, because AFAIK they aren't true objects. Meaning there's nothing more than the content of the VALUE. There's no associated object slot that can be moved.

Same for Integer and Float etc. But other than that, yes all static VALUE should be marked.

[sodabrew]

I'm not terribly familiar with these changes in Ruby 2.7, so I'll take your testing-word for it. If the other variables should also be marked that's easy enough to add in a subsequent PR!

[casperisfine]

Thanks!