Explicit handling of frozen strings in IO::Buffer#for.

[ioquatix]

It turns out that we can't touch attributes during GC free, so unlocking the buffer's source during GC is undefined (in a bad way). So, we change the API for IO::Buffer#for slightly to avoid this case. It still works in the scenarios we expect, e.g.

Defining global constant buffers:

# frozen_string_literal: true (optional but slightly more efficient)

TOKEN = IO::Buffer.for("my-token") # TOKEN.readonly? => true

def manipulate(string)
  IO::Buffer.for(string) do |buffer|
    buffer.set_string("Hello World") # Mutate string.
  end
end

[ioquatix]

rbs assumes the previous behaviour and it seems like we can't merge this PR until it is fixed: ruby/rbs#989

[ioquatix]

@soutaro looks good now!

[casperisfine]

Is the backport label on GitHub enough? Or should we create a Redmine ticket (asking because a colleague just ran into this bug).

[jeremyevans]

@casperisfine All backport requests should be submitted as closed bugs on Redmine with the appropriate backport field set to REQUIRED for the ruby versions you would like the fix backported to.

[casperisfine]

@jeremyevans thank you for confirming, that's what I remembered. I'll create the backport issue then.