safely create cache with tmp locked string object

[ggmichaelgo]

Types of Changes

• Bug fix.

PR Summary

When Memory::Sampler iterates String objects from the ObjectSpace, it truncates String to reduce memory usage and caches the results.

During this process, it is possible for the String objects to be internally locked. (Ruby String#rb_str_locktmp)

For example, when Socket#read is called, Ruby creates a buffer string that is temporarily locked. (Ruby Socket#rsock_s_recvfrom)
If the Memory::Sampler creates a report when the Socket#read is processing, it will raise a RuntimeError: can't modify string; temporarily locked error:

module Memory
  class Cache
    ...
    def lookup_string(obj)
      # since obj is internally locked, it can't be used as a Hash key
      @string_cache[obj] ||= String.new << obj[0, 64]
    end
  end
end

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[samuel-williams-shopify]

This looks good to me. We can probably make a simpler / faster / more reliable test case by using the following behaviour:

fiber = Fiber.new do |string|
	IO::Buffer.for(string) do
		# string is now locked for the duration of this block.
		Fiber.yield
	end
end

# Lock the string
key = "foo"
fiber.resume(key)
{key => "bar"} # can't modify string; temporarily locked (RuntimeError)