Implement Thread#fetch

[nomadium]

Hi folks,

This is another feature targeting Ruby 2.5 [1]: Thread#fetch (feature #13009).

Note: The tests are copied from MRI.

Thanks for your review and feedback.

1. Ruby 2.5 Support #4876

[nomadium]

I modified the PR because updated MRI 2.5.0 tests were merged in ruby-2.5 branch with 14a2aca.

I reverified the associated test with this new method and it works as expected:

  def test_thread_local_fetch
    t = Thread.new { sleep }

    assert_equal(false, t.key?(:foo))

    t["foo"] = "foo"
    t["bar"] = "bar"
    t["baz"] = "baz"

    x = nil
    assert_equal("foo", t.fetch(:foo, 0))
    assert_equal("foo", t.fetch(:foo) {x = true})
    assert_nil(x)
    assert_equal("foo", t.fetch("foo", 0))
    assert_equal("foo", t.fetch("foo") {x = true})
    assert_nil(x)

    x = nil
    assert_equal(0, t.fetch(:qux, 0))
    assert_equal(1, t.fetch(:qux) {x = 1})
    assert_equal(1, x)
    assert_equal(2, t.fetch("qux", 2))
    assert_equal(3, t.fetch("qux") {x = 3})
    assert_equal(3, x)

    assert_raise(KeyError) {t.fetch(:qux)}
  ensure
    t.kill if t
  end

miguel@alice:~/jruby$ PATH=$PWD/bin:$PATH jruby test/mri/runner.rb -v --color=never --tty=no --excludes=test/mri/excludes -q -n test_thread_local_fetch -- ruby/test_thread.rb
Run options: -v --color=never --tty=no --excludes=test/mri/excludes -q -n test_thread_local_fetch --

# Running tests:

[1/1] TestThread#test_thread_local_fetch = 0.01 s
Finished tests in 0.085849s, 11.6484 tests/s, 186.3737 assertions/s.
1 tests, 16 assertions, 0 failures, 0 errors, 0 skips

ruby -v: jruby 9.3.0.0-SNAPSHOT (2.5.0) 2018-03-04 4e258ca Java HotSpot(TM) 64-Bit Server VM 25.102-b14 on 1.8.0_102-b14 +jit [darwin-x86_64]

[nomadium]

Closing it in favor of #5089.