Make RSpec::Support.thread_local_data thread but not fiber local

Changelog.md

@@ -1,6 +1,11 @@
 ### Development
 [Full Changelog](http://github.com/rspec/rspec-support/compare/v3.12.0...main)
 
+Bug Fixes:
+
+* Fix `RSpec::Support.thread_local_data` to be Thread local but not Fiber local.
+  (Jon Rowe, #581)
+
 ### 3.12.0 / 2022-10-26
 [Full Changelog](http://github.com/rspec/rspec-support/compare/v3.11.1...v3.12.0)
 Enhancements:

lib/rspec/support.rb

@@ -89,8 +89,14 @@ def self.class_of(object)
     end
 
     # A single thread local variable so we don't excessively pollute that namespace.
-    def self.thread_local_data
-      Thread.current[:__rspec] ||= {}
+    if RUBY_VERSION.to_f >= 2
+      def self.thread_local_data
+        Thread.current.thread_variable_get(:__rspec) || Thread.current.thread_variable_set(:__rspec, {})
+      end
+    else
+      def self.thread_local_data
+        Thread.current[:__rspec] ||= {}
+      end
     end
 
     # @api private

spec/rspec/support_spec.rb

@@ -186,6 +186,26 @@ def object.some_method
       end
     end
 
+    describe ".thread_local_data" do
+      it "contains data local to the current thread" do
+        RSpec::Support.thread_local_data[:__for_test] = :oh_hai
+
+        Thread.new do
+          expect(RSpec::Support.thread_local_data).to eq({})
+        end.join
+      end
+
+      if defined?(Fiber)
+        it "shares data across fibres" do
+          RSpec::Support.thread_local_data[:__for_test] = :oh_hai
+
+          Fiber.new do
+            expect(RSpec::Support.thread_local_data[:__for_test]).to eq(:oh_hai)
+          end.resume
+        end
+      end
+    end
+
     describe "failure notification" do
       before { @failure_notifier = RSpec::Support.failure_notifier }
       after  { RSpec::Support.failure_notifier = @failure_notifier }