Fix Fiber#transfer

Fiber#transfer previously made it impossible to resume the fiber
if it was transferred to (no resuming the target of Fiber#transfer).
However, the documentation specifies that you cannot resume a fiber
that has transferred to another fiber (no resuming the source of
Fiber#transfer), unless control is transferred back.

Fix the code by setting the transferred flag on the current/source
fiber, and unsetting the transferred flag on the target fiber.

Fixes [Bug #9664]
Fixes [Bug #12555]

1 files changed, 12 insertions, 3 deletions

diff --git a/spec/ruby/library/fiber/transfer_spec.rb b/spec/ruby/library/fiber/transfer_spec.rb
index d13053666c..fa094a022d 100644
--- a/spec/ruby/library/fiber/transfer_spec.rb
+++ b/spec/ruby/library/fiber/transfer_spec.rb
@@ -42,9 +42,18 @@ describe "Fiber#transfer" do
     fiber2.transfer.should == [:fiber2_start, :fiber1, :fiber2_end]
   end
 
-  it "raises a FiberError when transferring to a Fiber which resumes itself" do
-    fiber = Fiber.new { fiber.resume }
-    -> { fiber.transfer }.should raise_error(FiberError)
+  ruby_version_is ''...'2.7' do
+    it "raises a FiberError when transferring to a Fiber which resumes itself" do
+      fiber = Fiber.new { fiber.resume }
+      -> { fiber.transfer }.should raise_error(FiberError)
+    end
+  end
+
+  ruby_version_is '2.7' do
+    it "allows transferring to a Fiber which resumes itself" do
+      fiber = Fiber.new { fiber.resume 1 }
+      fiber.transfer.should == 1
+    end
   end
 
   it "works if Fibers in different Threads each transfer to a Fiber in the same Thread" do