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]

cont.c

@@ -2226,7 +2226,8 @@ static VALUE
 rb_fiber_m_transfer(int argc, VALUE *argv, VALUE fiber_value)
 {
     rb_fiber_t *fiber = fiber_ptr(fiber_value);
-    fiber->transferred = 1;
+    fiber_current()->transferred = 1;
+    fiber->transferred = 0;
     return fiber_switch(fiber, argc, argv, 0, PASS_KW_SPLAT);
 }
 

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

test/ruby/test_fiber.rb

@@ -222,8 +222,8 @@ class TestFiber < Test::Unit::TestCase
   end
 
   def test_resume_self
-    f = Fiber.new {f.resume}
-    assert_raise(FiberError, '[ruby-core:23651]') {f.transfer}
+    f = Fiber.new {f.resume 1}
+    assert_equal(1, f.transfer)
   end
 
   def test_fiber_transfer_segv
@@ -289,14 +289,12 @@ class TestFiber < Test::Unit::TestCase
     assert_raise(FiberError){
       g=nil
       f=Fiber.new{
-        g.resume
-        g.resume
+        g.transfer
       }
       g=Fiber.new{
         f.resume
-        f.resume
       }
-      f.transfer
+      f.resume
     }
   end