relax Fiber#transfer's restriction

Using Fiber#transfer with Fiber#resume for a same Fiber is
limited (once Fiber#transfer is called for a fiber, the fiber
can not be resumed more). This restriction was introduced to
protect the resume/yield chain, but we realized that it is too much
to protect the chain. Instead of the current restriction, we
introduce some other protections.

(1) can not transfer to the resuming fiber.
(2) can not transfer to the yielding fiber.
(3) can not resume transferred fiber.
(4) can not yield from not-resumed fiber.

[Bug #17221]

Also at the end of a transferred fiber, it had continued on root fiber.
However, if the root fiber resumed a fiber (and that fiber can resumed
another fiber), this behavior also breaks the resume/yield chain.
So at the end of a transferred fiber, switch to the edge of resume
chain from root fiber.
For example, root fiber resumed f1 and f1 resumed f2, transferred to
f3 and f3 terminated, then continue from the fiber f2 (it was continued
from root fiber without this patch).

spec/ruby/library/fiber/current_spec.rb

@@ -42,10 +42,22 @@ describe "Fiber.current" do
     fiber3 = Fiber.new do
       states << :fiber3
       fiber2.transfer
-      flunk
+      ruby_version_is '3.0' do
+        states << :fiber3_terminated
+      end
+      ruby_version_is '' ... '3.0' do
+        flunk
+      end
     end
 
     fiber3.resume
-    states.should == [:fiber3, :fiber2, :fiber]
+
+    ruby_version_is "" ... "3.0" do
+      states.should == [:fiber3, :fiber2, :fiber]
+    end
+
+    ruby_version_is "3.0" do
+      states.should == [:fiber3, :fiber2, :fiber, :fiber3_terminated]
+    end
   end
 end

spec/ruby/library/fiber/resume_spec.rb

@@ -3,10 +3,21 @@ require_relative '../../spec_helper'
 require 'fiber'
 
 describe "Fiber#resume" do
-  it "raises a FiberError if the Fiber has transferred control to another Fiber" do
-    fiber1 = Fiber.new { true }
-    fiber2 = Fiber.new { fiber1.transfer; Fiber.yield }
-    fiber2.resume
-    -> { fiber2.resume }.should raise_error(FiberError)
+  ruby_version_is '' ... '3.0' do
+    it "raises a FiberError if the Fiber has transferred control to another Fiber" do
+      fiber1 = Fiber.new { true }
+      fiber2 = Fiber.new { fiber1.transfer; Fiber.yield }
+      fiber2.resume
+      -> { fiber2.resume }.should raise_error(FiberError)
+    end
+  end
+
+  ruby_version_is '3.0' do
+    it "can work with Fiber#transfer" do
+      fiber1 = Fiber.new { true }
+      fiber2 = Fiber.new { fiber1.transfer; Fiber.yield 10 ; Fiber.yield 20; raise }
+      fiber2.resume.should == 10
+      fiber2.resume.should == 20
+    end
   end
 end

spec/ruby/library/fiber/transfer_spec.rb

@@ -11,7 +11,13 @@ describe "Fiber#transfer" do
   it "transfers control from one Fiber to another when called from a Fiber" do
     fiber1 = Fiber.new { :fiber1 }
     fiber2 = Fiber.new { fiber1.transfer; :fiber2 }
-    fiber2.resume.should == :fiber1
+
+    ruby_version_is '' ... '3.0' do
+      fiber2.resume.should == :fiber1
+    end
+    ruby_version_is '3.0' do
+      fiber2.resume.should == :fiber2
+    end
   end
 
   it "returns to the root Fiber when finished" do
@@ -34,12 +40,24 @@ describe "Fiber#transfer" do
     states.should == [:start, :end]
   end
 
-  it "can transfer control to a Fiber that has transferred to another Fiber" do
-    states = []
-    fiber1 = Fiber.new { states << :fiber1 }
-    fiber2 = Fiber.new { states << :fiber2_start; fiber1.transfer; states << :fiber2_end}
-    fiber2.resume.should == [:fiber2_start, :fiber1]
-    fiber2.transfer.should == [:fiber2_start, :fiber1, :fiber2_end]
+  ruby_version_is '' ... '3.0' do
+    it "can transfer control to a Fiber that has transferred to another Fiber" do
+      states = []
+      fiber1 = Fiber.new { states << :fiber1 }
+      fiber2 = Fiber.new { states << :fiber2_start; fiber1.transfer; states << :fiber2_end}
+      fiber2.resume.should == [:fiber2_start, :fiber1]
+      fiber2.transfer.should == [:fiber2_start, :fiber1, :fiber2_end]
+    end
+  end
+
+  ruby_version_is '3.0' do
+    it "can not transfer control to a Fiber that has suspended by Fiber.yield" do
+      states = []
+      fiber1 = Fiber.new { states << :fiber1 }
+      fiber2 = Fiber.new { states << :fiber2_start; Fiber.yield fiber1.transfer; states << :fiber2_end}
+      fiber2.resume.should == [:fiber2_start, :fiber1]
+      -> { fiber2.transfer }.should raise_error(FiberError)
+    end
   end
 
   it "raises a FiberError when transferring to a Fiber which resumes itself" do
@@ -83,4 +101,28 @@ describe "Fiber#transfer" do
     thread.join
     states.should == [0, 1, 2, 3]
   end
+
+  ruby_version_is "" ... "3.0" do
+    it "runs until Fiber.yield" do
+      obj = mock('obj')
+      obj.should_not_receive(:do)
+      fiber = Fiber.new { 1 + 2; Fiber.yield; obj.do }
+      fiber.transfer
+    end
+
+    it "resumes from the last call to Fiber.yield on subsequent invocations" do
+      fiber = Fiber.new { Fiber.yield :first; :second }
+      fiber.transfer.should == :first
+      fiber.transfer.should == :second
+    end
+
+    it "sets the block parameters to its arguments on the first invocation" do
+      first = mock('first')
+      first.should_receive(:arg).with(:first).twice
+
+      fiber = Fiber.new { |arg| first.arg arg; Fiber.yield; first.arg arg; }
+      fiber.transfer :first
+      fiber.transfer :second
+    end
+  end
 end