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).

cont.c

@@ -207,6 +207,7 @@ typedef struct rb_context_struct {
 
 
 /*
+ * Fiber status:
  * Fiber status:
  *    [Fiber.new] ------> FIBER_CREATED
  *                        | [Fiber#resume]
@@ -235,14 +236,11 @@ struct rb_fiber_struct {
     rb_context_t cont;
     VALUE first_proc;
     struct rb_fiber_struct *prev;
-    BITFIELD(enum fiber_status, status, 2);
-    /* If a fiber invokes by "transfer",
-     * then this fiber can't be invoked by "resume" any more after that.
-     * You shouldn't mix "transfer" and "resume".
-     */
-    unsigned int transferred : 1;
+    VALUE resuming_fiber;
 
+    BITFIELD(enum fiber_status, status, 2);
     /* Whether the fiber is allowed to implicitly yield. */
+    unsigned int yielding : 1;
     unsigned int blocking : 1;
 
     struct coroutine_context context;
@@ -919,11 +917,13 @@ cont_mark(void *ptr)
     RUBY_MARK_LEAVE("cont");
 }
 
+#if 0
 static int
 fiber_is_root_p(const rb_fiber_t *fiber)
 {
     return fiber == fiber->cont.saved_ec.thread_ptr->root_fiber;
 }
+#endif
 
 static void
 cont_free(void *ptr)
@@ -2010,25 +2010,33 @@ fiber_current(void)
 }
 
 static inline rb_fiber_t*
-return_fiber(void)
+return_fiber(bool terminate)
 {
     rb_fiber_t *fiber = fiber_current();
     rb_fiber_t *prev = fiber->prev;
 
-    if (!prev) {
+    if (prev) {
+        fiber->prev = NULL;
+        prev->resuming_fiber = Qnil;
+        return prev;
+    }
+    else {
+        if (!terminate) {
+            rb_raise(rb_eFiberError, "attempt to yield on a not resumed fiber");
+        }
+
         rb_thread_t *th = GET_THREAD();
         rb_fiber_t *root_fiber = th->root_fiber;
 
         VM_ASSERT(root_fiber != NULL);
 
-        if (root_fiber == fiber) {
-            rb_raise(rb_eFiberError, "can't yield from root fiber");
+        // search resuming fiber
+        for (fiber = root_fiber;
+             RTEST(fiber->resuming_fiber);
+             fiber = fiber_ptr(fiber->resuming_fiber)) {
         }
-        return root_fiber;
-    }
-    else {
-        fiber->prev = NULL;
-        return prev;
+
+        return fiber;
     }
 }
 
@@ -2073,7 +2081,7 @@ fiber_store(rb_fiber_t *next_fiber, rb_thread_t *th)
 }
 
 static inline VALUE
-fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int is_resume, int kw_splat)
+fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int kw_splat, VALUE resuming_fiber, bool yielding)
 {
     VALUE value;
     rb_context_t *cont = &fiber->cont;
@@ -2120,11 +2128,21 @@ fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int is_resume, int
 
     VM_ASSERT(FIBER_RUNNABLE_P(fiber));
 
-    if (is_resume) {
+    rb_fiber_t *current_fiber = fiber_current();
+
+    VM_ASSERT(!RTEST(current_fiber->resuming_fiber));
+    if (RTEST(resuming_fiber)) {
+        current_fiber->resuming_fiber = resuming_fiber;
         fiber->prev = fiber_current();
+        fiber->yielding = 0;
     }
 
-    if (fiber_current()->blocking) {
+    VM_ASSERT(!current_fiber->yielding);
+    if (yielding) {
+        current_fiber->yielding = 1;
+    }
+
+    if (current_fiber->blocking) {
         th->blocking -= 1;
     }
 
@@ -2134,7 +2152,7 @@ fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int is_resume, int
 
     value = fiber_store(fiber, th);
 
-    if (is_resume && FIBER_TERMINATED_P(fiber)) {
+    if (RTEST(resuming_fiber) && FIBER_TERMINATED_P(fiber)) {
         fiber_stack_release(fiber);
     }
 
@@ -2152,7 +2170,7 @@ fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int is_resume, int
 VALUE
 rb_fiber_transfer(VALUE fiber_value, int argc, const VALUE *argv)
 {
-    return fiber_switch(fiber_ptr(fiber_value), argc, argv, 0, RB_NO_KEYWORDS);
+    return fiber_switch(fiber_ptr(fiber_value), argc, argv, RB_NO_KEYWORDS, Qfalse, false);
 }
 
 VALUE
@@ -2181,29 +2199,38 @@ rb_fiber_terminate(rb_fiber_t *fiber, int need_interrupt)
     fiber->cont.machine.stack = NULL;
     fiber->cont.machine.stack_size = 0;
 
-    next_fiber = return_fiber();
+    next_fiber = return_fiber(true);
     if (need_interrupt) RUBY_VM_SET_INTERRUPT(&next_fiber->cont.saved_ec);
-    fiber_switch(next_fiber, 1, &value, 0, RB_NO_KEYWORDS);
+    fiber_switch(next_fiber, 1, &value, RB_NO_KEYWORDS, Qfalse, false);
 }
 
 VALUE
 rb_fiber_resume_kw(VALUE fiber_value, int argc, const VALUE *argv, int kw_splat)
 {
     rb_fiber_t *fiber = fiber_ptr(fiber_value);
+    rb_fiber_t *current_fiber = fiber_current();
 
     if (argc == -1 && FIBER_CREATED_P(fiber)) {
         rb_raise(rb_eFiberError, "cannot raise exception on unborn fiber");
     }
-
-    if (fiber->prev != 0 || fiber_is_root_p(fiber)) {
-        rb_raise(rb_eFiberError, "double resume");
+    else if (FIBER_TERMINATED_P(fiber)) {
+        rb_raise(rb_eFiberError, "attempt to resume a terminated fiber");
+    }
+    else if (fiber == current_fiber) {
+        rb_raise(rb_eFiberError, "attempt to resume the current fiber");
+    }
+    else if (fiber->prev != NULL) {
+        rb_raise(rb_eFiberError, "attempt to resume a resumed fiber (double resume)");
+    }
+    else if (RTEST(fiber->resuming_fiber)) {
+        rb_raise(rb_eFiberError, "attempt to resume a resuming fiber");
+    }
+    else if (fiber->prev == NULL &&
+             (!fiber->yielding && fiber->status != FIBER_CREATED)) {
+        rb_raise(rb_eFiberError, "attempt to resume a transferring fiber");
     }
 
-    if (fiber->transferred != 0) {
-        rb_raise(rb_eFiberError, "cannot resume transferred Fiber");
-    }
-
-    return fiber_switch(fiber, argc, argv, 1, kw_splat);
+    return fiber_switch(fiber, argc, argv, kw_splat, fiber_value, false);
 }
 
 VALUE
@@ -2215,13 +2242,13 @@ rb_fiber_resume(VALUE fiber_value, int argc, const VALUE *argv)
 VALUE
 rb_fiber_yield_kw(int argc, const VALUE *argv, int kw_splat)
 {
-    return fiber_switch(return_fiber(), argc, argv, 0, kw_splat);
+    return fiber_switch(return_fiber(false), argc, argv, kw_splat, Qfalse, true);
 }
 
 VALUE
 rb_fiber_yield(int argc, const VALUE *argv)
 {
-    return fiber_switch(return_fiber(), argc, argv, 0, RB_NO_KEYWORDS);
+    return fiber_switch(return_fiber(false), argc, argv, RB_NO_KEYWORDS, Qfalse, true);
 }
 
 void
@@ -2362,8 +2389,13 @@ static VALUE
 rb_fiber_m_transfer(int argc, VALUE *argv, VALUE fiber_value)
 {
     rb_fiber_t *fiber = fiber_ptr(fiber_value);
-    fiber->transferred = 1;
-    return fiber_switch(fiber, argc, argv, 0, rb_keyword_given_p());
+    if (RTEST(fiber->resuming_fiber)) {
+        rb_raise(rb_eFiberError, "attempt to transfer to a resuming fiber");
+    }
+    if (fiber->yielding) {
+        rb_raise(rb_eFiberError, "attempt to transfer to a yielding fiber");
+    }
+    return fiber_switch(fiber, argc, argv, rb_keyword_given_p(), Qfalse, false);
 }
 
 /*
@@ -2411,8 +2443,8 @@ fiber_to_s(VALUE fiber_value)
     const rb_proc_t *proc;
     char status_info[0x20];
 
-    if (fiber->transferred) {
-        snprintf(status_info, 0x20, " (%s, transferred)", fiber_status_name(fiber->status));
+    if (RTEST(fiber->resuming_fiber)) {
+        snprintf(status_info, 0x20, " (%s by resuming)", fiber_status_name(fiber->status));
     }
     else {
         snprintf(status_info, 0x20, " (%s)", fiber_status_name(fiber->status));

spec/ruby/core/fiber/resume_spec.rb

@@ -6,9 +6,40 @@ describe "Fiber#resume" do
 end
 
 describe "Fiber#resume" do
-  it "raises a FiberError if the Fiber tries to resume itself" do
-    fiber = Fiber.new { fiber.resume }
-    -> { fiber.resume }.should raise_error(FiberError, /double resume/)
+  it "runs until Fiber.yield" do
+    obj = mock('obj')
+    obj.should_not_receive(:do)
+    fiber = Fiber.new { 1 + 2; Fiber.yield; obj.do }
+    fiber.resume
+  end
+
+  it "resumes from the last call to Fiber.yield on subsequent invocations" do
+    fiber = Fiber.new { Fiber.yield :first; :second }
+    fiber.resume.should == :first
+    fiber.resume.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.resume :first
+    fiber.resume :second
+  end
+
+  ruby_version_is '3.0' do
+    it "raises a FiberError if the Fiber tries to resume itself" do
+      fiber = Fiber.new { fiber.resume }
+      -> { fiber.resume }.should raise_error(FiberError, /current fiber/)
+    end
+  end
+
+  ruby_version_is '' ... '3.0' do
+    it "raises a FiberError if the Fiber tries to resume itself" do
+      fiber = Fiber.new { fiber.resume }
+      -> { fiber.resume }.should raise_error(FiberError, /double resume/)
+    end
   end
 
   it "returns control to the calling Fiber if called from one" do

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

spec/ruby/shared/fiber/resume.rb

@@ -35,32 +35,11 @@ describe :fiber_resume, shared: true do
     fiber.send(@method)
   end
 
-  it "runs until Fiber.yield" do
-    obj = mock('obj')
-    obj.should_not_receive(:do)
-    fiber = Fiber.new { 1 + 2; Fiber.yield; obj.do }
-    fiber.send(@method)
-  end
-
-  it "resumes from the last call to Fiber.yield on subsequent invocations" do
-    fiber = Fiber.new { Fiber.yield :first; :second }
-    fiber.send(@method).should == :first
-    fiber.send(@method).should == :second
-  end
-
   it "accepts any number of arguments" do
     fiber = Fiber.new { |a| }
     -> { fiber.send(@method, *(1..10).to_a) }.should_not raise_error
   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.send(@method, :first)
-    fiber.send(@method, :second)
-  end
-
   it "raises a FiberError if the Fiber is dead" do
     fiber = Fiber.new { true }
     fiber.send(@method)

test/ruby/test_fiber.rb

@@ -184,6 +184,34 @@ class TestFiber < Test::Unit::TestCase
     assert_equal([:baz], ary)
   end
 
+  def test_terminate_transferred_fiber
+    root_fiber = Fiber.current
+    log = []
+    fa1 = fa2 = fb1 = r1 = nil
+
+    fa1 = Fiber.new{
+      fa2 = Fiber.new{
+        log << :fa2_terminate
+      }
+      fa2.resume
+      log << :fa1_terminate
+    }
+    fb1 = Fiber.new{
+      fa1.transfer
+      log << :fb1_terminate
+    }
+
+    r1 = Fiber.new{
+      fb1.transfer
+      log << :r1_terminate
+    }
+
+    r1.resume
+    log << :root_terminate
+
+    assert_equal [:fa2_terminate, :fa1_terminate, :r1_terminate, :root_terminate], log
+  end
+
   def test_tls
     #
     def tvar(var, val)
@@ -278,29 +306,72 @@ class TestFiber < Test::Unit::TestCase
     assert_instance_of(Class, Fiber.new(&Class.new.method(:undef_method)).resume(:to_s), bug5083)
   end
 
-  def test_prohibit_resume_transferred_fiber
+  def test_prohibit_transfer_to_resuming_fiber
+    root_fiber = Fiber.current
+
     assert_raise(FiberError){
-      root_fiber = Fiber.current
-      f = Fiber.new{
-        root_fiber.transfer
-      }
-      f.transfer
-      f.resume
+      fiber = Fiber.new{ root_fiber.transfer }
+      fiber.resume
+    }
+
+    fa1 = Fiber.new{
+      fa2 = Fiber.new{ root_fiber.transfer }
+    }
+    fb1 = Fiber.new{
+      fb2 = Fiber.new{ root_fiber.transfer }
+    }
+    fa1.transfer
+    fb1.transfer
+
+    assert_raise(FiberError){
+      fa1.transfer
     }
     assert_raise(FiberError){
-      g=nil
-      f=Fiber.new{
-        g.resume
-        g.resume
-      }
-      g=Fiber.new{
-        f.resume
-        f.resume
-      }
-      f.transfer
+      fb1.transfer
     }
   end
 
+  def test_prohibit_transfer_to_yielding_fiber
+    root_fiber = Fiber.current
+    f1 = f2 = f3 = nil
+
+    f1 = Fiber.new{
+      f2 = Fiber.new{
+        f3 = Fiber.new{
+          p f3: Fiber.yield
+        }
+        f3.resume
+      }
+      f2.resume
+    }
+    f1.resume
+
+    assert_raise(FiberError){ f3.transfer 10 }
+  end
+
+  def test_prohibit_resume_to_transferring_fiber
+    root_fiber = Fiber.current
+
+    assert_raise(FiberError){
+      Fiber.new{
+        root_fiber.resume
+      }.transfer
+    }
+
+    f1 = f2 = nil
+    f1 = Fiber.new do
+      f2.transfer
+    end
+    f2 = Fiber.new do
+      f1.resume # attempt to resume transferring fiber
+    end
+
+    assert_raise(FiberError){
+      f1.transfer
+    }
+  end
+
+
   def test_fork_from_fiber
     skip 'fork not supported' unless Process.respond_to?(:fork)
     pid = nil