[ruby-core:100354] [Ruby master Bug#17221] Relax the Fiber#transfer's limitation

Issue #17221 has been updated by Eregon (Benoit Daloze).


I believe it's `a yielding Fiber` (the first sound in yield-ing is not a vowel sound)

----------------------------------------
Bug #17221: Relax the Fiber#transfer's limitation
https://bugs.ruby-lang.org/issues/17221#change-87965

* Author: ko1 (Koichi Sasada)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
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).

```ruby
require 'fiber'
f1 = nil
f2 = Fiber.new{
  f1.transfer
}
f1 = Fiber.new{
  f2.transfer
  Fiber.yield 10
  Fiber.yield 20
}
p f1.resume #=> 10
p f1.resume #=> `resume': cannot resume transferred Fiber (FiberError)
```

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.

```ruby
require 'fiber'

root = Fiber.current
f1 = f2 = nil

f1 = Fiber.new{
  f2 = Fiber.new{
    root.transfer(10)
  }
  f2.resume
}

p f1.transfer #=> 10

# root <-----+
#  |         |
#  v         | transfer
#  f1 -> f2 -+ # resume/yield chain


# horizontal direction: resume
# vertical direction: transfer

p f1.transfer #=> attempt to transfer to a resuming fiber (FiberError)

# f1 has it's own resume/yield chain, and f1.transfer breaks the chain

# root <-----+
#  || (error)|
#  vv        |
#  f1 -> f2 -+ # resume/yield chain
```

(2) can not transfer to the yielding fiber.

```ruby
require 'fiber'

f1 = f2 = nil

f1 = Fiber.new{
  f2 = Fiber.new{
    Fiber.yield
  }
  f2.resume
  10
}

p f1.transfer #=> 10

# root 
# | ^
# | | transfer
# v |
# f1 --> f2 # resume/yield chain
#    <--

p f2.transfer #=> `transfer': attempt to transfer to an yielding fiber (FiberError)

# f2 is waiting for the resume, so the transfer is not allowed.

# root --+
# | ^    | transfer (error)
# | |    |
# v |    v
# f1 --> f2 # resume/yield chain
#    <--

```

(3) can not resume transferring fiber.

```ruby
require 'fiber'

f1 = f2 = nil

f2 = Fiber.new{
  f1.resume #=> attempt to resume the transferring fiber (FiberError)
}
f1 = Fiber.new{
  f2.transfer
}
f1.transfer

# root
# |
# v
# f1 <-+
# |    |
# v    | resume (error)
# f2 --+

# f1 seems waiting for transfer from other fibers.
```

(4) can not yield from not-resumed fiber

```
require 'fiber'

f2 = Fiber.new do
  Fiber.yield #=> `yield': attempt to yield on not resumed fiber (FiberError)
end

f1 = Fiber.new
  f2.transfer
end

p f1.transfer

#     root
#     |
#     v
#     f1
#     |
#     v
#  <- f2
#  yield to where ...? (2.7 switches to root fiber)
```

and remove current restriction. The first example works fine:

```ruby
require 'fiber'
f1 = nil
f2 = Fiber.new{
  f1.transfer
}
f1 = Fiber.new{
  f2.transfer
  Fiber.yield 10
  Fiber.yield 20
}
p f1.resume #=> 10
p f1.resume #=> 20


# root -> f1 <-+
#         |    |
#         v    |
#         f2 --+
```

The basic idea is respect *programmer's intention*.

For (1), resuming fiber should be switched by the `Fiber.yield`.
For (2), yielding fiber should be switched by the `Fiber#resume`.
For (3), transferring fiber should be switched by the `Fiber#transfer`.

Mainly (1) can keep the resume/yield chain. Also (2) and (3) makes the chain and relationships with fibers cleanly.

----

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).

```ruby
require 'fiber'
f3 = Fiber.new{
  10
}
f2 = Fiber.new{
  f3.transfer + 20
}
f1 = Fiber.new{
  f2.resume
}
p f1.resume #=> 30

# without this patch:
#
# root -> f1 -> f2
#  ^             |
#  | exit        v
#  +----------- f3

# with this patch:
#
# root -> f1 -> f2 <-+  # keep resume/yield chain
#               |    |
#               v    |
#               f3 --+ exit
```

The patch is: https://github.com/ruby/ruby/pull/3636

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