Bug #12555: Cannot resume a fiber that was earlier transferred from, then transferred back to, and then yielded

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 2.3.0
• Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN

[amukher1 (Arindam Mukherjee)]

Consider the following code snippet:

require 'fiber'

fiber2 = nil

fiber1 = Fiber.new do
  puts "In Fiber 1"                 # 3
  fiber2.transfer                   # 4
end

fiber2 = Fiber.new do
  puts "In Fiber 2"                  # 1
  fiber1.transfer                    # 2
  puts "In Fiber 2 again"            # 5
  Fiber.yield                        # 6
  puts "Fiber 2 resumed"             # 10
end

fiber3 = Fiber.new do
  puts "In Fiber 3"                  # 8
end

fiber2.resume                        # 0
fiber3.resume                        # 7
fiber2.resume                        # 9

I have numbered the lines of code with the expected serial order of execution on the right. Once fiber3.resume returns and I call fiber2.resume, I expect the execution to continue inside fiber2 at the line marked # 10. Instead, I get the following error:

fiber2.rb:24:in `resume': cannot resume transferred Fiber (FiberError)
    from fiber2.rb:24:in `<main>'

That's an error reported from the last line of the listing: fiber2.resume.

[amukher1 (Arindam Mukherjee)]

• Subject changed from Cannot resume a fiber that was earlier transferred from, then transferred to, and then yielded to Cannot resume a fiber that was earlier transferred from, then transferred back to, and then yielded

[hsbt (Hiroshi SHIBATA)]

• Assignee deleted (core)

[jeremyevans0 (Jeremy Evans)]

• Is duplicate of Bug #9664: cannot resume transferred Fiber even if it should resume added

• Status changed from Open to Closed

Applied in changeset git|fa8ac91e957a076f6df1adaecad7896817138009.

---

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]

[ko1 (Koichi Sasada)]

• Status changed from Closed to Rejected

see https://bugs.ruby-lang.org/issues/9664#note-6