Fiber.cancel polling vs select timeout for bounded cooperative cancellation

Answer

A fiber that is blocked in Channel#receive(timeout:) (or inside a select with a timeout clause) does not observe Fiber.current.canceled? before the timeout elapses or a value is received. The timeout (or data arrival) is evaluated first; the cancellation flag is only checked after the fiber resumes from the yield.

Confirmed facts

• Fiber#cancel sets an internal flag; it does not interrupt any blocking operation or I/O.
• The standard Ruby Channel implementation yields control to the scheduler while waiting in receive(timeout:). While yielded, the fiber cannot run any Ruby code, so it cannot read Fiber.canceled?.
• A select with a timeout clause resumes the fiber as soon as either a case matches or the timeout expires. The scheduler chooses whichever condition becomes true first.
• After the fiber resumes, execution continues at the point after the receive (or select) and the cancellation flag can be examined normally.

Steps to verify (Ruby 3.2+)

require 'concurrent-ruby'

chan = Concurrent::Channel.new
fiber = Fiber.new do
  puts "Before receive: canceled? = #{Fiber.canceled?}"
  # Block with a 5‑second timeout
  result = chan.receive(timeout: 5)
  puts "After receive: result = #{result.inspect}, canceled? = #{Fiber.canceled?}"
end

# Start the fiber
fiber.resume

# Cancel after a short delay
Thread.new do
  sleep 1
  fiber.cancel
  puts "Main: called Fiber.cancel"
end

# Let the timeout expire (no value sent)
sleep 6

Expected output:

Before receive: canceled? = false
Main: called Fiber.cancel
After receive: result = nil, canceled? = true

The fiber sees canceled? = true only after the timeout returns nil.
When both a timeout and a cancellation flag are present
The scheduler evaluates the timeout clause and any data‑receiving clauses concurrently. Whichever triggers first causes the fiber to resume. The cancellation flag is examined only after that resumption, i.e., it is second in the evaluation order.
If you are using a non‑standard Channel implementation that does not yield while waiting, the flag could be observed sooner. Confirm whether you rely on the Ruby standard library’s Channel or a custom version.