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-# Fiber
-
-Fibers provide a mechanism for cooperative concurrency.
-
-## Context Switching
-
-Fibers execute a user-provided block. During the execution, the block may call `Fiber.yield` or `Fiber.transfer` to switch to another fiber. `Fiber#resume` is used to continue execution from the point where `Fiber.yield` was called.
-
-``` ruby
-#!/usr/bin/env ruby
-
-puts "1: Start program."
-
-f = Fiber.new do
-  puts "3: Entered fiber."
-  Fiber.yield
-  puts "5: Resumed fiber."
-end
-
-puts "2: Resume fiber first time."
-f.resume
-
-puts "4: Resume fiber second time."
-f.resume
-
-puts "6: Finished."
-```
-
-This program demonstrates the flow control of fibers.
-
-## Scheduler
-
-The scheduler interface is used to intercept blocking operations. A typical
-implementation would be a wrapper for a gem like `EventMachine` or `Async`. This
-design provides separation of concerns between the event loop implementation
-and application code. It also allows for layered schedulers which can perform
-instrumentation.
-
-To set the scheduler for the current thread:
-
-``` ruby
-Fiber.set_scheduler(MyScheduler.new)
-```
-
-When the thread exits, there is an implicit call to `set_scheduler`:
-
-``` ruby
-Fiber.set_scheduler(nil)
-```
-
-### Interface
-
-This is the interface you need to implement.
-
-``` ruby
-class Scheduler
-  # Wait for the specified process ID to exit.
-  # This hook is optional.
-  # @parameter pid [Integer] The process ID to wait for.
-  # @parameter flags [Integer] A bit-mask of flags suitable for `Process::Status.wait`.
-  # @returns [Process::Status] A process status instance.
-  def process_wait(pid, flags)
-    Thread.new do
-      Process::Status.wait(pid, flags)
-    end.value
-  end
-
-  # Wait for the given file descriptor to match the specified events within
-  # the specified timeout.
-  # @parameter event [Integer] A bit mask of `IO::READABLE`,
-  #   `IO::WRITABLE` and `IO::PRIORITY`.
-  # @parameter timeout [Numeric] The amount of time to wait for the event in seconds.
-  # @returns [Integer] The subset of events that are ready.
-  def io_wait(io, events, timeout)
-  end
-
-  # Sleep the current task for the specified duration, or forever if not
-  # specified.
-  # @parameter duration [Numeric] The amount of time to sleep in seconds.
-  def kernel_sleep(duration = nil)
-  end
-
-  # Execute the given block. If the block execution exceeds the given timeout,
-  # the specified exception `klass` will be raised. Typically, only non-blocking
-  # methods which enter the scheduler will raise such exceptions.
-  # @parameter duration [Integer] The amount of time to wait, after which an exception will be raised.
-  # @parameter klass [Class] The exception class to raise.
-  # @parameter *arguments [Array] The arguments to send to the constructor of the exception.
-  # @yields {...} The user code to execute.
-  def timeout_after(duration, klass, *arguments, &block)
-  end
-
-  # Block the calling fiber.
-  # @parameter blocker [Object] What we are waiting on, informational only.
-  # @parameter timeout [Numeric | Nil] The amount of time to wait for in seconds.
-  # @returns [Boolean] Whether the blocking operation was successful or not.
-  def block(blocker, timeout = nil)
-  end
-
-  # Unblock the specified fiber.
-  # @parameter blocker [Object] What we are waiting on, informational only.
-  # @parameter fiber [Fiber] The fiber to unblock.
-  # @reentrant Thread safe.
-  def unblock(blocker, fiber)
-  end
-
-  # Intercept the creation of a non-blocking fiber.
-  # @returns [Fiber]
-  def fiber(&block)
-    Fiber.new(blocking: false, &block)
-  end
-
-  # Invoked when the thread exits.
-  def close
-    self.run
-  end
-
-  def run
-    # Implement event loop here.
-  end
-end
-```
-
-Additional hooks may be introduced in the future, we will use feature detection
-in order to enable these hooks.
-
-### Non-blocking Execution
-
-The scheduler hooks will only be used in special non-blocking execution
-contexts. Non-blocking execution contexts introduce non-determinism because the
-execution of scheduler hooks may introduce context switching points into your
-program.
-
-#### Fibers
-
-Fibers can be used to create non-blocking execution contexts.
-
-``` ruby
-Fiber.new do
-  puts Fiber.current.blocking? # false
-
-  # May invoke `Fiber.scheduler&.io_wait`.
-  io.read(...)
-
-  # May invoke `Fiber.scheduler&.io_wait`.
-  io.write(...)
-
-  # Will invoke `Fiber.scheduler&.kernel_sleep`.
-  sleep(n)
-end.resume
-```
-
-We also introduce a new method which simplifies the creation of these
-non-blocking fibers:
-
-``` ruby
-Fiber.schedule do
-  puts Fiber.current.blocking? # false
-end
-```
-
-The purpose of this method is to allow the scheduler to internally decide the
-policy for when to start the fiber, and whether to use symmetric or asymmetric
-fibers.
-
-You can also create blocking execution contexts:
-
-``` ruby
-Fiber.new(blocking: true) do
-  # Won't use the scheduler:
-  sleep(n)
-end
-```
-
-However you should generally avoid this unless you are implementing a scheduler.
-
-#### IO
-
-By default, I/O is non-blocking. Not all operating systems support non-blocking
-I/O. Windows is a notable example where socket I/O can be non-blocking but pipe
-I/O is blocking. Provided that there *is* a scheduler and the current thread *is
-non-blocking*, the operation will invoke the scheduler.
-
-#### Mutex
-
-The `Mutex` class can be used in a non-blocking context and is fiber specific.
-
-#### ConditionVariable
-
-The `ConditionVariable` class can be used in a non-blocking context and is
-fiber-specific.
-
-#### Queue / SizedQueue
-
-The `Queue` and `SizedQueue` classes can be used in a non-blocking context and
-are fiber-specific.
-
-#### Thread
-
-The `Thread#join` operation can be used in a non-blocking context and is
-fiber-specific.