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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_schduler(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.
+  # @param duration [Numeric] The amount of time to sleep in seconds.
+  def kernel_sleep(duration = nil)
+  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` classses 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.