# 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.
  # @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` 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.
