Greenlet - Coroutines library for nim simmilar to python's greenlet.

Greenlet tries to be a FAST!, easy to use and yet simple to understand and library for using flavor of coroutines called "greenlets".

• It is based on the cgreenlet library: https://github.com/geertj/cgreenlet
• Which itself is based on the python's greenlet library: https://github.com/python-greenlet/greenlet

Very simple to use

Only a 3 procs do most of the work:

• newGreenlet()
• g.switchTo()
• g.destroy()

Only 2 extra convience procs to manage errors or gain speed:

• g.inject()
• g.reset()

And 5 getter procs to see state of things:

• rootGreenlet()
• currentGreenlet()
• parentGreenlet()
• g.started
• g.dead

Example

import greenlet

var g = newGreenlet proc (arg: pointer): pointer =
  echo "in func1 (greenlet 1)"
  discard rootGreenlet().switchTo()
  echo "exiting func1 (greenlet 1)"
  return nil

echo "entering greenlet 1"
discard g.switchTo()

echo "re-entering greenlet 1"
discard g.switchTo() 

echo "exiting program"
g.destroy()

output:

entering greenlet 1
in func1 (greenlet 1)
re-entering greenlet 1
exiting func1 (greenlet 1)
exiting program

Very fast switching:

Compared to other coroutines methods:

Compared to threads:

Basaed on CGreenlet

It is based on the cgreenlet library: https://github.com/geertj/cgreenlet

Following is the explanation of how cgreenlet library works.

Platform portability

Greenlet requires 4 platform and/or architecture specific pieces of functionality in order to be able to offer coroutines:

1. Thread-local storage.
2. Stack allocation and deallocation
3. Stack switching
4. Context switching

For each platform that is supported, these 4 areas of functionality have to be provided. What follows now is an overview on how each of these are implemented:

Thread-local storage

Currently two approaches are supported. The fastest and preferred way is to use a __thread allocation class keyword on thread-local variables. This is available on Linux and on Windows. This if is not available (e.g. on Mac OSX), the pthread_getspecific() API is used.

Thread-local storage is implemented in "greenlet-system.c".

Allocation of the stack

On Unix-like systems, greenlet uses mmap() and on Windows, it uses VirtualAlloc(). The stack is protected with one PROT_NONE guard page. Stack allocation is implemented in "greenlet-system.c".

Stack switching

There is really no portably way to do this. On Unix-like platforms, we could use sigaltstack() + longjmp(), or the deprecated POSIX makecontext(). On Windows, the Fiber API could be used.

The approach that greenlet has taken is to implement an architecture specific function to switch stacks:

  void _greenlet_callnewstack(void *stack, void *(*func)(void *), void *arg);

This function will switch stack to stack, and call func(arg) on the new stack. The function may not return. If it does try to return, a SIGSEGV will be raised because we overwrite the return address to NULL. The way to get out of this function is to switch context to a parent of the current context (see next section).

This function is implemented in assembly in "greenlet-asm.S".

Context switching

A relatively portably way to implement this would be setjmp()/longjmp(). The function is available on Unix-like platforms and Windows. However the performance of these functions varies greatly, as on some platforms this saves the signal stack and on others it doesn't. Also, setjmp()/longjmp() do not allow you to inject code just before execution is resumed in the target. Code injection is needed to propagate exceptions.

The approach taken by greenlet is to provide a fast architecture specific implementation of setjmp()/longjmp():

  int _greenlet_setjmp_fast(void *frame);
  void _greenlet_longjmp_fast(void *frame, void (*inject_func)(void *),
                              void *arg);

These functions work just like setjmp()/longjmp() only that they are about 2x faster and allow for code injection.

These functios are implemented in assemlby in "greenlet-asm.S".

Contributing

Feel free to add an issue on the github site, or fork it and send me a merge request.