[Vala] shortcut: Better co-routine support with fibers

["Sam Liddicott" <sam liddicott com>]

Some of you know I've been wanting the coroutine/continuation support
for better async handlers in Samba.

I've just read Ralf Engelschall's seminal "Portable Multithreading"
paper, which led to GNU Pthreads, and realised that he has enough in
there to do proper portable coroutine support in vala without
dependencies on particular event driven frameworks.

Rather than needing vala to declaring continuation structs to hold
automatic variables (which doesn't entirely solve the problem), it works
by allocating a new stack context from that point, which of course
automatically holds all automatic variable.

If the stack is allocated with mmap specifying that the region is to be
allocated as it is used, then we can be memory efficient without having
to worry about running out of stack.

As useful starting points I suggest:

libcoro - coroutines using Ralphs methods
http://software.schmorp.de/pkg/libcoro.html

coroutine usage based on libcoro - Jurg may be familiar with this one
http://felipec.wordpress.com/2008/09/28/continuations-in-c-easy-asynchronous-stuff/

Bug asking for fibers in GLib, mentioning the above two links
http://bugzilla.gnome.org/show_bug.cgi?id=565501

Libfiber - simple fiber library
http://www.rkeene.org/projects/info/wiki/22

PhuckThreads - thin skin around Ralph Engelschall's examples
https://lunastorm.dyndns.org/svn/PhuckThreads/

Implementing a thread library - See makecontext and longjmp implementations
http://evanjones.ca/software/threading.html

Windows setcontext stuff in case we need it
http://www.codeproject.com/KB/threads/ucontext.aspx

Of course we don't want a thread library, but coroutines are pretty much
directly scheduled fibres.


To continue a co-routine, just call sched_thread(thread_stack_ptr) -
which nicely requires calling a function that takes a single pointer -
something compatible with most callback types.


I'll be trying something like this on Samba and I expect it to be
straightforward, but not quite so simple for Vala, unless Vala intends
to support fibers?

With this method, "yield" will switch back to the main context, and
should only be called from a co-routine context. Something must generate
the co-routine instance. In samba it happens explicitly when the
function consciously decides to defer execution of a specific task, but
I'm not sure if this carries over well to some vala examples where yield
might be used during io selection and so forth. I haven't seen any vala
examples where  a corouting instance is  generated.

I hope someone knows what I'm talking about and speaks out.

Sam