std.concurrency.spawn does not accept delegates

[Jonathan M Davis]

On 2011-07-18 15:15, teo wrote:
> On Mon, 18 Jul 2011 18:14:45 +0000, Jonathan M Davis wrote:
> > On 2011-07-18 10:54, Simen Kjaeraas wrote:
> >> On Mon, 18 Jul 2011 18:06:46 +0200, Jonathan M Davis
> >> <jmdavisProg at gmx.com>
> >> 
> >> wrote:
> >> > On Monday 18 July 2011 15:55:52 teo wrote:
> >> >> On Mon, 18 Jul 2011 10:26:27 -0400, Steven Schveighoffer wrote:
> >> >> > On Sun, 17 Jul 2011 15:29:02 -0400, teo <teo.ubuntu at yahoo.com>
> >> >> > 
> >> >> > wrote:
> >> >> >> It looks like std.concurrency.spawn does not accept delegates. Is
> >> >> >> there
> >> >> >> any reason for that?
> >> >> > 
> >> >> > There is no type attached to the hidden 'this' pointer. So spawn
> >> >> 
> >> >> cannot
> >> >> 
> >> >> > guarantee it doesn't point to unshared data.
> >> >> > 
> >> >> > -Steve
> >> >> 
> >> >> Bad. I tried to pass as an argument a pointer to an instance of a
> >> >> class -
> >> >> a this pointer. That didn't work.
> >> > 
> >> > Only stuff that's immutable or implicitly convertible immutable can
> >> > be passed
> >> > across threads using spawn and send. Otherwise, there's a risk of it
> >> > ending up
> >> > getting altered by both threads (or altered by one when the other one
> >> > is using
> >> > at it). Sometimes, that can be a bit restrictive (particularly when
> >> > you _know_
> >> > that something isn't going to be altered by the thread sending it
> >> > after its
> >> > sent but the compiler doesn't), but it avoids all kinds of problems.
> >> > If you
> >> > want to send a class object across, then it needs to be immutable
> >> > (which tends
> >> > to be a bit of a pain to do for classes, since they need to have an
> >> > immutable
> >> > constructor, which is often a pain to do).
> >> 
> >> It could be that assumeUnique should handle this, by transforming the
> >> delegate into something spawn() and friends could handle.
> >> 
> >> Might be worth an enhancement request.
> > 
> > There have been discussions about how to do it in the past. Whether
> > assumeUnique will work depends on whether casting to immutable(C) will
> > work (where C is the class' type), and I don't know whether that cast
> > will work or not. If it does, then assumeUnique will do it, since all it
> > does is cast to immutable, but it's the sort of thing that requires
> > language support. There _might_ be a way to solve the problem with some
> > sort of Unique template that spawn and send knew about, but nothing of
> > the sort has been done yet. But until a number of the issues with const
> > and immutable in the compiler have been sorted out, that sort of thing
> > would probably be problematic anyway.
> > 
> > - Jonathan M Davis
> 
> This is a bit too restrictive in my opinion. Only the shared data between
> two threads should be immutable. But the threads can access all sorts of
> mutable data as well. And in this case we are actually talking about the
> control function (or start routine) of a thread. As long as it's address
> is fixed within the memory of a process its usage for that purpose should
> be fine. Nobody is going to pass that address around. Am I missing
> anything here?

When passing data between threads, it must be immutable. If it weren't, then 
you'd have to worry about mutexes and the like. Data is thread-local by 
default, so one thread does _not_ have access to the data in another thread 
unless it's shared. spawn starts a new thread with the data that it's given, 
and send allows you to send data to another thread, but if it's not immutable, 
then you're running into issues when multiple threads are dealing with mutabel 
data and could change it. And that's not allowed unless the data is shared - 
in which case there's no need for send, and you have to use mutexes or 
synchronized blocks to control access to it, ord you're going to have 
concurrency bugs.

- Jonathan M Davis