Message Passing and Shared Data

[Jonathan M Davis]

> I'm working on an application that makes use of message passing to
> separate things that need to be responsive from tasks that may take
> some time, and I'm having trouble because my message passing
> discipline isn't meshing with D's type system. I'm pretty much
> following the contract that when I send a message from one thread to
> another, the original thread doesn't store any references to the
> message or what's in it, IE ownership is transferred. The receiving
> thread may then send some of the same objects back after some
> processing, but again, ownership is transferred.
> This has the benefit that I avoid a lot of memory allocations, but it
> means that right now, I'm constructing everything as shared, then
> casting it away to populate the object, then sending the object, then
> casting shared away again in the receiver, and so on.
> The messages are generally passed around for things like requesting a
> large chunk of data that takes a while to generate and receiving
> objects from an IO thread that's decoding objects from sockets.
> 
> Any suggestions as to how I could avoid doing casts everywhere and/or
> restore some sort of safety while avoiding having to reallocate every
> object I mess with? I've tried fiddling with __gshared, but couldn't
> figure out any way to use it with message passing.

There has been some talk of making it possible to do what you want to do with 
a "unique" template or attribute, but it hasn't gone anywhere that I know of. 
In particular, if it requires actual language changes, then it's unlikely to 
change in D2. We might get some sort of template that solves the problem 
though. I don't know.

For the moment, however, message passing only works with immutable values. End 
of story. So, you have to do stuff like what you've been doing if you want to 
get around it. Essentially, you either deal just with immutable values, cast 
to and from immutable, or use shared. However, at the moment, there's no real 
way to "transfer ownership" of an object between threads, so message passing 
just doesn't do quite what you want to do. There may be a solution for it in 
the future though.

- Jonathan M Davis