Windows: Throwing Exceptions from Fibers in D2.059: Access Violation

[Sean Kelly]

On Apr 20, 2012, at 1:42 PM, Jameson Ernst wrote:

> On Friday, 20 April 2012 at 18:09:37 UTC, Sean Kelly wrote:
>> On Apr 20, 2012, at 10:59 AM, Andrew Lauritzen wrote:
>> 
>>> On Friday, 20 April 2012 at 17:49:52 UTC, Sean Kelly wrote:
>>>> There wouldn't me much of a performance hit, mostly an additional allocation and a bitcopy when creating a Fiber.  It's more that making this work on platforms with built-in TLS could be quite tricky.
>>> Note that this would somewhat sabotage their usefulness as coroutines, depending on how it was implemented. That's not to say the idea isn't good (but I'd frame it more like "tasks"; see Thread Building Blocks or similar), but fibers/coroutines as they stand now are useful so I'd hate to see that capability lost.
>> 
>> Ah, coroutines would actually want to share TLS with the executing thread?  That's good to know.
> 
> Yeah, I think there's a subtlety at play here, but it's an important one. In most languages and environments, a thread of execution and a call stack are conflated into a single entity. I think what Andrew and I are driving at is that we'd like them to be orthogonal concepts.
> 
> A "thread" is merely a lane of execution, and that lane can play host to any call-stack "fiber" context we'd like it to. For this reason, it's important that fibers not be bound to the thread that initially created them. Mono implemented a continuation system, but the stack context is tied to the thread that spawned it, which is frustrating and prevents efficient task pooling.

The weird thing in D is that static data is thread-local by default, so moving a fiber between threads would mean the value of statics would change between calls.  I had thought that by giving each fiber its own TLS space the programming model would be more predictable, but I can see how the current behavior may actually be desirable in some instances.  I wonder if there's any value in having a child of Fiber that has its own TLS, call it CoopThread or whatever.

For the record, there's absolutely nothing preventing you from moving a fiber across threads in Druntime.  It's absolutely legal behavior, despite the potential weirdness with statics.