Fix Issue 15104 - Fiber context switch in finally blocks breaks EH

[dnadlinger]

https://issues.dlang.org/show_bug.cgi?id=15104

[dnadlinger]

Ping @alexrp, @MartinNowak. This is quite a severe issue for any project using fibers (such as the Weka code base).

[ibuclaw]

👍 - Fix works for gdc too.

[MartinNowak]

Can we store it as Fiber field and on the stack instead? I want to get rid of the Context within chain at some point.

[dnadlinger]

Context seems like the most natural place right now, and this does not add any extra work for getting rid of it – just do the same thing to the EH context pointer that you are going to do to the stack top/bottom pointers.

[MartinNowak]

I want to keep the array of Contexts with stacktop/bottom for the GC, the nested push/pop Context is only needed to find out whether the stack pointer during a GC suspend is in the thread's stack or the current Fiber's stack. This can be done by checking the stack range instead.
But anyhow, keeping this variable on the stack is simple enough to do later.

[dnadlinger]

Hm, yeah, I thought I'd need to modify the asm code for this to push the extra value (which would have been annoying due to alignment, different platforms, etc.), but we could probably just save it on the stack in the context switching member functions.

[MartinNowak]

I think you can because the context switching itself does not throw.