errorVariable and thread safety

[elcritch]

While digging into #41 I noticed that binderror uses a global to store the last error state:

https://github.com/codex-storage/questionable/blob/e56cf86c4a089c78a1b7c3005f13343bfbbe3b48/questionable/private/binderror.nim#L4C10-L4C10

It should probably be made into var errorVariable: ptr ref CatchableError.

If it's not a threadvar issues might arise when used with multiple threads. I haven't worked through the precise logic, but some sequence similar to:

• thread1 and thread1 both encounter errors in a without clause
• thread1 uses errorVariable to store an error ptr
• thread1 is paused and thread2 runs
• thread2 stores its error in errorVariable
• thread2 is paused
• thread1 now reads errorVariable and gets the error meant for thread2

Probably not all too likely and shouldn't cause a false error case, just report the wrong error. That'd still be annoying.

[markspanbroek]

Good find, thanks! I was able to reproduce this in a test and adding {.threadvar.} indeed fixes the problem: #46