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spawn_system_task should not propagate ContextVar state #289

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njsmith opened this Issue Aug 16, 2017 · 2 comments

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njsmith commented Aug 16, 2017

Probably we should make spawn_impl have a system_task flag instead of its current ki_protection_enabled flag. We already have to special case init as not inheriting TaskLocals, so this would remove that special case (since init is a system task).

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Fuyukai commented Apr 14, 2018

Is this still relevant, given the ContextVar change? The system task context could potentially be reset to a regular context() rather than a copy_context() (if I understand the issue correctly), otherwise this can be closed.

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njsmith commented Apr 14, 2018

It's still as relevant as it was, just now it applies to ContextVars instead of TaskLocals :-).

The intuition is that a "system task" is a global resource that's not particularly associated with any task, so it shouldn't retain the context where it happened to be spawned. I think I was thinking about this in the context of #303, where the idea is to use system tasks to model the case where you want a single task to be part of multiple parts of the task tree at the same time; in this case it doesn't make sense for ContextVars to be inherited from whichever task happened to start the new one first.

Given how contextvars are more general than a single system, I guess the way to do it would be to stash a copy somewhere of the context that trio.run is called with, and then for system tasks copy that instead of the creator's context.

@njsmith njsmith changed the title from spawn_system_task should not propagate TaskLocals to spawn_system_task should not propagate ContextVar state Apr 14, 2018

@Fuyukai Fuyukai closed this in #520 May 6, 2018

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