Add the concept of a tree variable (TreeVar) #1543
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…nt nursery rather than from the spawner
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oremanj
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A TreeVar acts like a context variable that is inherited at nursery creation time (and then by child tasks of that nursery) rather than at task creation time. They are useful for providing 'ambient' access to a resource that is tied to an `async with` block in the parent task, such as an open file or trio-asyncio event loop. Prior art: python-trio/trio#1543 (never made it into mainline Trio). The implementation without Trio core support is somewhat less efficient, but still workable.
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TreeVars are like ContextVars except that their initial value in new tasks is determined differently: they are inherited from the new task's parent nursery rather than from its spawner. #1523 has some discussion of the possible uses.
Fixes #1523