task: Drop the join waker of a task eagerly when the JoinHandle gets dropped or the task completes #6986
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dropped or the task completes Currently, the waker registered with a JoinHandle is not dropped until the task allocation is dropped. This behaviour may cause the memory allocated by a task to not be freed when in the case of two tasks awaiting each others JoinHandle. This commit changes the behaviour by actively dropping the waker when the JoinHandle gets dropped (or the task completes in some cases).
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It may be nice to mention somewhere that JOIN_WAKER represents whether the runtime needs to call wake_by_ref, and not whether the waker field is set. This way, it makes intuitive sense that the runtime can unset JOIN_WAKER without setting waker to None.
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I thought that it is not needed to add this because JOIN_WAKER acts as an access control bit to the waker field.
But I could add it to the documentation as well to make the use cases for the JOIN_WAKER bit more clear.
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Ah, yeah, the "control bit" wording is okay, but I think what I proposed would be even better.
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I added the following to the docs of the JOIN_WAKER:
//! [...]After the runtime sets COMPLETE to one, it invokes the waker if there is one so in this
//! case when a task completes the `JOIN_WAKER` bit implicates to the runtime
//! whether it should invoke the waker or not. After the runtime is done with
//! using the waker during task completion, it unsets the `JOIN_WAKER` bit to give
//! the `JoinHandle` exclusive access again so that it is able to drop the waker
//! at a later point.
Would that be enough for you?
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Motivation
Currently, the waker registered with a JoinHandle is not dropped until the task allocation is dropped. This behaviour may cause the memory allocated by a task to not be freed when in the case of two tasks awaiting each others JoinHandle.
See #6505 for more details.
Solution
To prevent this we need to actively drop the waker when the
JoinHandlegets dropped (or the task completes in some cases).Closes #6505.