rt: add optional per-task user data exposed to hooks #7688
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In tokio-rs#7197 and tokio-rs#7306, improved capabilities of task hooks were discussed and an initial implementation provided. However, that involved quite wide-reaching changes, modifying every spawn site and introducing a global map to provide the full inheritance capabilities originally proposed. This is the first part of a more basic version where we only use the existing hooks and provide the capabilities for consumers to be able to implement more complex relationships if needed, just adding an optional user data ref to the task header. The additional data is 2*usize, and is not enough to result in the struct requiring more than one cache line. A user is now able to use their own global map to build inheritance capabilities if needed, and this would be made simpler by also exposing the current task user data to the on_task_spawn hook, which a followup will look to do.
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…ast helpers It's nicer syntactically to be able to directly pass whatever type and have tokio deal with how the user data is stored, but it makes on_task_spawn have a horrible wrapper function, so just take that out to keep things simpler and nicer for now.
Darksonn
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Oct 20, 2025
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| /// Custom user defined metadata for this task for use in hooks. | ||
| #[cfg(tokio_unstable)] | ||
| pub(super) user_data: UserData, |
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I believe the task header is supposed to fit in 64 bytes. Is that still the case? I'm thinking we might need to put this in the trailer.
ADD-SP
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Oct 20, 2025
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In #7197 and #7306, improved capabilities of task hooks were discussed and an initial implementation provided. However, that involved quite wide-reaching changes, modifying every spawn site and introducing a global map to provide the full inheritance capabilities originally proposed.
This is the first part of a more basic version where we only use the existing hooks and provide the capabilities for consumers to be able to implement more complex relationships if needed, just adding an optional user data ref to the task header.
The additional data is 2*usize, and is not enough to result in the struct requiring more than one cache line.
A user is now able to use their own global map to build inheritance capabilities if needed, and this would be made simpler by also exposing the current task user data to the on_task_spawn hook, which a followup will look to do.
Motivation
See #7306. The intent is to allow persistence of context for across evaluations of task hooks, which is potentially also accessible by the task itself.
For example you may want to just know how many poll()s a task took, metrics of each poll duration (or between each poll duration), or to set some flag to track what a task was about to do (e.g. read some bytes from a socket) to identify what a task has become blocked on.
Solution
By adding a fat pointer for task metadata, consumers are able to attack a reference to data of an arbitrary type for a task, which we then expose in hooks. This metadata can be configured at task spawn time, also allowing consumers to maintain some shared reference between the hooks and the task's future itself to maintain knowledge of what is going on.