fix(settings): mark drop guard as !Send #695
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Fixes mitsuhiko#694 Because the `SettingsDropGuard` modifies a `ThreadLocalKey` (it clones it in the `Settings::bind_to_scope` method, then restores it in its drop impl), it should not be moved to a different thread, as then it would modify that thread's `CURRENT_SETTINGS` instead of the original thread's. This is not an issue with normal code as the guard is usually unnameable (`let _guard = settings.bind_to_scope();`). but it is a problem with `async` code, as when held across `await` points is can be moved by the runtime to a separate thread. We fix it by adding a `PhantomData` field to `SettingsDropGuard` that does not implement `Send`. We verify it works by adding a `compile_fail` doctest to the struct that tries to send the drop guard to a different thread.
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This is technically a breaking change, but it is a logic error, so I would argue it is not breaking as users relying on this functionality had broken tests already |
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I'm not an expert here, but in principle it makes a lot of sense. I'll merge unless @mitsuhiko has any feedback in the next day or so. @jalil-salame if you'd be up for adding a one-sentence description to the Changelog (+ an attribution if you'd like), that would be great! |
Done! |
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Fixes #694
Because the
SettingsDropGuardmodifies aThreadLocalKey(it clones it in theSettings::bind_to_scopemethod, then restores it in its drop impl), it should not be moved to a different thread, as then it would modify that thread'sCURRENT_SETTINGSinstead of the original thread's.This is not an issue with normal code as the guard is usually unnameable (
let _guard = settings.bind_to_scope();). but it is a problem withasynccode, as when held acrossawaitpoints is can be moved by the runtime to a separate thread.We fix it by adding a
PhantomDatafield toSettingsDropGuardthat does not implementSend. We verify it works by adding acompile_faildoctest to the struct that tries to send the drop guard to a different thread.