Commits on Oct 12, 2022

1. turns out miri just hates localset

   

   hawkw committed Oct 12, 2022
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2. runtime: fix Stacked Borrows violation in LocalOwnedTasks …

   ## Motivation
   
   It turns out that `LocalSet` has...never actually passed Miri. This is
   because all of the tests for `LocalSet` are defined as integration tests
   in `tokio/tests/task_local_set.rs`, rather than lib tests in
   `tokio/src`, and we never run Miri for integration tests.
   
   PR #5095 added a new test reproducing an unrelated bug in `LocalSet`,
   which had to be implemented as a lib test, as it must make internal
   assertions about the state of the `LocalSet` that cannot be tested with
   just the public API. This test failed under Miri, and it turns out that
   the reason for this is unrelated to the change in #5095 --- `LocalSet`
   uses the `LocalOwnedTasks` type, which turns out to contain a stacked
   borrows violation due to converting an `&Header` into an
   `NonNull<Header>` when removing a task from the `LocalOwnedTasks` list.
   
   ## Solution
   
   Fortunately, this was actually quite easy to fix. The non-`Local`
   version of `OwnedTasks` already uses `Task::header_ptr()` to get a
   `NonNull<Header>` in its version of `remove`, which avoids this issue.
   Therefore, the fix was as simple as updating `LocalOwnedTasks` to do the
   same.
   
   I've also added a very simple lib test in `src/task/local_set.rs` that
   just creates a `LocalSet` and spawns a single task on it under a
   current-thread runtime. This test fails under Miri without the fix in
   this PR: https://github.com/tokio-rs/tokio/actions/runs/3237072694/jobs/5303682530
   
   We should probably keep the test so that we're always testing at least
   the most trivial `LocalSet` usage under Miri...

   

   hawkw committed Oct 12, 2022
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