wasmtime: add EngineWeak which has ::upgrade, created by Engine::weak#7797
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Engine is, internally, just an Arc<EngineInner>, so this is trivial to implement - EngineWeak is a Weak<EngineInner>. This behavior is desirable because `Engine::increment_epoch` typically happens in a worker thread, which in turn requires additional machinery to discard the `Engine` once it is no longer needed. If instead the worker thread holds an `EngineWeak`, it can stop ticking when all consumers of the `Engine` have dropped it. This has been documented as a suggestion in `increment_epoch`. For an example of additional machinery which is simplified by this change, see https://github.com/fastly/Viceroy/blob/25edee0700ec0b20b1b56db0a3a8d6f090397b3a/lib/src/execute.rs#L108-L116) Co-authored-by: John Van Enk <vanenkj@gmail.com>
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Sanity test for when it is expected to be upgradable and expected to not be?
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Engine is, internally, just an Arc, so this is trivial to implement - EngineWeak is a Weak.
This behavior is desirable because
Engine::increment_epochtypically happens in a worker thread, which in turn requires additional machinery to discard theEngineonce it is no longer needed. If instead the worker thread holds anEngineWeak, it can stop ticking when all consumers of theEnginehave dropped it. This has been documented as a suggestion inincrement_epoch.For an example of additional machinery which is simplified by this change, see https://github.com/fastly/Viceroy/blob/25edee0700ec0b20b1b56db0a3a8d6f090397b3a/lib/src/execute.rs#L108-L116)