Attempt to solve cyclic ref in WasmerEnv #2312
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2327: Fix cyclic ref in WasmerEnv r=syrusakbary a=MarkMcCaskey This is an alternative to #2312 to fix #2304 The core idea is to have a `WeakInstanceRef` too. The main concern is that `Memory` (from a `WasmerEnv`) should behave like it has a strong instance ref if it's cloned. Currently we always convert weak `VMMemory`s into strong ones and if we can't, we panic. 1 other edge case not handled yet: we need to make sure that when the `InstanceRef` is weak that everything still works as expected (for example when the `Instance` it points to is gone). Or maybe we don't. `WasmerEnv`s are always passed by `&` and `Clone`ing forces them to upgrade into `Strong` or panic. So there's no real case where this can happen except for when you're calling into a host function from Wasm while the Instance is freed. Doing that means that you're already executing uninitialized memory as code so it shouldn't be possible / if it is, the concern is not with`WasmerEnv`. # Review - [ ] Add a short description of the change to the CHANGELOG.md file Co-authored-by: Mark McCaskey <[email protected]> Co-authored-by: Syrus Akbary <[email protected]> Co-authored-by: Mark McCaskey <[email protected]>
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May 28, 2021
2327: Fix cyclic ref in WasmerEnv r=MarkMcCaskey a=MarkMcCaskey This is an alternative to #2312 to fix #2304 The core idea is to have a `WeakInstanceRef` too. The main concern is that `Memory` (from a `WasmerEnv`) should behave like it has a strong instance ref if it's cloned. Currently we always convert weak `VMMemory`s into strong ones and if we can't, we panic. 1 other edge case not handled yet: we need to make sure that when the `InstanceRef` is weak that everything still works as expected (for example when the `Instance` it points to is gone). Or maybe we don't. `WasmerEnv`s are always passed by `&` and `Clone`ing forces them to upgrade into `Strong` or panic. So there's no real case where this can happen except for when you're calling into a host function from Wasm while the Instance is freed. Doing that means that you're already executing uninitialized memory as code so it shouldn't be possible / if it is, the concern is not with`WasmerEnv`. # Review - [x] Add a short description of the change to the CHANGELOG.md file Co-authored-by: Mark McCaskey <[email protected]> Co-authored-by: Syrus Akbary <[email protected]> Co-authored-by: Mark McCaskey <[email protected]>
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Trying to fix #2304
I'm not sure we want to go with the strategy in this PR, it's very low level and unsafe. The idea is to decrement the strong count of
InstanceReffor each export in the Env because in the context of host functions, anything coming from the same instance is guaranteed to be alive. We must preserve the property that cloning these types then increments theInstanceRefcount though, so we can't get rid of it entirely. Additionally we have to handle the case of something being re-exported, so we can't assume that if something is exported from an Instance that it's owned by that Instance.The PR currently gets stuck in an infinite loop during instantiation.
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