Document and restrict some unsafe code in gix-pack #1137
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I found it confusing that `node` is not a `Node` but an `Item`.
The `Node` type is only used in `resolve.rs`, and using it safely depends on using the child items correctly, so let's define it there.
The `Clone` impl allows two `ItemSliceSend` to be backed by the same slice. That makes the behavior of `get_mut()` harder to reason about because we have to consider clones of the `ItemSliceSend` to determine if it's safe. By making it `Sync`, we make it more obvious that multiple threads referring to the same instance need to be considered. I renamed the type to `ItemSliceSync` since it's now also `Sync`. We could make the type also check the bounds and that an individual element has not been borrowed more than once. Then we could make `get_mut()` not `unsafe`.
`Node` is only safe if it doesn't repeat any indexes into the `ItemSliceSync`. Let's document this by giving it an unsafe constructor. To enforce that, I also moved it into a private module.
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Thanks a lot for your help with this! This portion of the code has seen a lot of revisions, probably more than anything else, which once more shows how hard it is to get |
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Thanks to @cramertj for highlighting these issues in our internal review of unsafe code at Google.