tustvold commented on PR #3839:
URL: https://github.com/apache/arrow-rs/pull/3839#issuecomment-1480775873
I'm having a somewhat hard time understanding the code you've linked but
please bear with me:
`CachedBasedObjectStoreRegistry`:
* Creates a `FileCacheObjectStore` combining a normal `ObjectStore` with a
`FileCacheLayer`,
* Computes a cache key from the URL
* Creates a `ObjectStoreWithKey` to extend the `FileCacheObjectStore` with
this key to wrap this construction
Calling get on this object store then:
* Calls through `ObjectStoreWithKey` to `FileCacheObjectStore`
* Looks up the value in the cache, passing the inner store as the `GetExtra`
argument
* This propagates through to `FileCacheLoader::load`
* Which then calls `CacheMedium::get_mapping_location`
* Which finally calls through to `get_key` which attempts to downcast the
provided `ObjectStore` to `ObjectStoreWithKey`
* This will always fail as the `ObjectStoreWithKey` is decorating the
`FileCacheObjectStore` not the other way round
A simple fix might be to swap the decoration order, but taking a step back,
I don't see the need for `ObjectStoreWithKey`. The cache key is known at the
time `FileCacheObjectStore` is constructed by `CachedBasedObjectStoreRegistry`.
I don't see a reason to not just store this key inside the
`FileCacheObjectStore` and pass it along with the inner `ObjectStore` as the
`GetExtra` argument to the cache. This would not only be simpler (and work) but
would also avoid any need for downcasting?
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