[GitHub] [arrow] pitrou edited a comment on pull request #8287: ARROW-10111: [Rust] Added new crate with code that consumes C Data interface

[GitBox <gi...@apache.org>]

pitrou edited a comment on pull request #8287:
URL: https://github.com/apache/arrow/pull/8287#issuecomment-704423199


   You're doing it wrong. I suggest again that you try to follow how C++ does things, otherwise you'll get lost.
   
   For example, your `release` callback assumes that buffers have been allocated by Rust. This is trivially wrong if e.g. roundtripping from Python to Rust to Python.
   
   So, what needs to happen is you have something like this (not necessarily working, but you get the idea):
   ```rust
   struct ExportedArrayData {
     buffers: Vec<Buffer> buffers,
     // other stuff perhaps...
   };
   ```
   
   Then your `private_data` must point to an heap-allocated `ExportedArrayData`. Your `release` callback will cast back `private_data` to `ExportedArrayData` and destroy it (releasing all the buffers). This can probably be done using:
   * `private_data = Box::new(ExportedArrayData...).into_raw() as *mut c_void` when exporting
   * `Box::from_raw(private_data as *mut ExportedArrayData)` in the release callback
   
   And again, I suggest you tackle importing and exporting separately. I cannot help you if you ignore what I tell you.


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