[I] `Path::from_absolute_path` discards UNC host on Windows, unprefixed `LocalFileSystem` cannot service UNC paths [arrow-rs-object-store]

["lachlancahill (via GitHub)" <gi...@apache.org>]

lachlancahill opened a new issue, #715:
URL: https://github.com/apache/arrow-rs-object-store/issues/715

   ### **Describe the bug**
   
   On Windows, given a UNC `PathBuf` like `\\server\share\foo`, `object_store::path::Path::from_absolute_path` returns `Path("share/foo")` the host (`server`) is dropped. Combined with `LocalFileSystem` (unprefixed), this means UNC paths cannot be used end-to-end: the host is gone before `LocalFileSystem` could resolve it back to a filesystem path.
   
   I'm filing this for visibility; whether the right fix is in `Path::from_absolute_path`, in `LocalFileSystem`, in documentation ("use `new_with_prefix` for UNC roots"), or somewhere else, I'd defer to maintainer judgement.
   
   ### **To Reproduce**
   
   ```toml
   [dependencies]
   url = "2.5"
   object_store = "0.13"
   ```
   
   ```rust
   fn main() {
       let input = r"\\<HOST>\TestShare\basic_partitioned\";
       let canonical = std::fs::canonicalize(input).unwrap();
       println!("canonical:    {canonical:?}");
       let url = url::Url::from_directory_path(&canonical).unwrap();
       println!("url:          {url}");
       let pathbuf = url.to_file_path().unwrap();
       println!("to_file_path: {pathbuf:?}");
       let store_path = object_store::path::Path::from_absolute_path(&pathbuf).unwrap();
       println!("object_store: {store_path}");
   }
   ```
   
   Output (Windows 11, `object_store 0.13.1`, `url 2.5.8`):
   
   ```
   canonical:    "\\\\?\\UNC\\<HOST>\\TestShare\\basic_partitioned"
   url:          file://<host>/TestShare/basic_partitioned/
   to_file_path: "\\\\<host>\\TestShare\\basic_partitioned\\"
   object_store: TestShare/basic_partitioned        <-- host dropped
   ```
   
   Lines 2 and 3 confirm `url` preserves the host across the round-trip. Line 4 is where it disappears.
   
   
   ### **Additional context**
   
   Surfaces as user-visible failures in:
   
   - delta-io/delta-kernel-rs#1482
   - delta-io/delta-rs#3551
   
   A `delta-kernel-rs` patch attempting to handle this above the `object_store` boundary did not help, since the host is stripped here regardless of how the caller routes the URL conversion. Documenting this either way (whether as a bug to fix or as expected behaviour with `new_with_prefix` as the prescribed workaround) would unblock those downstream fixes.
   
   ### **Potential workaround**
   
   `LocalFileSystem::new_with_prefix(r"\\server\share")` plus share-relative `Path`s appears to work, but requires the caller to know the UNC root in advance — which is awkward for libraries that accept user-supplied URIs.
   


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