[jackylee-ch]

vortex.io.ReadAt's docs say pyarrow files work with vx.open_readable, but none do. A pyarrow file has size() and read_at, so it is read as a ReadBytesAt, but pyarrow's read_at takes (nbytes, offset). For the new test's file, a read of 65535 bytes at offset 420089 read 420089 bytes at offset 65535, and vx.open_readable(pa.OSFile(path)) failed with RuntimeError: Python read of 420089..485624 failed: BufferError: read_at(420089, 65535) returned 420089 bytes.

open_readable now wraps a pyarrow file in a ReadBytesAt that passes the arguments in pyarrow's order. It is the only caller of the native open_readable, so readable.rs needs no pyarrow-specific mode. Reads through the wrapper are positional: an OS file or in-memory buffer serves several reads at once, with the GIL released while pyarrow reads, while a pa.PythonFile, which pyarrow returns for fsspec filesystems, still reads one at a time. Any pyarrow filesystem's open_input_file returns such a file, so this also covers HadoopFileSystem, for which Vortex has no store; it was not tested against HDFS. pyarrow-stubs types read_at without parameters, so the call carries a ty: ignore.

Tests

The new test reads a file through pa.OSFile, pa.memory_map and LocalFileSystem().open_input_file; it fails on develop. The vortex-python suite, ruff, ty and the docs build and doctests pass.

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