geotiff: route read_vrt(chunks=) XML read through size cap (#1831)

[brendancol]

Summary

PR #1818 capped VRT XML reads at 64 MiB (override via XRSPATIAL_VRT_MAX_XML_BYTES) in _vrt._read_vrt_xml so an attacker-supplied multi-GB VRT file cannot exhaust host memory at parse time. The lazy chunked dispatcher landed in #1822 (_read_vrt_chunked) parses the VRT independently with a bare open(source, 'r').read(), bypassing that cap; only the per-task worker reuses the capped reader.

This routes the chunked-path XML read through _read_vrt_xml so the same cap and env-var override apply on both eager and chunked entry points. Closes #1831.

Repro (pre-fix)

import os, tempfile, numpy as np
from xrspatial.geotiff import to_geotiff, read_vrt

td = tempfile.mkdtemp()
to_geotiff(np.zeros((10, 10), np.uint8),
           os.path.join(td, 'src.tif'), compression='none')

vrt = os.path.join(td, 'big.vrt')
pad = '<!-- ' + ('x' * (2 * 1024 * 1024)) + ' -->'
open(vrt, 'w').write(
    '<VRTDataset rasterXSize="10" rasterYSize="10">' + pad +
    '<VRTRasterBand dataType="Byte" band="1"><SimpleSource>'
    '<SourceFilename relativeToVRT="1">src.tif</SourceFilename>'
    '<SourceBand>1</SourceBand>'
    '<SrcRect xOff="0" yOff="0" xSize="10" ySize="10"/>'
    '<DstRect xOff="0" yOff="0" xSize="10" ySize="10"/>'
    '</SimpleSource></VRTRasterBand></VRTDataset>')

os.environ['XRSPATIAL_VRT_MAX_XML_BYTES'] = '1024'
read_vrt(vrt)              # eager: ValueError (cap enforced)
read_vrt(vrt, chunks=10)   # chunked: succeeds, cap bypassed

After this PR both raise the same ValueError referencing XRSPATIAL_VRT_MAX_XML_BYTES.

Test plan

• xrspatial/geotiff/tests/test_vrt_xml_size_cap_chunked_1831.py - new file, 3 tests pinning small-cap rejection, default-cap success, raised-cap success for chunks=.
• Existing test_vrt_xml_size_cap_1815.py (eager path) still passes.
• Existing test_vrt_lazy_chunks_1814.py (chunked happy path) still passes.

Found by automated security sweep (deep-sweep-security-geotiff-2026-05-13-s4), Cat 1 (unbounded allocation), severity MEDIUM.

[Copilot]

Pull request overview

This PR closes a security regression where read_vrt(..., chunks=...) bypassed the VRT XML size cap introduced in #1818 by performing an unbounded open().read(). It routes the chunked dispatcher’s upfront XML load through _vrt._read_vrt_xml so both eager and chunked entry points consistently enforce XRSPATIAL_VRT_MAX_XML_BYTES.

Changes:

• Update _read_vrt_chunked to read XML via _read_vrt_xml instead of open().read(), applying the same 64 MiB default cap and env-var override.
• Add regression tests covering cap rejection, default-cap success, and raised-cap success for the chunked (chunks=) path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
xrspatial/geotiff/__init__.py	Routes chunked-path VRT XML reading through _read_vrt_xml to enforce the size cap consistently.
xrspatial/geotiff/tests/test_vrt_xml_size_cap_chunked_1831.py	Adds regression tests ensuring chunks= honors XRSPATIAL_VRT_MAX_XML_BYTES.

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