geotiff: parallelise tile decode in _fetch_decode_cog_http_tiles (#1980)#1981
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The HTTP COG read path fetches tiles concurrently via ``read_ranges_coalesced`` (xarray-contrib#1480, xarray-contrib#1487) and then decoded them sequentially in a Python for-loop. Local ``_read_tiles`` already parallelises decode via ``ThreadPoolExecutor`` whenever ``tile_pixels >= 64 * 1024`` and ``n_tiles > 1`` (see _reader.py:2017); the HTTP path was structurally similar but never adopted the same gate, so wide windowed COG reads with many tiles left deflate/zstd decoding single-threaded after a parallel fetch. Mirror the local-file path's threshold and pool here. Codec extensions release the GIL inside their C implementations, so a ``ThreadPoolExecutor`` overlaps decode work across cores. The placement loop that writes pixels into ``result`` stays serial to avoid contended writes. Pass 10 of the geotiff performance sweep. Tests in test_cog_http_parallel_decode_2026_05_15.py: - parallel-branch end-to-end correctness (deflate, tile_size=256) - serial-branch end-to-end correctness (tile_size=128, single tile) - ThreadPoolExecutor instantiation above the threshold - single-tile path skips the decode pool - _decode_strip_or_tile invoked exactly once per placement
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Summary
forloop. The sibling local-file path in the same module (_read_tiles) already parallelises decode whenevertile_pixels >= 64 * 1024andn_tiles > 1; the HTTP path was structurally similar but never adopted the same gate.ThreadPoolExecutorin_fetch_decode_cog_http_tiles. Codec extensions (deflate, zstd, LZW) release the GIL inside their C implementations, so the pool overlaps decode work across cores. The placement loop that writes pixels intoresultstays serial to avoid contended writes.test_cog_http_parallel_decode_2026_05_15.py: parallel + serial round-trip correctness, structural pool-instantiation check above the threshold, single-tile path skips the decode pool,_decode_strip_or_tilecall count ==n_tiles.Pass 10 of /sweep-performance on the geotiff module. Closes #1980.
Test plan
python -m pytest xrspatial/geotiff/tests/test_cog_http_parallel_decode_2026_05_15.py.python -m pytest xrspatial/geotiff/tests/ -k 'cog or http'-> 262 passed.test_predictor2_big_endian_gpu_1517,test_size_param_validation_gpu_vrt_1776::test_tile_size_positive_works) flagged in prior sweep notes.