cp: open the source once per copy

[sylvestre]

Every regular-file copy on Linux opened the source three times: once in check_for_data, once in check_sparse_detection, then again in the copy itself. Open it once and thread the descriptor through the probe and the copy, so the strategy and the bytes come from the same fd.

before: open("src", O_RDONLY) x3 + open("dst", O_WRONLY|O_CREAT) x1
after: open("src", O_RDONLY) x1 + open("dst", O_WRONLY|O_CREAT) x1

Copying 3000 small files, 50 runs: 199.9ms +/- 14.9 -> 171.7ms +/- 23.6. The ratio's error bars overlap; the stable signal is system time, 177.3ms -> 146.9ms, with user time unchanged at ~18ms.

[codspeed-hq]

Merging this PR will improve performance by 32.93%

⚡ 5 improved benchmarks
✅ 6 untouched benchmarks
⏩ 400 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	cp_recursive_wide_tree[(6000, 800)]	333.8 ms	236 ms	+41.41%
⚡	Simulation	cp_recursive_balanced_tree[(5, 4, 10)]	130.1 ms	96.7 ms	+34.51%
⚡	Simulation	cp_preserve_metadata[(5, 4, 10)]	135.8 ms	102.7 ms	+32.24%
⚡	Simulation	cp_archive_balanced_tree[(5, 4, 10)]	143.2 ms	109.8 ms	+30.44%
⚡	Simulation	cp_recursive_deep_tree[(120, 4)]	22.9 ms	18.1 ms	+26.52%

Tip

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_{Comparing sylvestre:ls (b3f95a6) with main (bd240c0)}

Footnotes

1. 400 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 152.18 MB (+996 KB, +0.64%)

Significant per-binary changes:
  comm     1.12 MB ->    2.33 MB  (+1.21 MB, +107.29%)

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.

[sylvestre]

moved here #14272