uucore: add DirFd::open_anchor for write-only directories

[abendrothj]

Creating entries relative to a directory descriptor fails with EACCES when the directory has write and search permission but no read, because DirFd::open opens it for reading. install -D hits this for leading directories, and ln/cp/mv when replacing entries.

DirFd::open_anchor opens the directory readable and on EACCES retries with O_PATH (Linux, Android) or O_SEARCH (Apple targets, FreeBSD, NetBSD, illumos, Solaris), which is enough for the *at calls. The targets get has_o_path/has_o_search cfg aliases in uucore's build script. If the retry fails, its own error is returned.

Used by #14779 and #15122.

Part of #14778

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)

[codspeed]

Merging this PR will degrade performance by 4.8%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 3 improved benchmarks
❌ 15 regressed benchmarks
✅ 381 untouched benchmarks
⏩ 54 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	five_38_bit_primes	1.5 s	1.8 s	-17.84%
❌	Simulation	true_consecutive_calls	294.7 ns	350.2 ns	-15.86%
❌	Simulation	false_consecutive_calls	294.7 ns	350.2 ns	-15.86%
❌	Simulation	check_sorted_utf8_locale	422.9 ms	492.2 ms	-14.09%
❌	Simulation	sort_numeric_utf8_locale	36.1 ms	40.7 ms	-11.41%
❌	Simulation	merge_single_file_utf8_locale	119.3 ms	133.4 ms	-10.57%
❌	Simulation	tsort_complex_dag[50000]	87.6 ms	97.2 ms	-9.79%
❌	Simulation	tsort_tree_dag[(10, 3)]	36.5 ms	39.4 ms	-7.34%
❌	Simulation	tsort_wide_dag[100000]	157.6 ms	167 ms	-5.64%
❌	Simulation	merge_pre_sorted_files	238.7 ms	252.8 ms	-5.59%
❌	Simulation	merge_pre_sorted_files_utf8_locale	237.9 ms	251.9 ms	-5.54%
❌	Simulation	tsort_linear_chain[1000000]	1.9 s	2 s	-4.75%
❌	Simulation	sort_german_de_locale	282.1 ms	295.1 ms	-4.41%
❌	Simulation	sort_case_sensitive[500000]	321 ms	334.9 ms	-4.15%
❌	Simulation	sort_spill_to_tmp_files_utf8_locale	321 ms	332.1 ms	-3.34%
⚡	Simulation	three_39_bit_primes	901.6 ms	578.3 ms	+55.9%
⚡	Simulation	sort_dictionary_order[500000]	2.1 s	1.9 s	+7.02%
⚡	Simulation	rm_recursive_tree	14.4 ms	13.7 ms	+5.28%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing abendrothj:uucore-dirfd-open-anchor (028b336) with main (fe26d56)}

Footnotes

1. 54 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. ↩

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[abendrothj]

Landed on main with #14779, so closing.