implement xargs -P using rayon and a spin lock

[codspeed-hq]

Add CodSpeed benchmarks for the parallel xargs -P path

This PR introduces parallel execution for xargs -P (via rayon), but the existing benchmark suite (benches/xargs_bench.rs) only exercised the serial path — so CodSpeed reported the new code as untouched.

This change extends benches/xargs_bench.rs with three benchmarks that drive the new rayon-based dispatch in process_input:

• parallel_p2 — -n 10 -P 2: a low degree of parallelism. This is the spin-lock worst case noted earlier in the review (high contention between two workers).
• parallel_p8 — -n 10 -P 8: higher parallelism, exercising the scheduler under more contention.
• parallel_p8_batched_n — -n 50 -P 8: fewer, larger batches so the balance shifts back toward argument assembly while still going through the parallel bridge.

Implementation notes

• The parallel benchmarks use a smaller dedicated corpus (400 tokens) than the serial ones (4000). Each batch spawns a real true child, so a smaller corpus keeps the number of spawns reasonable under the CodSpeed simulation instrument while still producing enough batches to feed the parallel bridge.
• The serial benchmarks (split_whitespace, split_null, batched_n, batched_size) are unchanged.
• Measurements are dominated by xargs's own work (reading, splitting, batching, and parallel dispatch); child-process spawns are not traced under simulation.

All benchmarks build and run successfully under codspeed run --mode simulation.

[codecov]

Codecov Report

❌ Patch coverage is 93.38843% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.15%. Comparing base (2a3eac9) to head (3ec89f4).

Files with missing lines	Patch %	Lines
src/xargs/mod.rs	93.38%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #794   +/-   ##
=======================================
  Coverage   92.15%   92.15%           
=======================================
  Files          35       35           
  Lines        7377     7457   +80     
  Branches      383      384    +1     
=======================================
+ Hits         6798     6872   +74     
- Misses        438      442    +4     
- Partials      141      143    +2     

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[github-actions]

Commit 017bea8 has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 413 / FAILED: 81 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 417 / FAILED: 77 / SKIPPED: 1

Changes from main branch:
  TOTAL: +0
  PASSED: -4
  FAILED: +4

New test failures (4):
  - idef-s26-0
  - s14
  - s14-0
  - s42

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6
  Reference: TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6

New test failures (1):
  - gnu/files0_from_ok

Test improvements (1):
  + gnu/okdir_path_empty

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks
🆕 3 new benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	parallel_p2	N/A	564.3 ms	N/A
🆕	parallel_p8	N/A	22.4 ms	N/A
🆕	parallel_p8_batched_n	N/A	6 ms	N/A

---

_{Comparing push-zlxwkxlkpsls (3ec89f4) with main (2a3eac9)}

[sylvestre]

could you please fix the conflicts? thanks

[codspeed-hq]

could you please fix the conflicts? thanks

Done — main is merged into push-zlxwkxlkpsls and the conflicts are resolved.

Conflicts and how they were resolved

• src/xargs/mod.rs
  • Import block: kept the rayon::iter imports from this branch and main's uucore::error::strip_errno.
  • --arg-file opening: kept the Box<dyn Read + Send> type (the Send bound is required by the parallel path) while adopting main's strip_errno-based error message.
• tests/test_xargs.rs: kept both sides — the new xargs_parallel_one / xargs_parallel_many tests from this branch, plus main's xargs_trailing_blanks / xargs_quoted_empty_argument.

main's rework of the whitespace-delimited argument reader (the new in_argument tracking) merged cleanly and is preserved unchanged.

Verification

• cargo build --all-targets: OK
• cargo test --test test_xargs: 34 passed, 0 failed
• cargo clippy --all-targets: no new warnings (only a pre-existing one in src/locate/mod.rs coming from main)
• codspeed run --mode simulation -- cargo codspeed run --bench xargs_bench: all 7 benchmarks (4 serial + 3 parallel) run successfully

One note from that local run: parallel_p2 is by far the heaviest benchmark (~207 ms locally vs ~3.5 ms for parallel_p8), which matches the expectation that two spinning workers are the worst case for the spin lock. The upcoming CI report will give the reference numbers on the CodSpeed runners.

Summary

• Warming up the performance wizard...
• Handling comment — Comment handling finished successfully
• Resolving merge conflicts with main — Conflicts in src/xargs/mod.rs and tests/test_xargs.rs resolved; build and xargs tests pass
• Validating benchmarks — All 7 xargs benchmarks ran successfully under the simulation instrument
• Pushing the merge — Merge commit pushed to push-zlxwkxlkpsls

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[github-actions]

Commit 3ec89f4 has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 417 / FAILED: 77 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 421 / FAILED: 73 / SKIPPED: 1

Changes from main branch:
  TOTAL: +0
  PASSED: -4
  FAILED: +4

New test failures (4):
  - idef-s26-0
  - s14
  - s14-0
  - s42

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 316 / PASSED: 273 / FAILED: 37 / SKIPPED: 6
  Reference: TOTAL: 317 / PASSED: 274 / FAILED: 37 / SKIPPED: 6

Changes from main branch:
  TOTAL: -1
  PASSED: -1
  FAILED: +0

No result in this run (1) - hung, crashed, or renamed:
  ? posix/o (was PASS)