stty: do not write settings back for query-only arguments

[m0g3r]

Fixes #13722.

stty size only reads the terminal, but every invocation carrying settings arguments finished with an unconditional tcsetattr. From a background process group POSIX requires the kernel to raise SIGTTOU, whose default disposition stops the process, so stty size off the foreground hangs with no diagnostic and never reaps.

tcsetattr is now called only when at least one parsed argument actually changes the terminal settings. ArgOptions::Print does not, so stty size takes the same path it already took for printing and then stops.

Test

A unit test covers the predicate directly, since reproducing SIGTTOU needs a controlling terminal and a background process group, which the test harness does not provide.

Checked by hand against a pty that size still reports dimensions, that rows/columns still apply, and that -a is unaffected:

stty -F pty size          -> 0 0
stty -F pty rows 40 columns 100
stty -F pty size          -> 40 100
stty -F pty -a            -> speed 38400 baud; rows 40; columns 100; line = 0; ...

cargo test -p uu_stty 27 passed, the stty integration suite 53 passed / 33 ignored, cargo clippy -p uu_stty --all-targets -- -D warnings clean.

Note this touches the same line as #13893. If that lands first the verification it adds should move inside this guard, since there is nothing to verify when the settings were never written.

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Written with AI assistance. I have reviewed the diff and verified the behaviour against a pty myself.

[github-actions]

Binary size comparison:

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

Total size of compared binaries: 149.02 MB (+92 KB, +0.06%)

Significant per-binary changes:
  [           1.10 MB ->    1.18 MB  (+76 KB, +6.74%)
  mkdir       1.10 MB ->    1.17 MB  (+76 KB, +6.76%)
  mknod       1.09 MB ->    1.17 MB  (+76 KB, +6.79%)
  test        1.10 MB ->    1.18 MB  (+76 KB, +6.74%)
  install     1.25 MB ->    1.32 MB  (+72 KB, +5.62%)
  mkfifo      1.09 MB ->    1.16 MB  (+72 KB, +6.45%)
  chmod       1.15 MB ->    1.22 MB  (+68 KB, +5.76%)

[sylvestre]

Please also add a test in test_stty.rs

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/rm/isatty (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will degrade performance by 3.02%

⚠️ 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

❌ 1 regressed benchmark
✅ 352 untouched benchmarks
⏩ 50 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	du_wide_tree[(5000, 500)]	19.7 ms	20.3 ms	-3.02%

Tip

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

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_{Comparing m0g3r:stty-no-tcsetattr-for-queries (93d7b2d) with main (5633f03)}

Footnotes

1. 50 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. ↩