Fix uptime on macOS using sysctl kern.boottime fallback #8908
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If utmpx BOOT_TIME is unavailable, derive boot time via sysctl CTL_KERN.KERN_BOOTTIME to reduce intermittent macOS failures (e.g., uutils#3621). Context (blame/history): - 2774274 ("uptime: Support files in uptime (uutils#6400)"): added macOS utmpxname validation and non-fatal 'unknown uptime' fallback with tests (tests/by-util/test_uptime.rs). - 920d29f ("uptime: add support for OpenBSD using utmp"): reorganized uptime.rs and solidified utmp/utmpx-driven paths.
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Add unit tests for sysctl boottime availability and get_uptime reliability on macOS, verifying the fallback mechanism works correctly when utmpx BOOT_TIME is unavailable. Add integration tests to ensure uptime command consistently succeeds on macOS with various flags (default, --since) and produces properly formatted output. Enhance documentation of the sysctl fallback code with detailed comments explaining why it exists, the issue it addresses (uutils#3621), and comprehensive SAFETY comments for the unsafe sysctl call. All tests are properly gated with #[cfg(target_os = "macos")] to ensure they only run on macOS and don't interfere with other platforms.
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On macOS, the uptime command would sometimes fail in CI when utmpx couldn't provide a BOOT_TIME entry. This was causing flaky test failures, especially for uptime --since.
Fixes #3621
The fix adds a fallback using sysctl to query kern.boottime directly from the kernel. If we can't get boot time from utmpx, we now try sysctl before giving up. This makes uptime much more reliable on macOS.
blame:
2774274 added file support to uptime in, they added macOS-specific validation for utmpx paths but only implemented a graceful degradation fallback that printed "couldn't get boot time". Later when 047ec99 refactored uptime into uucore in, this utmpx-only approach was preserved. 6818bbe also had to relax error messages in due to macOS errno differences, showing ongoing macOS-specific issues.
None of these commits implemented the sysctl kern.boottime fallback that macOS has always provided as the canonical way to get boot time. This PR finally adds that missing piece that should have been there from the start.