Without privilege, write() clears S_ISUID/S_ISGID, and the save applied
the original mode before writing the data, so the bits were lost; a
root-run test environment hid it. Apply ownership and mode after the
data is written (still before fsync and rename), and restore the mode
after an in-place write (files with hard links).
The test now expects the mode read back after chmod: BSD silently drops
setgid when the file's group, inherited from its directory, is not one
of the caller's groups. A hard-link variant covers the in-place path.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
kyleandClaude Opus 5.5
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