isWritableDirectory (permissions bits) is not enough: a OneDrive
Files-On-Demand directory whose provider domain is signed out can report as
existing and POSIX-writable while an actual write fails. probeWritable
writes a small ".redline-probe-<uuid>" file into the folder via
AtomicFile.write (open+write+fsync+rename+directory-fsync), then removes it;
any failure at write, fsync, or removal means false.
Three unit tests: an ordinary writable directory (true, and no probe file
left behind), a chmod 500 directory (false; permissions restored in
teardown), and a plain file path (false).
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ZiTXPbPCSjzPVsfoweAbp
kua-agentandClaude Fable 5.1
723cd7dcea