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fix: rename transfers via a nofollow parent dirfd - #17
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Closes M1: the final partial-to-destination rename used
fs::rename, which follows a symlinked parent directory. Between thevalidate_destination_pathlstat and the rename, a concurrent local process could swap a parent directory for a symlink, causing the file to be written outside the destination root.safe_renamenow opens the destination parent withopenat(O_DIRECTORY|O_NOFOLLOW|O_CLOEXEC)(refusing symlinks) and renames into the pinned dirfd viarenameat, so the destination directory cannot be swapped between validation and rename.Verified the mechanism (a rename through a symlinked parent escapes the root) and that
O_NOFOLLOWrefuses the symlink; the normal transfer path still works.