The /tmp-fallback case did check-then-create on a fixed /tmp name, a
TOCTOU/symlink race, and the EXIT trap only cleaned $TMPDIR. Create the decoy
atomically with noclobber (O_EXCL) so it refuses to overwrite an existing file
or follow a symlink at that path, and remove it on exit only when this test
created it. The fixed path is required (it is exactly the path the old code
would form), so a random mktemp name cannot replace it. Addresses the Copilot
review on #8198; the wrapper fix is unchanged.
The cached device path was trusted for merely existing, not for being a hiddev node, and fell back to a predictable /tmp path when XDG_RUNTIME_DIR was unset. Validate the cache shape (hiddev char device) and cache only under the user-private runtime dir; asdcontrol already gates non-Apple devices downstream, so this is defense-in-depth in the layer Omarchy owns.
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