_install_tirith() uses shutil.move() to place the binary from tmpdir
to ~/.hermes/bin/. When these are on different filesystems (common in
Docker, NFS), shutil.move() falls back to copy2 + unlink, but copy2's
metadata step can raise PermissionError. This exception propagated
past the fail_open guard, crashing the terminal tool entirely.
Additionally, a failed install could leave a non-executable tirith
binary at the destination, causing a retry loop on every subsequent
terminal command.
Fix:
- Catch OSError from shutil.move() and fall back to shutil.copy()
(skips metadata/xattr copying that causes PermissionError)
- If even copy fails, clean up the partial dest file to prevent
the non-executable retry loop
- Return (None, 'cross_device_copy_failed') so the failure routes
through the existing install-failure caching and fail_open logic
Closes#10127
18396af31e