fn sysroot_is_read_only(d: &Dir) -> Result<bool>Expand description
Determine whether /sysroot (given as d) is on a physically read-only
block device, remounting it read-write as a side effect when necessary.
ostree mounts /sysroot read-only by default even on writable systems and
remounts it writable on demand, so the read-only mount flag alone is not
meaningful. The authoritative signal is the backing block device’s read-only
flag, which is set for a live ISO (a read-only loop device over an immutable
rootfs image) and propagates from a whole disk to its partitions. We check
that first and avoid even attempting a remount that is bound to fail.