The v4 migration made the kernel write v2 log entries (flags field) but
append only wrote a log header when the region had no magic. A store
formatted by userspace already carries a v1 header, so the kernel appended
v2 entries under it and every reader framed them as v1: the length landed
on the flags field, the walk stopped at the first entry, and get/fold/boot
saw an empty log.
Now append insists the header matches what it writes: a v1 log that still
holds entries is folded into the image first (the actual v4 migration),
then the entry lands in a fresh v2 log; an empty v1 log is upgraded in
place. The fold is factored into fold_now, shared by append and sync.
surface-camera-build
0267831b18