`full_object_plaintext_len` decides whether a body-cache hook hit may serve
bytes in place of the erasure read. It is a fail-closed allow-list: it excludes
every read whose `ReadPlan::build` applies some other transform (ranged/part,
raw/data-movement, restore, encrypted, remote) with an early `return None`,
then returns a `Some(..)` length only for the whole-plaintext cases. A newly
added `ReadPlan` branch that nobody teaches this gate about falls through to
`None` and safely bypasses the cache. Flip it to a deny-list and the same new
branch silently serves bytes in the wrong representation — the backlog#1108 /
#1109 / #1146 class of bug.
The existing unit and e2e tests only cover the branches that exist today. This
adds `scripts/check_body_cache_whitelist.sh`, a structural guard wired into
pre-commit / pre-pr / dev-check and CI, that asserts every exclusion predicate
and a `return None` still precede the first `Some(..)`. Reordering a predicate,
dropping one, moving the positive return ahead of the gate, or renaming/removing
the function all fail; wording, formatting, and adding a new exclusion in the
same gate do not. Mutation-tested against all four regression shapes.
This machine-enforces the structural invariant that backlog#1146 was kept open
to guard by hand.
Co-authored-by: heihutu <heihutu@gmail.com>
houseme
e742a540a4