`verify-kernel-append` asks the question a log exists to answer: four writes are acknowledged, the
VM is killed with `SIGKILL` with no shutdown at all, and the log must still hold them. It is red on
`#95` and on `#87` — the kernel this box runs — and green on the kernel built at 18:41, and the one
commit between them is e0218ec96: the collapse to a single `fsync` per append, where the entry goes
down unsynced and the control write that counts it carries the flush for both.
That collapse is right where its sentence is true, and the sentence assumes there is a control block
to write. *A store with no control block has no count write*, so nothing on the append path was
flushed at all — and the caller was told a write it may never get back. That is not an exotic shape:
it is what a store the CLI builds looks like, and it is the shape that gate's own device has.
Measured, same device, same four writes, the host's copy of the device read while the guest is still
hung:
no control block the log region holds its header and zeros where the entry should be, and
the fold returns the base store: records=2
a formatted device the entry is on the device, and the fold reproduces the userspace store
exactly: records=6, fnv1a64=6b679d39597a62b3
and a pre-collapse kernel keeps the entry in *both* shapes, because the entry's own `fsync` is what
made it durable there.
So the fix is not the revert. `append` takes the entry's own `fsync` exactly when no later write in
the same operation will carry one — `layout.control.is_some()`, one boolean, both branches gated by
the two shapes above. The box's store is a formatted file with a control block, so it keeps the
measured 4.96 ms win; a bare store pays its entry's flush, which is what it always did before the
collapse. The count still goes second, so a crash can still lose an unacknowledged mutation rather
than count one that is not there.
The gate was wrong in the same way the first diagnosis was, and is fixed with the kernel: it ran its
crash check over one device shape, which is the shape that made the collapse look safe everywhere. It
now runs it over both — a store with no control block and a formatted device — folds each the way
userspace reads it, and compares both against the same userspace store, so "it survived" means the
same thing for each. It also stops SIGKILLing the `timeout` wrapper rather than QEMU: SIGKILL cannot
be forwarded, so every run of this gate left a 512 MiB VM spinning in `pause()` for the rest of the
wall, twice found beside a gate that measures latency.
Built as #96. The wall, whole: 26 of 26.
surface-camera-build
8c24a7ff0d