A decode ends by deleting the shards it read, and the only thing standing
between that and a bad reconstruction is verifyDecodedVolumeBeforeDelete,
which asks whether .dat and .idx are non-empty. A .dat truncated to a
single byte passes, and the shards -- the only other copy of everything
past the cut -- are deleted on the strength of it.
The server already knows the answer it never checks: FindDatFileSize
returns the extent the EC index references, and WriteDatFile rebuilds to
it. Compare the two once the file is written and fail the decode instead
of reporting a short volume as a good one.
Longer than the extent still verifies -- padding is not missing data --
so only a genuinely short rebuild is rejected.
Needle counts cannot answer this: .idx is written from .ecx, so the count
matches by construction and a truncated .dat still reports every needle.
Chris LuandGitHub
fbd85d31b0