Layers which wrap backends, such as the hasher backend and the VFS
cache, use fs.Fingerprint to detect whether an object has changed.
This only works if the object a backend hands back from a write
operation is identical to the object a fresh listing returns. If a
backend keeps the source's nanosecond precision modtime in memory
while the server stores milliseconds, or omits a hash the server
knows, every fingerprint comparison fails and cached hashes or files
are silently invalidated.
The existing tests compared modtimes within f.Precision() so they
could not detect these sub-precision divergences.
Add a checkFingerprint helper which asserts the fast and slow
fs.Fingerprint of the object returned from a write operation exactly
match those of the object read back from the remote, and wire it into
all the write paths which return or mutate an object: Put (all
variants), PutStream, large and streamed uploads, Update, SetModTime,
server side Copy and Move, and the metadata mutating Copy/Move
subtests.
Rework the ObjectOpenFingerprint test to use the same helper against
a fully read reference object, with subtests reporting which kind of
read is being checked.
Nick Craig-Wood
7dd42c93cf