Comment 10 for bug 1809174

"reporting bugs to the effect that a no-change update should finish instantly."

By definition, if the on-disk copy is corrupt, there is an change-update available.

The algorithm for update should look like this:
- does the local copy exist? No -> update available
- is the local copy valid (checksums match)? No -> update available
- does the remote repo report a change? Yes -> update available

"That said, files of size 0 could be made always invalid"

My case happened to be corruption of the form of an empty file. Checking the checksum will detect all forms of corruption.