>> Pure, yes.  Simple, no.  Who gets to decide what counts as corrupt?
> Yes, actually it is very simple in this case.  If the st_mtime value
> for the file changes from the time it was first opened to the time
> its "last" buffer was written to the backup then the backup is
> obviously corrupt.

That's one possible meaning for "corrupt" here.

Despite what you appear to be convinced of, it is not the only one; it
is not even the only useful one.

Since you are unlikely to be convinced of that, and I haven't seen
anyone else supporting that position, I see nothing to be gained from
elaborating.  I'll be happy to do so off-list with anyone interested.

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