I'm not sure what you mean by flush, fprint is synchronous.
I could retry the write if you like, though there's not
much likelyhood that it would work the second time if
it didn't the first, there's no lock on .longnames,
it's append only, so a failure is probably the fs or
someone having removed the file.

.longnames is also written before the file is created,
so the name should be there should you find the hashed
file when reading the dir.

However, in rreaddir, I don't reread the file while
translating the short names to long ones.  Someone
could create a file while I'm busy converting the
dir.  There's plenty of window there for someone to
sneak through.