This can happen if we create the .tmp file in the same directory as the
target, and the .do file first does "rm -rf" on that directory, then
re-creates it. The result is that the stdout file is lost.
We'll make this a warning if the .do script *didn't* write to stdout
(so the loss is harmless, just weird), and an error if they *did* write
to stdout, which we can detect because we still have an open fd on the
file, so we can fstat() it.
