pkhuong / kismet-cache / 6207687088

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stack: check close(2) before publishing NamedTempFile to cache

Close *can* fail for real I/O reasons, and while it would be ideal
to always fsync:

1. that's an (opt-out) option
2. it can still be nice to detect post-fsync failures
3. with NFS close-to-open consistency, closing is the most reliable
   way to make the file's contents globally visible.

In general, Kismet works on paths, not files, so there's not much
we can do about closing (or not) the underlying File object before
publishing it.

For `NamedTempFile`s though, we get both a file and a path!  We can
extract the file, make sure it's ready for publication (world readable,
fsync-ed if necessary, and then closed) before passing the `TempPath`
to Kismet's usual path-based machinery.

This `TempPath` is what keeps the temporary file path alive, so everything
will still be cleaned up on error, even though the file descriptor
itself is long gone.

Of course, Rust doesn't yet have an `Error`-ful `File::close`, so we
roll our own with libc...

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