making a file sparse - in-place?

[Bernhard Voelker]

Inspired by a recent post on util-linux ML [1], talking about turning
a file into a sparse file in-place, i.e. not using a 2-step approach
like `cp --sparse file file2 && mv file2 file`), I thought, hey, don't
we have this in coreutils already?

a)
Therefore, I tried
  $ dd if=file of=file conv=sparse

... well, the data in 'file' was lost because the file got truncated
by the open(...,O_TRUNC) call - even without a warning.
POSIX [2] does not say anything about what dd(1) should do if the
input and output file are the same.
Is there a reason why dd should not deny to take "if=file of=file"
without "conv=notrunc", or why we shouldn't warn at least?
(Okay, the latter is not much of use because it's too late then
anyway with a simple warning.) So I'd favor outputting an error
diagnostic and exit(EXIT_FAILURE) in this case.

b)
Then, I tried
  $ dd if=file of=file conv=sparse,notrunc
to avoid truncating the output file. That didn't corrupt the data,
but the file still was not sparse afterward.
What's the reason for conv=sparse not to work in this situation?
BTW: generally, writing to the same file seems to work, e.g.:
  dd if=file of=file conv=ucase,notrunc

c)
As a last method, I tried
  $ cp --force --backup --sparse=always file file
Most interestingly, the *backup* file was sparse afterward:
  $ du file*
  2688    file
  256     file~
That's surprising, and I'd almost consider this a little bug.
But from the implementation of cp(1), that behavior is quite clear.

Apart from my tries above: do we already have a possibility in coreutils
to turn a file to a sparse file in-place?

[1] http://article.gmane.org/gmane.linux.utilities.util-linux-ng/8415
[2] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/dd.html

Have a nice day,
Berny