bug#11816: sort -o: error comes late if opening the outfile fails

[Bernhard Voelker]

If opening the output file for writing is not possible - e.g. because the user
doesn't have sufficient privileges, then sort issues an error. The problem
is that the whole - then useless - computation is already done.

In the following little example, it took ~15s to sort the data,
and then to realize that it can't write the result:

  $ time seq 1000000 | src/sort --random-sort -o /cantwritehere
  src/sort: open failed: /cantwritehere: Permission denied

  real    0m14.955s
  user    0m14.936s
  sys     0m0.042s

I'd have expected sort to give the error immediately after startup
instead of wasting time for sorting.
Shouldn't sort open the outfile right at the beginning (unless in==out),
or is this behavior required by some standard?

Have a nice day,
Berny