hbal: return exit status 0 in case of early exit

This derives from an internal bug, but the story is consistent across
both internal and external usage of hbal.

Basically right now, hbal returns exit code 1 if requested to exit
early, even if all jobs are successful. This is counter-intuitive due
to two reasons:

- hbal did what it was requested (exit early), so it shouldn't return error
- there were no job failures, so there's nothing to "cleanup" or
  investigate on the Ganeti cluster, so again it shouldn't return
  error

Therefore the new behaviour is as follows:

- for cases where all jobs were successful, even if terminated early
  via SIGINT or via --limit, we exit with code 0
- for cases where jobs have failed or there were other errors in
  running hbal, the exit code is 1
- for cases were hbal is requested an immediate termination (SIGTERM),
  exit code is 2, denoting "unknown whether the Ganeti cluster is
  consistent or not"

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

htools/Ganeti/HTools/Program/Hbal.hs

@@ -180,7 +180,7 @@ execWrapper master nl il cref alljss = do
     then do
       hPrintf stderr "Exiting early due to user request, %d\
                      \ jobset(s) remaining." (length alljss)::IO ()
-      return False
+      return True
     else execJobSet master nl il cref alljss
 
 -- | Execute an entire jobset.

man/hbal.rst

@@ -415,9 +415,15 @@ EXIT STATUS
 -----------
 
 The exit status of the command will be zero, unless for some reason the
-algorithm fatally failed (e.g. wrong node or instance data), or (in case
-of job execution) either one of the jobs has failed or the balancing was
-interrupted early.
+algorithm failed (e.g. wrong node or instance data), invalid command
+line options, or (in case of job execution) one of the jobs has failed.
+
+Once job execution via Luxi has started (``-X``), if the balancing was
+interrupted early (via *SIGINT*, or via ``--max-length``) but all jobs
+executed successfully, then the exit status is zero; a non-zero exit
+code means that the cluster state should be investigated, since a job
+failed or we couldn't compute its status and this can also point to a
+problem on the Ganeti side.
 
 BUGS
 ----