Save and close stdout on exit

Redirecting our output through outfilter.py is inherently a bit racy,
since the disk-image-create process will exit, and then you might get
outfilter.py flushing any remaining output as it closes.

On an interactive prompt this might lead to final output overwriting
the prompt, etc.  This can be a bit confusing when you start running
things in a loop.

If we save the original fd, then on the exit path close the redirected
fd's and wait a little bit for final output (as a result of the
close), we get a more consistent output.

Change-Id: I8efe57ab421c1941e99bdecab62c6e21a87e4584

diskimage_builder/lib/disk-image-create

@@ -197,6 +197,10 @@ if [[ -n "${LOGFILE}" ]]; then
     echo "Output logs going to: ${LOGFILE}"
     _LOGFILE_FLAG="-o ${LOGFILE}"
 fi
+
+# Save the existing stdout to fd3
+exec 3>&1
+
 exec 1> >( ${DIB_PYTHON_EXEC:-python} $_LIB/outfilter.py ${_TS_FLAG} ${_QUIET_FLAG} ${_LOGFILE_FLAG} ) 2>&1
 
 
@@ -550,5 +554,18 @@ fi
 # Remove the leftovers, i.e. the temporary image directory.
 cleanup_image_dir
 
+# Restore fd 1&2 from the outfilter.py redirect back to the original
+# saved fd.  Note small hack that we can't really wait properly for
+# outfilter.py so put in a sleep (might be possible to use coproc for
+# this...?)
+#
+# TODO(ianw): probably better to cleanup the exit handler a bit for
+# this?  We really want some helper functions that append to the exit
+# handler so we can register multiple things.
+set +o xtrace
+echo "Build completed successfully"
+exec 1>&3 2>&3
+sleep 1
+
 # All done!
 trap EXIT