Good (or morning afternoon evening night)

When `dired-do-shell-command' runs COMMAND over FILES in
parallel, the `shell-command' process may finish before all the
files have being processed.

emacs -Q ~

* / & du SPC -s RET
; If the number of lines in #<buffer *Async Shell Command*> is less
; than (1+ (length (dired-get-marked-files))) then NOT all files
; has being processed.

Attached patch fix this issue adding 'wait' at the end of COMMAND
when FILES are procesed in parallel.

In GNU Emacs 25.0.92.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.30)
  of 2016-04-01 built on calancha-pc
Repository revision: a3daa34336da158555d96d670077eedb9eaeeb9c