Previously, the pids type was list(), which raised a ValueError in case if
pid was not in pids during the pids.remove(pid) call. The fix changes
the type of pids to set() and substitutes remove() with discard(), which
doesn't raise an Exception if a value is not in the set.
Change-Id: I8d41cb2a8ec14d29e5c7411ddfe48ae7a41deb45
Closes-Bug: #1567638
1a18079a15