Fix zombie phantomjs processes (#338)

After running wallaby as part of a separate application, I confirmed we
were still leaving around zombie phantom processes. It turns out if we
kill the script's process group, it will kill all of the child processes
as well. This appears to fix the zombie issue once and for all.

One thing to note, I kill the process group by using the negative value
of the wrapper script's pid, instead of using `ps axo pid, pgid` to look
up the process group id. This is because I was occasionally running into
issues where the process group lookup wasn't returning a result, causing
more zombie processes. Hopefully this approach won't cause any issues.

Fixes #224.

priv/run_phantom.sh

@@ -9,6 +9,8 @@
 
 set -e
 
+script_status="running"
+
 create_pipe(){
   local pipe=$(mktemp -u)
   mkfifo -m 600 "$pipe"
@@ -39,10 +41,13 @@ shutdown(){
   local my_pid=$1
   local program_pid=$2
 
-  children=$(ps xao pid,pgid | grep $my_pid | awk '{print $1}' | grep -v $my_pid)
-  kill $program_pid 2>/dev/null
+  if [ $script_status = "running" ]; then
+    script_status="shutting down"
+
+    # Kill this script's process group
+    kill -TERM $(($my_pid * -1)) 2>/dev/null
+  fi
 
-  wait_for_pids_to_exit $children
   exit 0
 }