notify from a file change that is not managed by puppet

example usage:

snack(~) % rm /tmp/flag
snack(~) % echo "Hello" > /tmp/original
snack(~) % sudo puppet apply unmanaged-notify.pp
notice: /Stage[main]//File[/tmp/flag]/ensure: defined content as '{md5}09f7e02f1290be211da707a266f153b3'
notice: /Stage[main]//Exec[hello world]: Triggered 'refresh' from 1 events

# Won't exec 'hello world' again because no change occured to /tmp/original:
snack(~) % sudo puppet apply unmanaged-notify.pp

# Now change /tmp/original:
snack(~) % echo "testing" > /tmp/original  
snack(~) % sudo puppet apply unmanaged-notify.pp
notice: /Stage[main]//File[/tmp/flag]/content: content changed '{md5}09f7e02f1290be211da707a266f153b3' to '{md5}eb1a3227cdc3fedbaec2fe38bf6c044a'
notice: /Stage[main]//Exec[hello world]: Triggered 'refresh' from 1 events

unmanaged-notify.pp

# Manually manage /tmp/original
# Each puppet run willy copy it to /tmp/flag if there's a change and notify
# the exec when it changes.
file {
  "/tmp/flag":
    source => "file:///tmp/original",
    notify => Exec["hello world"];
}

exec {
  "hello world":
    command => "/bin/echo hello world",
    refreshonly => true;
}