[BUG] perf stat: hangs with -p and process completes

[Stephane Eranian]

Hi,

I am running into a perf stat issue with the -p option which allows you
to attach to a running process. If that process happens to terminate
while under monitoring
perf hangs in there and never terminates. The proper behavior would be to stop.
I can see the issue in that the attached process is not a child, so
wait() would not work.

To reproduce:
  $ sleep 10 &
  $ perf stat -p $!

doing the same with perf record works, so there is a solution to this problem.