One easy solution is to simply close the pipe in the parent after starting both processes, before calling p1.wait():

p1.stdout.close()

It's not "perfect", though, p1 will execute a while before recieving SIGPIPE. For a perfect solution, it would be necessary to close the pipe end in the parent after the fork but before the exec in the child. This would require some kind of synchronization. 

Moving to feature request.