How can one build all ports?

[Diomidis Spinellis]

Is there a way to run a tinderbox for all the FreeBSD ports?  Ideally, I 
would like each port to be built exactly once.

I can obtain a list of all ports with something like

find /usr/ports -type d -maxdepth 2 | sed -n 's/\/usr\/ports\///;/\//p'

but I'm not sure how tinderbuild will react if it is invoked multiple 
times through xargs, and what will happen when I ask tinderbuild to 
build a port that has already been built through a dependency.

Is there a better way?

Diomidis - dds@