In case of an error in the writer, the packbuilder will stay around
waiting for someone to read from its channel. The state associated
with a packbuilder is non-trivial and it will keep a reference to the
object, so the GC won't be able to free it.
Change the ForEach interface to also return a "stop" channel. Closing
the channel or writing into it will cause the first receive clause to
act, making the callback to return -1, aborting the operation and
ending the goroutine, freeing its hold on the packbuilder.
Carlos Martín Nieto