multibuild.eclass: Reap stray subjobs before exiting sub-phase

Alike the PMS-defined phases, sub-phases do not support leaving jobs
behind. To avoid leaving something accidentally, make sure to reap all
jobs (i.e. wait) post running the command. Warn if any stray processes
are found since that indicates a bug in ebuild.

eclass/multibuild.eclass

@@ -125,6 +125,18 @@ multibuild_foreach_variant() {
 		_multibuild_run "${@}" \
 			> >(exec tee -a "${T}/build-${MULTIBUILD_ID}.log") 2>&1
 		lret=${?}
+
+		# make sure no processes are left over
+		local leftovers=
+		while :; do
+			wait -n
+			[[ ${?} -eq 127 ]] && break || leftovers=1
+		done
+
+		if [[ ${leftovers} ]]; then
+			ewarn "The multibuild function has left one or more processes running"
+			ewarn "in the background. Please report a bug."
+		fi
 	done
 	[[ ${ret} -eq 0 && ${lret} -ne 0 ]] && ret=${lret}