regenerating autotools in packages that have a lot of AC_CONFIG_SUBDIRS is
really slow due to the serialization of all the dirs (which really isn't
required).  so i took some code that i merged into portage semi-recently
(which is based on work by Brian, although i'm not sure he wants to admit it)
and put it into a new multiprocessing.eclass.  this way people can generically
utilize this in their own eclasses/ebuilds.

it doesn't currently support nesting.  not sure if i should fix that.

i'll follow up with an example of parallelizing of eautoreconf.  for
mail-filter/maildrop on my 4 core system, it cuts the time needed to run from
~2.5 min to ~1 min.
-mike

# Copyright 1999-2012 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: $

# @ECLASS: multiprocessing.eclass
# @MAINTAINER:
# base-system@g.o
# @AUTHORS:
# Brian Harring <ferringb@g.o>
# Mike Frysinger <vapier@g.o>
# @BLURB: parallelization with bash (wtf?)
# @DESCRIPTION:
# The multiprocessing eclass contains a suite of functions that allow ebuilds
# to quickly run things in parallel using shell code.

if [[ ${___ECLASS_ONCE_MULTIPROCESSING} != "recur -_+^+_- spank" ]] ; then
___ECLASS_ONCE_MULTIPROCESSING="recur -_+^+_- spank"

# @FUNCTION: makeopts_jobs
# @USAGE: [${MAKEOPTS}]
# @DESCRIPTION:
# Searches the arguments (defaults to ${MAKEOPTS}) and extracts the jobs number
# specified therein.  Useful for running non-make tools in parallel too.
# i.e. if the user has MAKEOPTS=-j9, this will show "9".
# We can't return the number as bash normalizes it to [0, 255].  If the flags
# haven't specified a -j flag, then "1" is shown as that is the default `make`
# uses.  Since there's no way to represent infinity, we return 999 if the user
# has -j without a number.
makeopts_jobs() {
	[[ $# -eq 0 ]] && set -- ${MAKEOPTS}
	# This assumes the first .* will be more greedy than the second .*
	# since POSIX doesn't specify a non-greedy match (i.e. ".*?").
	local jobs=$(echo " $* " | sed -r -n \
		-e 's:.*[[:space:]](-j|--jobs[=[:space:]])[[:space:]]*([0-9]+).*:\2:p' \
		-e 's:.*[[:space:]](-j|--jobs)[[:space:]].*:999:p')
	echo ${jobs:-1}
}

# @FUNCTION: multijob_init
# @USAGE: [${MAKEOPTS}]
# @DESCRIPTION:
# Setup the environment for executing things in parallel.
# You must call this before any other multijob function.
multijob_init() {
	# Setup a pipe for children to write their pids to when they finish.
	mj_control_pipe="${T}/multijob.pipe"
	mkfifo "${mj_control_pipe}"
	exec {mj_control_fd}<>${mj_control_pipe}
	rm -f "${mj_control_pipe}"

	# See how many children we can fork based on the user's settings.
	mj_max_jobs=$(makeopts_jobs "$@")
	mj_num_jobs=0
}

# @FUNCTION: multijob_child_init
# @DESCRIPTION:
# You must call this first in the forked child process.
multijob_child_init() {
	[[ $# -eq 0 ]] || die "${FUNCNAME} takes no arguments"

	trap 'echo ${BASHPID} $? >&'${mj_control_fd} EXIT
	trap 'exit 1' INT TERM
}

# @FUNCTION: multijob_post_fork
# @DESCRIPTION:
# You must call this in the parent process after forking a child process.
# If the parallel limit has been hit, it will wait for one to finish and
# return the child's exit status.
multijob_post_fork() {
	[[ $# -eq 0 ]] || die "${FUNCNAME} takes no arguments"

	: $(( ++mj_num_jobs ))
	if [[ ${mj_num_jobs} -ge ${mj_max_jobs} ]] ; then
		multijob_finish_one
	fi
	return $?
}

# @FUNCTION: multijob_finish_one
# @DESCRIPTION:
# Wait for a single process to exit and return its exit code.
multijob_finish_one() {
	[[ $# -eq 0 ]] || die "${FUNCNAME} takes no arguments"

	local pid ret
	read -r -u ${mj_control_fd} pid ret
	: $(( --mj_num_jobs ))
	return ${ret}
}

# @FUNCTION: multijob_finish
# @DESCRIPTION:
# Wait for all pending processes to exit and return the bitwise or
# of all their exit codes.
multijob_finish() {
	[[ $# -eq 0 ]] || die "${FUNCNAME} takes no arguments"

	local ret=0
	while [[ ${mj_num_jobs} -gt 0 ]] ; do
		multijob_finish_one
		: $(( ret |= $? ))
	done
	# Let bash clean up its internal child tracking state.
	wait
	return ${ret}
}

fi