sfeed_update: use xargs -P -0

Some of the options, like -P are as of writing (2023) non-POSIX:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/xargs.html. However
many systems support this useful extension for many years now.

Some historic context:

The xargs -0 option was added on 1996-06-11, about a year after the NetBSD
import (over 27 years ago at the time of writing):

http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/xargs/xargs.c?rev=1.2&content-type=text/x-cvsweb-markup

On OpenBSD the xargs -P option was added on 2003-12-06 by syncing the FreeBSD
code:

http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/xargs/xargs.c?rev=1.14&content-type=text/x-cvsweb-markup

Looking at the imported git history log of GNU findutils (which has xargs), the
very first commit already had the -0 and -P option on Sun Feb 4 20:35:16 1996
+0000.

Tested on many systems, old and new, some notable:

- OpenBSD 7.4
- Void Linux
- FreeBSD 12
- NetBSD 9.3
- HaikuOS (uses GNU tools).
- Slackware 11
- OpenBSD 3.8
- NetBSD 5.1

Some shells:
- oksh
- bash
- dash
- zsh

During testing there are some incompatibilities found in parsing the fields so
the arguments are passed as one argument which is split later on by the child
program.

1 files changed, 32 insertions, 16 deletions

diff --git a/sfeed_update b/sfeed_update
index 0628e2e..309b348 100755
--- a/sfeed_update
+++ b/sfeed_update
@@ -163,14 +163,12 @@ _feed() {
 # fetch and process a feed in parallel.
 # feed(name, feedurl, [basesiteurl], [encoding])
 feed() {
-	# wait until ${maxjobs} are finished: will stall the queue if an item
-	# is slow, but it is portable.
-	[ ${signo} -ne 0 ] && return
-	[ $((curjobs % maxjobs)) -eq 0 ] && wait
-	[ ${signo} -ne 0 ] && return
-	curjobs=$((curjobs + 1))
-
-	_feed "$@" &
+	# Job parameters for xargs.
+	# Specify fields as a single parameter separated by the NUL separator.
+	# These fields are split later by the child process, this allows xargs
+	# with empty fields across many implementations.
+	printf '%s\037%s\037%s\037%s\037%s\037%s\0' \
+		"${config}" "${sfeedtmpdir}" "$1" "$2" "$3" "$4"
 }
 
 cleanup() {
@@ -201,8 +199,6 @@ feeds() {
 }
 
 main() {
-	# job counter.
-	curjobs=0
 	# signal number received for parent.
 	signo=0
 	# SIGINT: signal to interrupt parent.
@@ -217,16 +213,36 @@ main() {
 	touch "${sfeedtmpdir}/ok" || die
 	# make sure path exists.
 	mkdir -p "${sfeedpath}"
-	# fetch feeds specified in config file.
-	feeds
-	# wait till all feeds are fetched (concurrently).
-	[ ${signo} -eq 0 ] && wait
-	# check error exit status indicator for parallel jobs.
-	[ -f "${sfeedtmpdir}/ok" ]
+
+	# print feeds for parallel processing with xargs.
+	feeds > "${sfeedtmpdir}/jobs" || die
+	SFEED_UPDATE_CHILD="1" xargs -s 65535 -x -0 -P "${maxjobs}" -n 1 \
+		"$(readlink -f "${argv0}")" < "${sfeedtmpdir}/jobs"
 	statuscode=$?
+
+	# check error exit status indicator for parallel jobs.
+	[ -f "${sfeedtmpdir}/ok" ] || statuscode=1
 	# on signal SIGINT and SIGTERM exit with signal number + 128.
 	[ ${signo} -ne 0 ] && die $((signo+128))
 	die ${statuscode}
 }
 
+# process a single feed.
+# parameters are: config, tmpdir, name, feedurl, basesiteurl, encoding
+if [ "${SFEED_UPDATE_CHILD}" = "1" ]; then
+	IFS="" # "\037"
+	[ "$1" = "" ] && exit 0 # must have an argument set
+	printf '%s\n' "$1" | \
+	while read -r config tmpdir name feedurl basesiteurl encoding; do
+		# load config file, sets $config.
+		loadconfig "${config}"
+		sfeedtmpdir="${tmpdir}"
+		_feed "${name}" "${feedurl}" "${basesiteurl}" "${encoding}"
+		exit "$?"
+	done
+	exit 0
+fi
+
+# ...else parent mode:
+argv0="$0" # remember $0, in shells like zsh $0 is the function name.
 [ "${SFEED_UPDATE_INCLUDE}" = "1" ] || main "$@"