rpmbuild fails for unknown reason when using --target - build

Question

I recently updated my dev machine from Fedora 30 to 31, but I still need to target a Fedora 30 machine.
I am trying to build a SPEC using the parameter --target f30, but I get an error for 'xargs' out of nowhere...
rpmbuild -bb ~/rpmbuild/SPECS/Project.spec --target f30
Building target platforms: f30
Building for target f30
Executing(%prep): /bin/sh -e /var/tmp/rpm-tmp.hUazCA
+ umask 022
+ cd /home/matthew/rpmbuild/BUILD
+ cd /home/matthew/rpmbuild/BUILD
+ rm -rf Project-1.9.1
+ /usr/bin/gzip -dc /home/matthew/rpmbuild/SOURCES/Project-1.9.1.tar.gz
+ /usr/bin/tar -xof -
+ STATUS=0
+ '[' 0 -ne 0 ']'
+ cd Project-1.9.1
+ /usr/bin/chmod -Rf a+rX,u+w,g-w,o-w .
+ RPM_EC=0
++ jobs -p
+ exit 0
Executing(%build): /bin/sh -e /var/tmp/rpm-tmp.TnKydD
+ umask 022
+ cd /home/matthew/rpmbuild/BUILD
+ cd Project-1.9.1
+ RPM_EC=0
++ jobs -p
+ exit 0
Executing(%install): /bin/sh -e /var/tmp/rpm-tmp.GIPsyB
+ umask 022
+ cd /home/matthew/rpmbuild/BUILD
+ '[' '/home/matthew/rpmbuild/BUILDROOT/Project-1.9.1-1.fc31.%{_arch}' '!=' / ']'
+ rm -rf '/home/matthew/rpmbuild/BUILDROOT/Project-1.9.1-1.fc31.%{_arch}'
++ dirname '/home/matthew/rpmbuild/BUILDROOT/Project-1.9.1-1.fc31.%{_arch}'
+ mkdir -p /home/matthew/rpmbuild/BUILDROOT
+ mkdir '/home/matthew/rpmbuild/BUILDROOT/Project-1.9.1-1.fc31.%{_arch}'
+ cd Project-1.9.1
+ mkdir -p '/home/matthew/rpmbuild/BUILDROOT/Project-1.9.1-1.fc31.%{_arch}/opt/Project'
+ cp -a README.md __dump__ __files__ __scripts__ __wheels__ apps dbms dbtools graph.png loc manage.py meta program requirements.txt rgs static veh '/home/matthew/rpmbuild/BUILDROOT/Project-1.9.1-1.fc31.%{_arch}/opt/Project'
+ /usr/lib/rpm/check-buildroot
xargs: invalid number for -P option
Usage: xargs [OPTION]... COMMAND [INITIAL-ARGS]...
Run COMMAND with arguments INITIAL-ARGS and more arguments read from input.
Mandatory and optional arguments to long options are also
mandatory or optional for the corresponding short option.
-0, --null items are separated by a null, not whitespace;
disables quote and backslash processing and
logical EOF processing
-a, --arg-file=FILE read arguments from FILE, not standard input
-d, --delimiter=CHARACTER items in input stream are separated by CHARACTER,
not by whitespace; disables quote and backslash
processing and logical EOF processing
-E END set logical EOF string; if END occurs as a line
of input, the rest of the input is ignored
(ignored if -0 or -d was specified)
-e, --eof[=END] equivalent to -E END if END is specified;
otherwise, there is no end-of-file string
-I R same as --replace=R
-i, --replace[=R] replace R in INITIAL-ARGS with names read
from standard input; if R is unspecified,
assume {}
-L, --max-lines=MAX-LINES use at most MAX-LINES non-blank input lines per
command line
-l[MAX-LINES] similar to -L but defaults to at most one non-
blank input line if MAX-LINES is not specified
-n, --max-args=MAX-ARGS use at most MAX-ARGS arguments per command line
-P, --max-procs=MAX-PROCS run at most MAX-PROCS processes at a time
-p, --interactive prompt before running commands
--process-slot-var=VAR set environment variable VAR in child processes
-r, --no-run-if-empty if there are no arguments, then do not run COMMAND;
if this option is not given, COMMAND will be
run at least once
-s, --max-chars=MAX-CHARS limit length of command line to MAX-CHARS
--show-limits show limits on command-line length
-t, --verbose print commands before executing them
-x, --exit exit if the size (see -s) is exceeded
--help display this help and exit
--version output version information and exit
Report bugs to <bug-findutils#gnu.org>.
+ /usr/lib/rpm/redhat/brp-ldconfig
+ /usr/lib/rpm/brp-compress
+ /usr/lib/rpm/brp-strip /usr/bin/strip
+ /usr/lib/rpm/brp-strip-comment-note /usr/bin/strip /usr/bin/objdump
+ /usr/lib/rpm/brp-strip-static-archive /usr/bin/strip
xargs: invalid number for -P option
Usage: xargs [OPTION]... COMMAND [INITIAL-ARGS]...
Run COMMAND with arguments INITIAL-ARGS and more arguments read from input.
Mandatory and optional arguments to long options are also
mandatory or optional for the corresponding short option.
-0, --null items are separated by a null, not whitespace;
disables quote and backslash processing and
logical EOF processing
-a, --arg-file=FILE read arguments from FILE, not standard input
-d, --delimiter=CHARACTER items in input stream are separated by CHARACTER,
not by whitespace; disables quote and backslash
processing and logical EOF processing
-E END set logical EOF string; if END occurs as a line
of input, the rest of the input is ignored
(ignored if -0 or -d was specified)
-e, --eof[=END] equivalent to -E END if END is specified;
otherwise, there is no end-of-file string
-I R same as --replace=R
-i, --replace[=R] replace R in INITIAL-ARGS with names read
from standard input; if R is unspecified,
assume {}
-L, --max-lines=MAX-LINES use at most MAX-LINES non-blank input lines per
command line
-l[MAX-LINES] similar to -L but defaults to at most one non-
blank input line if MAX-LINES is not specified
-n, --max-args=MAX-ARGS use at most MAX-ARGS arguments per command line
-P, --max-procs=MAX-PROCS run at most MAX-PROCS processes at a time
-p, --interactive prompt before running commands
--process-slot-var=VAR set environment variable VAR in child processes
-r, --no-run-if-empty if there are no arguments, then do not run COMMAND;
if this option is not given, COMMAND will be
run at least once
-s, --max-chars=MAX-CHARS limit length of command line to MAX-CHARS
--show-limits show limits on command-line length
-t, --verbose print commands before executing them
-x, --exit exit if the size (see -s) is exceeded
--help display this help and exit
--version output version information and exit
Report bugs to <bug-findutils#gnu.org>.
error: Bad exit status from /var/tmp/rpm-tmp.GIPsyB (%install)
RPM build errors:
Bad exit status from /var/tmp/rpm-tmp.GIPsyB (%install)
Using the very verbose -vv option with 'rpmbuild' didn't provide anything useful. Any ideas what the issue could be?
Even when targeting f31 (the platform itself) specifically, the build fails, but when not adding a target such that it defaults to f31, it succeeds.

Answer 1

The target option is for architecture. And even that works only if you do some other low level magic. You should do:
rpmbuild -bs ~/rpmbuild/SPECS/Project.spec
# this will produce Project.src.rpm
mock -r fedora-30-x86_64 Project.src.rpm
And it will does not matter whether you are on F30 or 31 or on CentOS.