pkgdiff-mg: under systemd use /var/tmp as default TMPDIR

pkgdiff can cause large files in the used temprorary directory that
are not cleaned up on exit. Using /tmp for those, which is often a
tmpfs, which means that the files consume main memory at first. Later
the system may move the unused files on the tmpfs into swap.

Under systemd, using /var/tmp for files that are not deleted by the
creator, which is pkgdiff-mg in our case, is safe, since systemd will
automatically delete old unused files from /var/tmp.

Signed-off-by: Florian Schmaus <flow@gentoo.org>

Author: Flowdalic

pkgdiff-mg

@@ -36,7 +36,13 @@ fn2=${2##*/}
 ver1=$(patom -F '%{fullver}' "=${cat1}/${fn1%.ebuild}")
 ver2=$(patom -F '%{fullver}' "=${cat2}/${fn2%.ebuild}")
 
-export PORTAGE_TMPDIR="${TMPDIR:-/tmp}"/mgorny-dev-scripts
+DEFAULT_TMPDIR=/tmp
+if [[ -d /run/systemd/system ]]; then
+	# System booted via systemd, which automatically cleans up stale
+	# files in /var/tmp.
+	DEFAULT_TMPDIR=/var/tmp
+fi
+export PORTAGE_TMPDIR="${TMPDIR:-${DEFAULT_TMPDIR}}"/mgorny-dev-scripts
 export USE="${USE} test"
 # use noauto to skip pkg_setup
 export FEATURES="${FEATURES} noauto"