dev-lang/python-exec: Install and migrate configuration file

dev-lang/python-exec/python-exec-2.9999.ebuild

@@ -27,7 +27,7 @@ SLOT="2"
 KEYWORDS="~alpha ~amd64 ~arm ~arm64 ~hppa ~ia64 ~m68k ~mips ~ppc ~ppc64 ~s390 ~sh ~sparc ~x86 ~ppc-aix ~amd64-fbsd ~x86-fbsd ~x64-freebsd ~x86-freebsd ~amd64-linux ~arm-linux ~x86-linux ~ppc-macos ~x64-macos ~x86-macos ~m68k-mint ~sparc-solaris ~sparc64-solaris ~x64-solaris ~x86-solaris"
 IUSE=""
 
-# eselect-python because of /usr/bin/python* collisions
+# eselect-python because of /usr/bin/python* collisions and new config
 # python versions because of missing $scriptdir/python* symlinks
 RDEPEND="
 	!<app-eselect/eselect-python-20160206
@@ -63,6 +63,12 @@ src_configure() {
 src_install() {
 	default
 
+	# Prepare and own the template
+	sed -n -e '/^#/p' config/python-exec.conf.example \
+		> "${T}"/python-exec.conf || die
+	insinto /etc/python-exec
+	doins "${T}"/python-exec.conf
+
 	local f
 	for f in python{,2,3}; do
 		# can't use symlinks here since random stuff
@@ -75,18 +81,45 @@ src_install() {
 }
 
 pkg_preinst() {
-	local py
+	if [[ -e ${EROOT}etc/python-exec/python-exec.conf ]]; then
+		# preserve current configuration
+		cp "${EROOT}"etc/python-exec/python-exec.conf \
+			"${ED}"etc/python-exec/python-exec.conf || die
+	else
+		# preserve previous Python version preference
+		local py old_pythons=()
+		local config_base=${EROOT}etc/env.d/python
 
-	# Copy python[23] selection from the old format (symlink)
-	for py in 2 3; do
-		if [[ -L ${EROOT}/usr/bin/python${py} ]]; then
-			local target=$(readlink "${EROOT}/usr/bin/python${py}")
-
-			# check if it's actually old eselect symlink
-			if [[ ${target} == python?.? ]]; then
-				einfo "Preserving Python${py} as ${target}"
-				echo "${target}" > "${EROOT}/etc/env.d/python/python${py}" || die
-			fi
+		# start with the 'global' preference (2 vs 3)
+		if [[ -f ${config_base}/config ]]; then
+			old_pythons+=( "$(<${config_base}/config)" )
 		fi
-	done
+
+		# then try specific py2/py3 selections
+		for py in 3 2; do
+			local target=
+
+			if [[ -f ${config_base}/python${py} ]]; then
+				# try the newer config files
+				target=$(<${config_base}/python${py})
+			elif [[ -L ${EROOT}/usr/bin/python${py} ]]; then
+				# check the older symlink format
+				target=$(readlink "${EROOT}/usr/bin/python${py}")
+
+				# check if it's actually old eselect symlink
+				[[ ${target} == python?.? ]] || target=
+			fi
+
+			# add the extra target if found and != global
+			if [[ ${target} && ${old_pythons[0]} != ${target} ]]; then
+				old_pythons+=( "${target}" )
+			fi
+		done
+
+		einfo "Keeping the following Python preference: ${old_pythons[*]}"
+
+		local IFS=$'\n'
+		echo "${old_pythons[*]}" \
+			>> "${ED}"etc/python-exec/python-exec.conf || die
+	fi
 }