eclass/optfeature: Sync with gentoo

It's from gentoo commit 3b2d830d52a9e6bee1a803566b360b28aa759c9d.

sdk_container/src/third_party/portage-stable/eclass/optfeature.eclass

@@ -1,19 +1,72 @@
-# Copyright 1999-2020 Gentoo Authors
+# Copyright 1999-2021 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: optfeature.eclass
 # @MAINTAINER:
 # base-system@gentoo.org
+# @SUPPORTED_EAPIS: 6 7 8
 # @BLURB: Advertise optional functionality that might be useful to users
 
-case ${EAPI:-0} in
-	[0-7]) ;;
-	*)     die "Unsupported EAPI=${EAPI} (unknown) for ${ECLASS}" ;;
+case ${EAPI} in
+	6|7|8) ;;
+	*) die "${ECLASS}: EAPI=${EAPI:-0} is not supported" ;;
 esac
 
 if [[ -z ${_OPTFEATURE_ECLASS} ]]; then
 _OPTFEATURE_ECLASS=1
 
+# @ECLASS-VARIABLE: _OPTFEATURE_DEFAULT_HEADER
+# @INTERNAL
+# @DESCRIPTION:
+# Default header printed ahead of optfeature output. Can be overridden
+# by calling optfeature_header function. Will not be displayed if all optional
+# dependencies are present.
+_OPTFEATURE_DEFAULT_HEADER="Install additional packages for optional runtime features:"
+
+readonly _OPTFEATURE_DEFAULT_HEADER
+
+# @ECLASS-VARIABLE: _OPTFEATURE_HEADER
+# @INTERNAL
+# @DESCRIPTION:
+# Default empty. Custom header printed ahead of optfeature output.
+# Set by calling optfeature_header function with the desired output, or reset
+# by optfeature_header without argument. Will not be displayed if all optional
+# dependencies are present.
+_OPTFEATURE_HEADER=
+
+# @ECLASS-VARIABLE: _OPTFEATURE_DOHEADER
+# @INTERNAL
+# @DESCRIPTION:
+# If true, print header ahead of the first optfeature output.
+_OPTFEATURE_DOHEADER=true
+
+# @FUNCTION: optfeature_header
+# @USAGE: [custom header for follow-up optfeature calls]
+# @DESCRIPTION:
+# Set a custom header for follow-up optfeature calls, or reset to default
+# header by calling it without argument. This can not only be used to customize
+# the header but also to distinguish optfeature "groups", e.g. to list a number
+# of different possible database backends, and then a number of optional
+# regular runtime features.
+#
+# The following snippet will leave the default header untouched for the first
+# two optfeature calls. Then a custom header is set that is going to be
+# displayed in case dev-db/a or dev-db/b are not installed.
+# @CODE
+# pkg_postinst() {
+# 	optfeature "foo support" app-misc/foo
+# 	optfeature "bar support" app-misc/bar
+# 	optfeature_header "Install optional database backends:"
+# 	optfeature "a DB backend" dev-db/a
+# 	optfeature "b DB backend" dev-db/b
+# }
+# @CODE
+optfeature_header() {
+	debug-print-function ${FUNCNAME} "$@"
+	_OPTFEATURE_HEADER="${1}"
+	_OPTFEATURE_DOHEADER=true
+}
+
 # @FUNCTION: optfeature
 # @USAGE: <short description> <package atom to match> [other atoms]
 # @DESCRIPTION:
@@ -24,19 +77,23 @@ _OPTFEATURE_ECLASS=1
 # app-misc/bar or app-misc/baz[bar] for optional bar support
 # and either both app-misc/a and app-misc/b or app-misc/c for alphabet support.
 # @CODE
-#	optfeature "foo support" app-misc/foo
-#	optfeature "bar support" app-misc/bar app-misc/baz[bar]
-#	optfeature "alphabet support" "app-misc/a app-misc/b" app-misc/c
+# pkg_postinst() {
+# 	optfeature "foo support" app-misc/foo
+# 	optfeature "bar support" app-misc/bar app-misc/baz[bar]
+# 	optfeature "alphabet support" "app-misc/a app-misc/b" app-misc/c
+# }
 # @CODE
 optfeature() {
 	debug-print-function ${FUNCNAME} "$@"
 
 	local i j msg
+	local -a arr
 	local desc=$1
 	local flag=0
 	shift
 	for i; do
-		for j in ${i}; do
+		read -r -d '' -a arr <<<"${i}"
+		for j in "${arr[@]}"; do
 			if has_version "${j}"; then
 				flag=1
 			else
@@ -49,9 +106,14 @@ optfeature() {
 		fi
 	done
 	if [[ ${flag} -eq 0 ]]; then
+		if [[ ${_OPTFEATURE_DOHEADER} == true ]]; then
+			elog ${_OPTFEATURE_HEADER:-${_OPTFEATURE_DEFAULT_HEADER}}
+			_OPTFEATURE_DOHEADER=false
+		fi
 		for i; do
+			read -r -d '' -a arr <<<"${i}"
 			msg=" "
-			for j in ${i}; do
+			for j in "${arr[@]}"; do
 				msg+=" ${j} and"
 			done
 			msg="${msg:0: -4} for ${desc}"