dev-lang/python-exec: Add USE=native-symlinks to break stuff

Following toolchain packages, let's add USE=native-symlinks (the name
is far from perfect but consistency) that removes convenience python
and python3 symlinks.  This should help test that ebuilds don't rely
on implicit Python versioning and use Python eclasses correctly.
In other words, semi-broken ebuilds will fail early.

Signed-off-by: Michał Górny <mgorny@gentoo.org>

dev-lang/python-exec/metadata.xml

@@ -13,4 +13,11 @@
     <bugs-to>https://github.com/mgorny/python-exec/issues/</bugs-to>
     <remote-id type="github">mgorny/python-exec</remote-id>
   </upstream>
+  <use>
+    <flag name="native-symlinks">
+      Install generic symlinks like python and python3.  If this flag
+      is disabled, only versioned python3.X executables will
+      be available to end users and ebuilds not using python eclasses.
+    </flag>
+  </use>
 </pkgmetadata>

dev-lang/python-exec/python-exec-2.4.6-r3.ebuild

@@ -0,0 +1,128 @@
+# Copyright 1999-2020 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=7
+
+inherit python-utils-r1
+
+DESCRIPTION="Python script wrapper"
+HOMEPAGE="https://github.com/mgorny/python-exec/"
+SRC_URI="https://github.com/mgorny/python-exec/releases/download/v${PV}/${P}.tar.bz2"
+
+LICENSE="BSD-2"
+SLOT="2"
+KEYWORDS="~alpha ~amd64 ~arm ~arm64 ~hppa ~ia64 ~m68k ~mips ~ppc ~ppc64 ~riscv ~s390 ~sparc ~x86 ~ppc-aix ~x64-cygwin ~amd64-linux ~x86-linux ~ppc-macos ~x64-macos ~x86-macos ~m68k-mint ~sparc-solaris ~sparc64-solaris ~x64-solaris ~x86-solaris"
+# Internal Python project hack.  Do not copy it.  Ever.
+IUSE="${_PYTHON_ALL_IMPLS[@]/#/python_targets_} +native-symlinks"
+
+RDEPEND="!<=dev-lang/python-2.7.18-r3:2.7"
+
+src_configure() {
+	local pyimpls=() i EPYTHON
+	for i in "${_PYTHON_ALL_IMPLS[@]}"; do
+		if use "python_targets_${i}"; then
+			python_export "${i}" EPYTHON
+			pyimpls+=( "${EPYTHON}" )
+		fi
+	done
+
+	local myconf=(
+		--with-fallback-path="${EPREFIX}/usr/local/sbin:${EPREFIX}/usr/local/bin:${EPREFIX}/usr/sbin:${EPREFIX}/usr/bin:${EPREFIX}/sbin:${EPREFIX}/bin"
+		--with-python-impls="${pyimpls[*]}"
+	)
+
+	econf "${myconf[@]}"
+}
+
+src_install() {
+	default
+
+	# Prepare and own the template
+	insinto /etc/python-exec
+	newins - python-exec.conf \
+		< <(sed -n -e '/^#/p' config/python-exec.conf.example)
+
+	if use native-symlinks; then
+		local programs=( python python3 )
+		local scripts=( python-config python3-config 2to3 idle pydoc pyvenv )
+
+		local f
+		for f in "${programs[@]}"; do
+			# symlink the C wrapper for python to avoid shebang recursion
+			# bug #568974
+			dosym python-exec2c /usr/bin/"${f}"
+		done
+		for f in "${scripts[@]}"; do
+			# those are python scripts (except for new python-configs)
+			# so symlink them via the python wrapper
+			dosym ../lib/python-exec/python-exec2 /usr/bin/"${f}"
+		done
+	fi
+}
+
+pkg_preinst() {
+	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
+
+		# start with the 'global' preference (2 vs 3)
+		if [[ -f ${config_base}/config ]]; then
+			old_pythons+=( "$(<${config_base}/config)" )
+		fi
+
+		# then try specific py3 selection
+		for py in 3; 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
+
+		if [[ ${old_pythons[@]} ]]; then
+			elog "You seem to have just upgraded into the new version of python-exec"
+			elog "that uses python-exec.conf for configuration. The ebuild has attempted"
+			elog "to convert your previous configuration to the new format, resulting"
+			elog "in the following preferences (most preferred version first):"
+			elog
+			for py in "${old_pythons[@]}"; do
+				elog "  ${py}"
+			done
+			elog
+			elog "Those interpreters will be preferred when running Python scripts or"
+			elog "calling wrapped Python executables (python, python2, pydoc...)."
+			elog "If none of the preferred interpreters are supported, python-exec will"
+			elog "fall back to the newest supported Python version."
+			elog
+			elog "Please note that due to the ambiguous character of the old settings,"
+			elog "you may want to modify the preference list yourself. In order to do so,"
+			elog "open the following file in your favorite editor:"
+			elog
+			elog "  ${EROOT}/etc/python-exec/python-exec.conf"
+			elog
+			elog "For more information on the new configuration format, please read"
+			elog "the comment on top of the installed configuration file."
+
+			local IFS=$'\n'
+			echo "${old_pythons[*]}" \
+				>> "${ED}"/etc/python-exec/python-exec.conf || die
+		fi
+	fi
+}