[gentoo-python] [PATCH 2/2] Re-enable python-r1 -> python-single-r1 dep prevention in EAPI 5+.

[Michał Górny]

---
 gx86/eclass/python-r1.eclass | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/gx86/eclass/python-r1.eclass b/gx86/eclass/python-r1.eclass
index cfd0a36..3cae4f0 100644
--- a/gx86/eclass/python-r1.eclass
+++ b/gx86/eclass/python-r1.eclass
@@ -133,12 +133,35 @@ fi
 
 _python_set_globals() {
 	local flags=( "${PYTHON_COMPAT[@]/#/python_targets_}" )
-	#local flags_st=( "${PYTHON_COMPAT[@]/#/-python_single_target_}" )
 	local optflags=${flags[@]/%/?}
-	#optflags+=,${flags_st[@]/%/(-)}
 
-	# PYTHON_SINGLE_TARGET safety check temporarily disabled
-	# because of issues with paludis, bug #447524.
+	# Here's a nice trick to prevent python-r1 packages from depending
+	# on python-single-r1 packages.
+	#
+	# python-single-r1 enforces at least one PYTHON_SINGLE_TARGET
+	# through REQUIRED_USE. Therefore, we can assume that
+	# python-single-r1 packages have at least one flag set.
+	#
+	# If packages does not use python-single-r1, it should have no
+	# PYTHON_SINGLE_TARGET flags.
+	#
+	# The '-python_single_target...(-)' construct is always true when
+	# there's no PYTHON_SINGLE_TARGET flags. If there is at least one
+	# such flag, there will be at least one set, and the relevant
+	# constraint will prevent the package from being merged.
+	#
+	# Therefore, developer trying to depend on python-single-r1 package
+	# will be unable to merge the package. This should be enough to
+	# prevent him from committing one.
+	#
+	# In EAPI < 5, an implicit IUSE is added for all USE_EXPAND
+	# variables, making '(-)' deps unusable on them. This can be seen
+	# in Paludis (bug #447524). We need at least EAPI=5 for this to work
+	# correctly.
+	if ! has "${EAPI}" 4; then
+		local flags_st=( "${PYTHON_COMPAT[@]/#/-python_single_target_}" )
+		optflags+=,${flags_st[@]/%/(-)}
+	fi
 
 	IUSE=${flags[*]}
 	REQUIRED_USE="|| ( ${flags[*]} )"
-- 
1.8.0.2