[Buildroot] [RFC] python: fix wrong library linking for CFFI backend

[yegorslists at googlemail.com]

From: Yegor Yefremov <yegorslists at googlemail.com>

If python-cffi target variant is installed before a CFFI backend
package is built, the cross-compiler tries to link the target
_cffi_backend.so and fails. This workaround removes target
_cffi_backend.so before package building and restores it after
the package was installed. This is only needed for Python 2.x.

Signed-off-by: Yegor Yefremov <yegorslists at googlemail.com>
---
 package/python-pynacl/python-pynacl.mk | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/package/python-pynacl/python-pynacl.mk b/package/python-pynacl/python-pynacl.mk
index e1769b8f7..feb5245a7 100644
--- a/package/python-pynacl/python-pynacl.mk
+++ b/package/python-pynacl/python-pynacl.mk
@@ -13,4 +13,21 @@ PYTHON_PYNACL_SETUP_TYPE = setuptools
 PYTHON_PYNACL_DEPENDENCIES = libsodium host-python-cffi
 PYTHON_PYNACL_ENV = SODIUM_INSTALL=system
 
+# Remove _cffi_backend.so before building the package and restore it
+# after installation. Without target _cffi_backend.so the cross-compilation
+# process will use the host _cffi_backend.so and produce correct target
+# *.so file. This workaround is only required for Python 2.x.
+ifeq ($(BR2_PACKAGE_PYTHON),y)
+define PYTHON_PYNACL_CFFI_HOOK
+	mv $(TARGET_DIR)/usr/lib/python2.7/site-packages/_cffi_backend.so \
+	$(TARGET_DIR)/usr/lib/python2.7/site-packages/br_cffi_backend.so 2>/dev/null || true
+endef
+define PYTHON_PYNACL_RESTORE_CFFI_BACKEND_HOOK
+	mv $(TARGET_DIR)/usr/lib/python2.7/site-packages/br_cffi_backend.so \
+	$(TARGET_DIR)/usr/lib/python2.7/site-packages/_cffi_backend.so 2>/dev/null || true
+endef
+PYTHON_PYNACL_PRE_BUILD_HOOKS = PYTHON_PYNACL_CFFI_HOOK
+PYTHON_PYNACL_POST_INSTALL_TARGET_HOOKS = PYTHON_PYNACL_RESTORE_CFFI_BACKEND_HOOK
+endif
+
 $(eval $(python-package))
-- 
2.11.0