[Buildroot] [PATCH 3/6] pkg-python: support host-python dependency different from the python in the target

[Samuel Martin]

Some packages need a host-python interpreter with a version different
from the one installed in the target to run some build scripts (eg.
scons requires python2 to run, so build any kind of packages even if
the python interpreter selected for the target is python3).

In such cases, we need to add the right host-python dependency to the
package using the host-python-package infrastructure, and we also want
to invoke the right host python interpreter during the build steps.

This patch add a *_FORCE_HOST_PYTHON variable that can be set either
to 'python2' or 'python3'. This variable can be set by any package
using the host-python-package infrastructure can set to force the
python interpreter for the build. This variable also takes care of
setting the right host-python dependency.

This *_FORCE_HOST_PYTHON variable only affects packages using the
host-python-package infrastructure.

If some configure/build/install commands are overloaded in the *.mk
file, the right python interpreter should be explicitly called.

If the package define some tool variable (eg.: SCONS), the variable
should explicitly called the right python interpreter.

Signed-off-by: Samuel Martin <s.martin49 at gmail.com>
Cc: Gustavo Zacarias <gustavo at zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
---
 docs/manual/adding-packages-python.txt |  8 ++++++
 package/pkg-python.mk                  | 50 +++++++++++++++++++++++++++++++---
 2 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/docs/manual/adding-packages-python.txt b/docs/manual/adding-packages-python.txt
index b8d5331..da97654 100644
--- a/docs/manual/adding-packages-python.txt
+++ b/docs/manual/adding-packages-python.txt
@@ -144,6 +144,14 @@ therefore only use a few of them, or none.
   setuptools packages) and +HOST_PKG_PYTHON_SETUPTOOLS_INSTALL_OPT+
   (for host setuptools packages).
 
+* +HOST_PYTHON_FOO_FORCE_HOST_PYTHON+, to define the host python
+  interpreter. The usage of this variable is limited to the host
+  packages. The two supported value are +python2+ and +python3+.
+  It will may sure the right host python package is available for
+  the build and will invoke it for the build. If some build steps
+  are overloaded, the right python interpreter must be explicitly
+  called in the commands.
+
 With the Python infrastructure, all the steps required to build and
 install the packages are already defined, and they generally work well
 for most Python-based packages. However, when required, it is still
diff --git a/package/pkg-python.mk b/package/pkg-python.mk
index 512ef66..178b494 100644
--- a/package/pkg-python.mk
+++ b/package/pkg-python.mk
@@ -143,11 +143,29 @@ $(2)_DEPENDENCIES ?= $(filter-out host-python host-python3 host-python-setuptool
 # Target packages need both the python interpreter on the target (for
 # runtime) and the python interpreter on the host (for
 # compilation). However, host packages only need the python
-# interpreter on the host.
+# interpreter on the host, whose version may be enforced by setting
+# the *_FORCE_HOST_PYTHON variable.
+#
+# So:
+# - for target packages, we always depend on the default python interpreter
+#   (the one selected by the config);
+# - for host packages:
+#   - if *_FORCE_HOST_PYTHON is not set, then we depend on use the default
+#     interperter;
+#   - otherwise, we depend on the one requested by *_FORCE_HOST_PYTHON.
+#
 ifeq ($(4),target)
 $(2)_DEPENDENCIES += $(if $(BR2_PACKAGE_PYTHON3),host-python3 python3,host-python python)
 else
+ifeq ($($(2)_FORCE_HOST_PYTHON),)
 $(2)_DEPENDENCIES += $(if $(BR2_PACKAGE_PYTHON3),host-python3,host-python)
+else
+ifeq ($($(2)_FORCE_HOST_PYTHON),python2)
+$(2)_DEPENDENCIES += host-python
+else
+$(2)_DEPENDENCIES += host-$$($(2)_FORCE_HOST_PYTHON)
+endif
+endif
 endif
 
 # Setuptools based packages will need host-python-setuptools (both
@@ -161,6 +179,30 @@ $(2)_DEPENDENCIES += host-python-setuptools
 endif
 endif
 
+# Python interpreter to use for building the package.
+#
+# We may want to specify the python interpreter toi be used for building a
+# package, especially for host-packages (target packages must be built using
+# the same version of the interpreter as the one installed on the target).
+#
+# So:
+# - for target packages, we always use the default python interpreter (which
+#   is the same version as the one built and installed on the target);
+# - for host packages:
+#   - if *_FORCE_HOST_PYTHON is not set, then we use use the default
+#     interperter;
+#   - otherwise, we use the one requested by *_FORCE_HOST_PYTHON.
+#
+ifeq ($(4),target)
+$(2)_PYTHON_INTERPRETER = $(HOST_DIR)/usr/bin/python
+else
+ifeq ($($(2)_FORCE_HOST_PYTHON),)
+$(2)_PYTHON_INTERPRETER = $(HOST_DIR)/usr/bin/python
+else
+$(2)_PYTHON_INTERPRETER = $(HOST_DIR)/usr/bin/$($(2)_FORCE_HOST_PYTHON)
+endif
+endif
+
 #
 # Build step. Only define it if not already defined by the package .mk
 # file.
@@ -169,7 +211,7 @@ ifndef $(2)_BUILD_CMDS
 define $(2)_BUILD_CMDS
 	(cd $$($$(PKG)_BUILDDIR)/; \
 		$$($$(PKG)_BASE_ENV) $$($$(PKG)_ENV) \
-		$(HOST_DIR)/usr/bin/python setup.py \
+		$$($(2)_PYTHON_INTERPRETER) setup.py \
 		$$($$(PKG)_BASE_BUILD_TGT) \
 		$$($$(PKG)_BASE_BUILD_OPT) $$($$(PKG)_BUILD_OPT))
 endef
@@ -183,7 +225,7 @@ ifndef $(2)_INSTALL_CMDS
 define $(2)_INSTALL_CMDS
 	(cd $$($$(PKG)_BUILDDIR)/; \
 		$$($$(PKG)_BASE_ENV) $$($$(PKG)_ENV) \
-		$(HOST_DIR)/usr/bin/python setup.py install \
+		$$($(2)_PYTHON_INTERPRETER) setup.py install \
 		$$($$(PKG)_BASE_INSTALL_OPT) $$($$(PKG)_INSTALL_OPT))
 endef
 endif
@@ -196,7 +238,7 @@ ifndef $(2)_INSTALL_TARGET_CMDS
 define $(2)_INSTALL_TARGET_CMDS
 	(cd $$($$(PKG)_BUILDDIR)/; \
 		$$($$(PKG)_BASE_ENV) $$($$(PKG)_ENV) \
-		$(HOST_DIR)/usr/bin/python setup.py install \
+		$$($(2)_PYTHON_INTERPRETER) setup.py install \
 		$$($$(PKG)_BASE_INSTALL_OPT) $$($$(PKG)_INSTALL_OPT))
 endef
 endif
-- 
1.9.0