kernel plugin: properly generate Ubuntu kernel .config

Recent Ubuntu kernels (>= Jammy) are now using a new format to store
their .config, called annotations [1].

The kernel plugin is trying to assemble a .config using the old config
chunks stored in `debian.<kernel>/config/*` that are not available
anymore in the kernels that are using this new annotations model.

Instead of trying to manually assemble the config we should rely on
`debian/rules genconfigs` that is more portable across different Ubuntu
kernels and releases.

[1] https://lists.ubuntu.com/archives/kernel-team/2023-June/140230.html

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

snapcraft_legacy/plugins/v2/_kernel_build.py

@@ -278,17 +278,20 @@ def _do_base_config_cmd(
         logger.info("Using ubuntu config flavour %s", config_flavour)
         conf_cmd = textwrap.dedent(
             """	echo "Assembling Ubuntu config..."
-	branch=$(cut -d'.' -f 2- < ${{KERNEL_SRC}}/debian/debian.env)
-	baseconfigdir=${{KERNEL_SRC}}/debian.${{branch}}/config
-	archconfigdir=${{KERNEL_SRC}}/debian.${{branch}}/config/${{DEB_ARCH}}
-	ubuntuconfig=${{baseconfigdir}}/config.common.ubuntu
-	archconfig=${{archconfigdir}}/config.common.${{DEB_ARCH}}
-	flavourconfig=${{archconfigdir}}/config.flavour.{config_flavour}
-	cat ${{ubuntuconfig}} ${{archconfig}} ${{flavourconfig}} \
-> {dest_dir}/.config 2>/dev/null || true
 	if [ -f ${{KERNEL_SRC}}/debian/rules ] && [ -x ${{KERNEL_SRC}}/debian/rules ]; then
+		# Generate Ubuntu kernel configs
+		pushd ${{KERNEL_SRC}}
+		fakeroot debian/rules clean genconfigs || true
+		popd
+
+		# Pick the right kernel .config for the target arch and flavour
+		ubuntuconfig=${{KERNEL_SRC}}/CONFIGS/${{DEB_ARCH}}-config.flavour.{config_flavour}
+		cat ${{ubuntuconfig}} > ${dest_dir}/.config
+
+		# Clean up kernel source directory
 		pushd ${{KERNEL_SRC}}
 		fakeroot debian/rules clean
+		rm -rf CONFIGS/
 		popd
 	fi""".format(
                 config_flavour=config_flavour, dest_dir=dest_dir

tests/legacy/unit/plugins/v2/test_kernel.py

@@ -1302,18 +1302,14 @@ def _is_sub_array(array, sub_array):
     "if [ ! -e ${SNAPCRAFT_PART_BUILD}/.config ]; then",
     textwrap.dedent(
         """	echo "Assembling Ubuntu config..."
-	branch=$(cut -d'.' -f 2- < ${KERNEL_SRC}/debian/debian.env)
-	baseconfigdir=${KERNEL_SRC}/debian.${branch}/config
-	archconfigdir=${KERNEL_SRC}/debian.${branch}/config/${DEB_ARCH}
-	ubuntuconfig=${baseconfigdir}/config.common.ubuntu
-	archconfig=${archconfigdir}/config.common.${DEB_ARCH}
-	flavourconfig=${archconfigdir}/config.flavour.raspi
-	cat ${ubuntuconfig} ${archconfig} ${flavourconfig} \
-> ${SNAPCRAFT_PART_BUILD}/.config 2>/dev/null || true
 	if [ -f ${KERNEL_SRC}/debian/rules ] && [ -x ${KERNEL_SRC}/debian/rules ]; then
+		# Generate Ubuntu kernel configs
 		pushd ${KERNEL_SRC}
-		fakeroot debian/rules clean
+		fakeroot debian/rules clean genconfigs || true
 		popd
+		# Pick the right kernel .config for the target arch and flavour
+		ubuntuconfig=${KERNEL_SRC}/CONFIGS/${DEB_ARCH}-config.flavour.${flavourconfig}
+		cat ${ubuntuconfig} > ${SNAPCRAFT_PART_BUILD}/.config
 	fi"""
     ),
     "fi",

tests/unit/parts/plugins/test_kernel.py

@@ -1401,18 +1401,14 @@ def _is_sub_array(array, sub_array):
     "if [ ! -e ${CRAFT_PART_BUILD}/.config ]; then",
     textwrap.dedent(
         """	echo "Assembling Ubuntu config..."
-	branch=$(cut -d'.' -f 2- < ${KERNEL_SRC}/debian/debian.env)
-	baseconfigdir=${KERNEL_SRC}/debian.${branch}/config
-	archconfigdir=${KERNEL_SRC}/debian.${branch}/config/${DEB_ARCH}
-	ubuntuconfig=${baseconfigdir}/config.common.ubuntu
-	archconfig=${archconfigdir}/config.common.${DEB_ARCH}
-	flavourconfig=${archconfigdir}/config.flavour.raspi
-	cat ${ubuntuconfig} ${archconfig} ${flavourconfig} \
-> ${CRAFT_PART_BUILD}/.config 2>/dev/null || true
 	if [ -f ${KERNEL_SRC}/debian/rules ] && [ -x ${KERNEL_SRC}/debian/rules ]; then
+		# Generate Ubuntu kernel configs
 		pushd ${KERNEL_SRC}
-		fakeroot debian/rules clean
+		fakeroot debian/rules clean genconfigs || true
 		popd
+		# Pick the right kernel .config for the target arch and flavour
+		ubuntuconfig=${KERNEL_SRC}/CONFIGS/${DEB_ARCH}-config.flavour.${flavourconfig}
+		cat ${ubuntuconfig} > ${CRAFT_PART_BUILD}/.config
 	fi"""
     ),
     "fi",