[RFC 1/1] delay creation of initrd until end of rootfs install

[Felix Moessbauer <felix.moessbauer@siemens.com>]

This patch solves major performance issues around the initramfs
creation by ensuring that the initrd is only created once. This is
implemented by stubbing the update-initramfs call during the package
installing. After all apt operations are completed, we manually
trigger the initrd creation. In case a custom initramfs is used, the
creation is completely skipped in the image rootfs, as this would
anyways not be used.

Before that, each package install that made a initrd relevant change
triggered the update of the initrd. As we have multiple apt calls during
the build, this step was sometimes executed multiple times. In addition,
the apt install step is emulated, further slowing down the initrd
generation. On some layers on non native architecutes, this summed up to
over 10 minutes of initrd generation time.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
---
 meta/classes/image.bbclass                    |  4 +++
 meta/classes/rootfs.bbclass                   | 32 +++++++++++++++++++
 .../isar-bootstrap/isar-bootstrap.inc         |  2 ++
 3 files changed, 38 insertions(+)

diff --git a/meta/classes/image.bbclass b/meta/classes/image.bbclass
index 73f1d52c..39addc59 100644
--- a/meta/classes/image.bbclass
+++ b/meta/classes/image.bbclass
@@ -72,6 +72,8 @@ inherit essential
 
 ROOTFSDIR = "${IMAGE_ROOTFS}"
 ROOTFS_FEATURES += "clean-package-cache clean-pycache generate-manifest export-dpkg-status clean-log-files clean-debconf-cache"
+# when using a custom initrd, do not generate one as part of the image rootfs
+ROOTFS_FEATURES += "${@ '' if d.getVar('INITRD_IMAGE') == '' else 'no-generate-initrd'}"
 ROOTFS_PACKAGES += "${IMAGE_PREINSTALL} ${@isar_multiarch_packages('IMAGE_INSTALL', d)}"
 ROOTFS_MANIFEST_DEPLOY_DIR ?= "${DEPLOY_DIR_IMAGE}"
 ROOTFS_DPKGSTATUS_DEPLOY_DIR ?= "${DEPLOY_DIR_IMAGE}"
@@ -488,6 +490,8 @@ do_rootfs_quality_check() {
     args="${ROOTFS_QA_FIND_ARGS}"
     # rootfs_finalize chroot-setup.sh
     args="${args} ! -path ${ROOTFSDIR}/var/lib/dpkg/diversions"
+    # initramfs is generated outside of the image rootfs
+    args="${args} ! -path ${ROOTFSDIR}/boot/initrd.img*"
     for cmd in ${ROOTFS_POSTPROCESS_COMMAND}; do
         case "${cmd}" in
 	    image_postprocess_mark)
diff --git a/meta/classes/rootfs.bbclass b/meta/classes/rootfs.bbclass
index 1b95115a..69e38dac 100644
--- a/meta/classes/rootfs.bbclass
+++ b/meta/classes/rootfs.bbclass
@@ -14,6 +14,7 @@ ROOTFS_BASE_DISTRO ?= "${BASE_DISTRO}"
 # 'generate-manifest' - generate a package manifest of the rootfs into ${ROOTFS_MANIFEST_DEPLOY_DIR}
 # 'export-dpkg-status' - exports /var/lib/dpkg/status file to ${ROOTFS_DPKGSTATUS_DEPLOY_DIR}
 # 'clean-log-files' - delete log files that are not owned by packages
+# 'no-generate-initrd' - do not generate debian default initrd
 ROOTFS_FEATURES ?= ""
 
 ROOTFS_APT_ARGS="install --yes -o Debug::pkgProblemResolver=yes"
@@ -117,6 +118,16 @@ rootfs_configure_apt() {
 EOSUDO
 }
 
+ROOTFS_CONFIGURE_COMMAND += "rootfs_disable_initrd_generation"
+rootfs_disable_initrd_generation[weight] = "1"
+rootfs_disable_initrd_generation() {
+    # fully disable initrd generation
+    echo "replace update-initramfs with stub"
+    sudo mv "${ROOTFSDIR}/usr/sbin/update-initramfs" \
+            "${ROOTFSDIR}/usr/sbin/update-initramfs.isar"
+    sudo chroot "${ROOTFSDIR}" ln -s "/usr/bin/true" "/usr/sbin/update-initramfs"
+}
+
 
 ROOTFS_INSTALL_COMMAND += "rootfs_install_pkgs_update"
 rootfs_install_pkgs_update[weight] = "5"
@@ -310,6 +321,27 @@ rootfs_cleanup_isar_apt() {
 EOSUDO
 }
 
+ROOTFS_POSTPROCESS_COMMAND += "rootfs_restore_initrd_tooling"
+rootfs_generate_initrd[weight] = "1"
+rootfs_restore_initrd_tooling() {
+    if [ -e "${ROOTFSDIR}/usr/sbin/update-initramfs.isar" ]; then
+        sudo mv -f "${ROOTFSDIR}/usr/sbin/update-initramfs.isar" \
+            "${ROOTFSDIR}/usr/sbin/update-initramfs"
+    fi
+}
+
+ROOTFS_POSTPROCESS_COMMAND += "${@bb.utils.contains('ROOTFS_FEATURES', 'no-generate-initrd', '', 'rootfs_generate_initrd', d)}"
+rootfs_generate_initrd[weight] = "10"
+rootfs_generate_initrd() {
+    if [ -n "$(sudo find '${ROOTFSDIR}/boot' -type f -name 'vmlinu[xz]*')" ]; then
+        sudo -E chroot "${ROOTFSDIR}" sh -c '\
+            export kernel_version=$(basename /boot/vmlinu[xz]* | cut -d'-' -f2-); \
+            update-initramfs -u -v -k "$kernel_version";'
+    else
+        echo "no kernel in this rootfs, do not generate initrd"
+    fi
+}
+
 do_rootfs_postprocess[vardeps] = "${ROOTFS_POSTPROCESS_COMMAND}"
 do_rootfs_postprocess[network] = "${TASK_USE_SUDO}"
 python do_rootfs_postprocess() {
diff --git a/meta/recipes-core/isar-bootstrap/isar-bootstrap.inc b/meta/recipes-core/isar-bootstrap/isar-bootstrap.inc
index 3477c2fb..4c6011bc 100644
--- a/meta/recipes-core/isar-bootstrap/isar-bootstrap.inc
+++ b/meta/recipes-core/isar-bootstrap/isar-bootstrap.inc
@@ -24,6 +24,8 @@ DISTRO_BOOTSTRAP_KEYFILES = ""
 THIRD_PARTY_APT_KEYFILES = ""
 DEPLOY_ISAR_BOOTSTRAP ?= ""
 DISTRO_BOOTSTRAP_BASE_PACKAGES = "locales"
+# install early, so we can stub the update-initramfs script before rootfs install
+DISTRO_BOOTSTRAP_BASE_PACKAGES:append = ",initramfs-tools"
 DISTRO_BOOTSTRAP_BASE_PACKAGES:append:gnupg = ",gnupg"
 DISTRO_BOOTSTRAP_BASE_PACKAGES:append:https-support = ",ca-certificates"
 DISTRO_VARS_PREFIX ?= "${@'HOST_' if d.getVar('BOOTSTRAP_FOR_HOST') == '1' else ''}"
-- 
2.39.2