buildsys, systemd: add unified-cgroup-hierarchy image feature

Dockerfile

@@ -68,6 +68,7 @@ ARG VARIANT_FLAVOR
 ARG REPO
 ARG GRUB_SET_PRIVATE_VAR
 ARG SYSTEMD_NETWORKD
+ARG UNIFIED_CGROUP_HIERARCHY
 ENV SYSTEMD_NETWORKD=${SYSTEMD_NETWORKD}
 ENV VARIANT=${VARIANT}
 WORKDIR /home/builder
@@ -102,6 +103,7 @@ RUN rpmdev-setuptree \
    && echo "%bcond_without $(V=${VARIANT_FLAVOR:-no}; V=${V,,}; echo ${V//-/_})_flavor" >> .bconds \
    && echo -e -n "${GRUB_SET_PRIVATE_VAR:+%bcond_without grub_set_private_var\n}" >> .bconds \
    && echo -e -n "${SYSTEMD_NETWORKD:+%bcond_without systemd_networkd\n}" >> .bconds \
+   && echo -e -n "${UNIFIED_CGROUP_HIERARCHY:+%bcond_without unified_cgroup_hierarchy\n}" >> .bconds \
    && cat .bconds ${PACKAGE}.spec >> rpmbuild/SPECS/${PACKAGE}.spec \
    && find . -maxdepth 1 -not -path '*/\.*' -type f -exec mv {} rpmbuild/SOURCES/ \; \
    && echo ${NOCACHE}

packages/systemd/Cargo.toml

@@ -10,6 +10,7 @@ path = "pkg.rs"
 
 [package.metadata.build-package]
 releases-url = "/~https://github.com/systemd/systemd-stable/releases"
+package-features = ["unified-cgroup-hierarchy"]
 
 [[package.metadata.build-package.external-files]]
 url = "/~https://github.com/systemd/systemd-stable/archive/v250.9/systemd-stable-250.9.tar.gz"

packages/systemd/systemd.spec

@@ -176,7 +176,11 @@ CONFIGURE_OPTS=(
  -Dpkgconfigdatadir='%{_cross_pkgconfigdir}'
  -Dpkgconfiglibdir='%{_cross_pkgconfigdir}'
 
+ %if %{with unified_cgroup_hierarchy}
+ -Ddefault-hierarchy=unified
+ %else
  -Ddefault-hierarchy=hybrid
+ %endif
 
  -Dadm-group=false
  -Dwheel-group=false

tools/buildsys/src/manifest.rs

@@ -204,6 +204,16 @@ flag is meant primarily for development, and will be removed when development ha
 [package.metadata.build-variant.image-features]
 systemd-networkd = true
 ```
+
+`unified-cgroup-hierarchy` makes systemd set up a unified cgroup hierarchy on
+boot, i.e. the host will use cgroup v2 by default. This feature flag allows
+old variants to continue booting with cgroup v1 and new variants to move to
+cgroup v2, while users will still be able to override the default via command
+line arguments set in the boot configuration.
+```
+[package.metadata.build-variant.image-features]
+unified-cgroup-hierarchy = true
+```
 */
 
 mod error;
@@ -494,6 +504,7 @@ impl SupportedArch {
 pub enum ImageFeature {
     GrubSetPrivateVar,
     SystemdNetworkd,
+    UnifiedCgroupHierarchy,
 }
 
 impl TryFrom<String> for ImageFeature {
@@ -502,6 +513,7 @@ impl TryFrom<String> for ImageFeature {
         match s.as_str() {
             "grub-set-private-var" => Ok(ImageFeature::GrubSetPrivateVar),
             "systemd-networkd" => Ok(ImageFeature::SystemdNetworkd),
+            "unified-cgroup-hierarchy" => Ok(ImageFeature::UnifiedCgroupHierarchy),
             _ => error::ParseImageFeatureSnafu { what: s }.fail()?,
         }
     }
@@ -512,6 +524,7 @@ impl fmt::Display for ImageFeature {
         match self {
             ImageFeature::GrubSetPrivateVar => write!(f, "GRUB_SET_PRIVATE_VAR"),
             ImageFeature::SystemdNetworkd => write!(f, "SYSTEMD_NETWORKD"),
+            ImageFeature::UnifiedCgroupHierarchy => write!(f, "UNIFIED_CGROUP_HIERARCHY"),
         }
     }
 }