Create rpm/README.md

rpm/README.md

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+# HOW-TO Maintain RPM lists for containers when adding new target platforms and architectures
+
+## Overview
+
+The file `rpms/BUILD.bazel` provides a list pinned rpms to be built in each category of CDI containers
+
+These are maintained with bazeldnf with rpm repos and names of packages  specified in the following three files:
+
+ 1. `.bazelrc`
+ 2. `repo.yaml`
+ 3. `hack/build/rpm-deps.sh` 
+
+Then, running `make rpm-deps` invokes bazeldnf installed in the bazel cdi builder container volume to populate the `rpm/BUILD.bazel` file 
+
+## Prerequisites and Caveats
+
+`bazeldnf` is not provided in the persistent CDI build container; it is built and installed from a pinned source tarball in the `make rpm-defs` step as per https://github.com/kubevirt/containerized-data-importer/blob/main/WORKSPACE#L83-L95 -- so if you _exec_ into the running container, you won't find the executable of bazeldnf in your bash path; rather it is only made available to bazel build as a package, e.g.:
+
+```
+[containerized-data-importer]$ ./hack/build/bazel-docker.sh bash
+CDI_CRI: podman, CDI_CONTAINER_BUILDCMD: buildah
+Making sure output directory exists...
+go version go1.21.5 linux/s390x
+go version go1.21.5 linux/s390x
+Starting rsyncd
+
+Rsyncing /home/cfillekes/projects/containerized-data-importer to container
+8def2759249580cd4b7880fa2a79c554611a1033fe2d3f9bcc15aa3a79008c89
+Starting bazel server
+go version go1.21.5 linux/s390x
+go version go1.21.5 linux/s390x
+[root@m1325001 containerized-data-importer]# which bazeldnf
+/usr/bin/which: no bazeldnf in (/root/.local/bin:/root/bin:/gimme/.gimme/versions/go1.21.5.linux.s390x/bin:/root/go/bin:/go/bin:/opt/gradle/gradle-6.6/bin:/gimme/.gimme/versions/go1.21.5.linux.s390x/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin)
+[root@m1325001 containerized-data-importer]# find / -name bazeldnf
+find: ‘/sys/fs/pstore’: Permission denied
+find: ‘/sys/fs/bpf’: Permission denied
+/root/.cache/bazel/_bazel_root/e03bf9038fc5089c0dbc8615812d9838/execroot/__main__/bazel-out/host/bin/external/bazeldnf
+/root/.cache/bazel/_bazel_root/e03bf9038fc5089c0dbc8615812d9838/execroot/__main__/bazel-out/host/bin/external/bazeldnf/pkg/api/bazeldnf
+/root/.cache/bazel/_bazel_root/e03bf9038fc5089c0dbc8615812d9838/execroot/__main__/bazel-out/s390x-fastbuild/bin/bazeldnf.bash.runfiles/__main__/external/bazeldnf
+/root/.cache/bazel/_bazel_root/e03bf9038fc5089c0dbc8615812d9838/execroot/__main__/bazel-out/s390x-fastbuild/bin/bazeldnf.bash.runfiles/bazeldnf
+/root/.cache/bazel/_bazel_root/e03bf9038fc5089c0dbc8615812d9838/execroot/__main__/external/bazeldnf
+/root/.cache/bazel/_bazel_root/e03bf9038fc5089c0dbc8615812d9838/external/bazeldnf
+/root/.cache/bazel/_bazel_root/e03bf9038fc5089c0dbc8615812d9838/external/bazeldnf/pkg/api/bazeldnf
+```
+
+
+## Configuring the platform, build, run and test target to maintain pinned rpm lists
+
+### add platform targets to `.bazelrc`
+
+This defines which platforms we are going to build for, and how.  For example, adding `aarch64` and `s390x` native and cross-built platform targets:
+
+```
+build:aarch64 --platforms=@io_bazel_rules_go//go/toolchain:linux_arm64_cgo --incompatible_use_cc_configure_from_rules_cc
+run:aarch64 --platforms=@io_bazel_rules_go//go/toolchain:linux_arm64_cgo --incompatible_use_cc_configure_from_rules_cc
+test:aarch64 --platforms=@io_bazel_rules_go//go/toolchain:linux_arm64_cgo --host_javabase=@local_jdk//:jdk
+
+build:crossbuild-aarch64 --incompatible_enable_cc_toolchain_resolution --platforms=//bazel/platforms:aarch64-none-linux-gnu --platforms=@io_bazel_rules_go//go/toolchain:lin
+ux_arm64_cgo
+run:crossbuild-aarch64  --incompatible_enable_cc_toolchain_resolution --platforms=//bazel/platforms:aarch64-none-linux-gnu --platforms=@io_bazel_rules_go//go/toolchain:linu
+x_arm64_cgo
+test:crossbuild-aarch64 --platforms=@io_bazel_rules_go//go/toolchain:linux_arm64_cgo --host_javabase=@local_jdk//:jdk
+``` 
+
+NB in future, when we add native and cross-compiled targets for ppc64le and s390x, they are similarly defined in `.bazelrc`
+
+### add targets to `repo.yaml`
+
+`repo.yaml` specifies the URLs of the repositories in which the packages to be installed via rpm are to be found, e.g.:
+
+```
+- arch: aarch64
+  baseurl: http://mirror.stream.centos.org/9-stream/BaseOS/aarch64/os/
+  name: centos/stream9-baseos-aarch64
+  gpgkey: https://www.stream.centos.org/keys/RPM-GPG-KEY-CentOS-Official
+```
+
+### add targets to `hack/build/rpm-deps.sh`
+
+`hack/build/rpm-deps.sh` provides the list of dnf packages from which the rpm meta data is to be extracted for each class of container targets, and the flags to run bazeldnf with from inside the bazel build container. 
+
+```
+centos_base="
+  ca-certificates
+  crypto-policies
+  acl
+  curl
+  vim-minimal
+  util-linux-core
+"
+```
+where `centos_base` is referred to in the `bazel run //:bazeldnf` job like so:
+
+```
+bazel run \
+    --config=aarch64 \
+    //:bazeldnf -- rpmtree \
+    --public --nobest \
+    --name centos_base_aarch64 --arch aarch64 \
+    --basesystem centos-stream-release \
+    ${bazeldnf_repos} \
+    $centos_base \
+    $centos_extra
+```
+
+For further documentation on how `bazeldnf` works, please consult https://github.com/rmohr/bazeldnf