Installer/MCO: store pointer ignition customizations in MachineConfig

This is an alternative to #467

enhancements/machine-config/custom-ignition-machineconfig.md

@@ -0,0 +1,199 @@
+---
+title: Store user ignition customizations in MachineConfig
+authors:
+  - "@hardys"
+  - "@celebdor"
+reviewers:
+  - "@celebdor"
+  - "@cgwalters"
+  - "@crawford"
+  - "@kirankt"
+  - "@runcom"
+  - "@stbenjam"
+  - "@yuqi-zhang"
+approvers:
+  - "@cgwalters"
+  - "@crawford"
+  - "@runcom"
+  - "@yuqi-zhang"
+
+creation-date: 2020-08-21
+last-updated: 2020-11-13
+status: implementable
+---
+
+# Store user ignition customizations in MachineConfig
+
+## Release Signoff Checklist
+
+- [x] Enhancement is `implementable`
+- [ ] Design details are appropriately documented from clear requirements
+- [ ] Test plan is defined
+- [ ] Graduation criteria for dev preview, tech preview, GA
+- [ ] User-facing documentation is created in [openshift-docs](https://github.com/openshift/openshift-docs/)
+
+## Summary
+
+This enhancement proposes changing the way installer ignition customizations
+are stored, so that instead of storing the modified pointer ignition in a
+Secret we include the user changes in a MachineConfig, such that the MCO
+can manage it, and it is included in the MCO rendered config.
+
+## Motivation
+
+The installer supports user [modification of the pointer ignition config](https://github.com/openshift/installer/blob/master/docs/user/customization.md#os-customization-unvalidated) it generates.  While this interface is marked as unvalidated, we know from [previous bug reports](https://bugzilla.redhat.com/show_bug.cgi?id=1881703) that some users are using it.
+
+This presents a problem for the plans to have the [MCO manage the pointer config](https://github.com/openshift/enhancements/blob/master/enhancements/machine-config/user-data-secret-managed.md)
+because it uses a static template that does not consider any user customizations, and for that reason the work was [reverted](https://github.com/openshift/machine-config-operator/pull/2126).
+
+Additionally, in some situations it is necessary to perform network configuration
+before it is possible to download the rendered config from the MCS in the ramdisk
+where ignition evaluates the pointer config.
+
+This leads to a chicken-egg problem, where a user could configure the network
+via ignition, but ignition cannot apply any config until it has downloaded
+any append/merge directives.
+
+We could solve that problem on some platforms (baremetal in particular) by
+just providing the fully rendered config to each host, bypassing the pointer
+config where config-drive size limits allow.  However this results in the [same
+issue with losing any user customizations](https://bugzilla.redhat.com/show_bug.cgi?id=1833483) provided directly via the pointer config.
+
+Discussions indicate we may want to deprecate/remove this ignition-config method of
+customization and mandate MachineConfig manifests instead.  We could in the
+meantime do that internally e.g have the installer detect any customization
+to the pointer config, and inject a MachineConfig manifest containing that data
+instead of writing it via the current user-data Secret.
+
+### Goals
+
+ * Unblock the MCO managed pointer ignition work that was previously reverted
+
+ * Provide a solution to customer requirements for baremetal IPI around bond/VLAN
+   and other configurations for controlplane networking
+
+ * Provide a means by which we might warn the user if we decide to deprecate customization via the pointer ignition config.
+
+### Non-Goals
+
+This work may give us some options wrt warnings around this interface but it
+does not aim to formally deprecate the ignition-configs customization, that
+may be separately discussed in a future enhancement.
+
+The discussion of interfaces here and how they relate to network configuration
+ only considers the existing interfaces, not proposals around a future
+[declarative network configuration](https://github.com/openshift/enhancements/pull/399) although we need to ensure the approach taken in each case is aligned.
+
+### User Stories
+
+#### Story 1
+
+As a user I would like my existing customizations to pointer ignition files
+to work, but be warned if there is a preferable interface I should be using.
+
+#### Story 2
+
+As a baremetal IPI user, I need to deploy in an environment where network
+configuration is required before access to the controlplane network is possible.
+
+Specifically I wish to deploy in an environment where the controlplane is
+on a non-default VLAN (a common configuration for baremetal).
+
+Currently I cannot perform this configuration via ignition or MachineConfig,
+because the pointer config requires network access prior to any configuration
+being performed on the host.
+
+## Design Details
+
+If the installer can detect the case where the pointer config loaded at `create cluster` contains
+user customizations we can create a new MachineConfig object to encapsulate this config,
+instead of persisting it to the user-data secret directly.
+
+This would be equivalent to the user creating those same customizations via a MachineConfig
+manifests, and we could potentially warn users to guide them to that interface,
+but avoid breaking any existing users performing customizations directly via
+the pointer ignition.
+
+This would mean we can potentially restore the [MCO managed pointer ignition](openshift/machine-config-operator#1792)
+work, where the MCO will maintain a templated pointer config, and any user customizations
+will be persisted in the existing rendered config.
+
+This would also solve the issue for IPI baremetal without any [MCO API changes](https://github.com/openshift/enhancements/pull/467), since we could consume the existing rendered config directly.
+
+### Test Plan
+
+  * Ensure no regressions in existing deployments via existing e2e-metal-ipi CI coverage
+  * Prove we can do deployments where the controlplane network requires configuration, such as that described in [previous bug reports](https://bugzilla.redhat.com/show_bug.cgi?id=1824331) - we can add a periodic CI job based on e2e-metal-ipi to test this.
+
+
+### Upgrade / Downgrade Strategy
+
+This change only impacts the day1 deployment, since the changes are in
+the installer.
+
+If the MCO managed pointer ignition config work is restored, the upgrade
+strategy will be defined in https://github.com/openshift/enhancements/blob/master/enhancements/machine-config/user-data-secret-managed.md
+
+### Version Skew Strategy
+
+## Alternatives
+
+### Native ignition support for early-network config
+
+This was initially proposed via [an ignition issue](https://github.com/coreos/ignition/issues/979)
+where we had some good discussion, but ultimately IIUC the idea of adding a new
+interface for early network configuration to ignition was rejected.
+
+### Implement MCO support for a flattened ignition config
+
+This was discussed in https://github.com/openshift/enhancements/pull/467 and
+the solution documented here is derived from discussion on that PR.
+
+### Implement config flattening in platform-specific repos
+
+The ignition configuration is consumed at two points during deployment for IPI baremetal
+deployments:
+
+  * Via terraform (when the installer deploys the controlplane
+    nodes using [terraform-provider-ironic](https://github.com/openshift-metal3/terraform-provider-ironic).
+  * Workers are then deployed via the machine-api, using a
+    [baremetal Cluster API Provider](https://github.com/openshift/cluster-api-provider-baremetal/)
+
+The ironic terraform provider is designed to be generic, and thus we would prefer
+not to add OS specific handling of the user-data there, and in addition [previous
+prototying of a new provider](https://github.com/openshift-metal3/terraform-provider-openshift)
+indicates that due to terraform limitations it is not possible to
+pass the data between providers/resources in the way that would be required.
+
+### Add support for injecting NM config to IPI
+
+This might be possible using a recently added [ironic python agent feature](https://specs.openstack.org/openstack/ironic-specs/specs/approved/in-band-deploy-steps.html)
+using that interface, it could be possible to inject network configuration in a
+similar way to coreos-install.
+
+However this is a very bleeding edge feature (not yet documented), and it means
+maintaining a custom interface that would be different to both the proven
+uncustomized OSP deploy ramdisk, and the coreos-deploy toolchain.
+
+### Convert IPI to coreos-install based deployment
+
+Long term, from an OpenShift perspective, this makes sense as there is overlap
+between the deploy tooling.
+
+However in the short/medium term, the IPI deployment components are based on
+Ironic, and that is only really tested with the ironic deploy ramdisk
+(upstream and downstream) - we need to investigate how IPI deployments may be
+adapted to converge with the coreos-install based workflow, but this is likely
+to require considerable planning and development effort to achieve, thus is not
+implementable as an immediate solution.
+
+### Customize images
+
+Currently the only option for IPI based deployments is to customize the
+OS image, then provide a locally cached copy of this custom image to the
+installer via the `clusterOSImage` install-config option.
+
+This is only a stopgap solution, it is inconvenient for the user, and
+inflexible as it will only work if the configuration required is common
+to every node in the cluster (or for multiple worker machinepools, you
+would require a custom image per pool).