OCPBUGS-80970: MCS fallback to latest v3

[pablintino]

- What I did

When an Ignition client requests a v3 minor version higher than the one MCS supports (e.g. 3.6.0), return the latest supported v3 version (currently 3.5.0) instead of erroring. This is safe because Ignition v3 spec versions are backward-compatible within the same major. Requests for a different major (e.g. v4) still error as before.

- How to verify it

E2Es should suffice

- Description for the changelog

MCS now returns the latest supported Ignition v3 config instead of erroring when a client requests an unsupported v3.x minor version.

[openshift-ci-robot]

@pablintino: This pull request references MCO-2182 which is a valid jira issue.

Details

In response to this:

- What I did

When an Ignition client requests a v3 minor version higher than the one MCS supports (e.g. 3.6.0), return the latest supported v3 version (currently 3.5.0) instead of erroring. This is safe because Ignition v3 spec versions are backward-compatible within the same major. Requests for a different major (e.g. v4) still error as before.

- How to verify it

E2Es should suffice

- Description for the changelog

MCS now returns the latest supported Ignition v3 config instead of erroring when a client requests an unsupported v3.x minor version.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.

[coderabbitai]

Walkthrough

The pull request exports three previously unexported error sentinel variables from the ignition converter module, updates all internal references to use these exported constants, and adds fallback logic in the API layer to return the maximum supported Ignition version when unsupported minor versions are requested on a supported major version.

Changes

Cohort / File(s)	Summary
Ignition Converter Error Export pkg/controller/common/ignition_converter.go	Renamed three package-level error variables from unexported to exported: errIgnitionConverterWrongSourceType → ErrIgnitionConverterWrongSourceType, errIgnitionConverterUnknownVersion → ErrIgnitionConverterUnknownVersion, errIgnitionConverterUnsupportedConversion → ErrIgnitionConverterUnsupportedConversion. Updated all return statements and error checks to reference the new exported constants. Updated interface documentation comments accordingly.
Controller Test Updates pkg/controller/common/helpers_test.go	Updated test assertions in TestIgnitionConverterGetSupportedMinorVersion and TestIgnitionConverterConvert to check against the newly exported error constants instead of their previous unexported counterparts.
API Layer Version Fallback Logic pkg/server/api.go	Added conditional logic to detectSpecVersionFromAcceptHeader that handles ErrIgnitionConverterUnknownVersion by checking if the requested major version matches the maximum supported major version with a higher minor; if so, returns the maximum supported version instead of propagating the error.
API Test Coverage pkg/server/api_test.go	Expanded TestAcceptHeaders with two new test scenarios: one for unsupported higher minor version on supported major version (expects successful fallback to max version), and one for completely unsupported major version (expects error). Updated test struct to include errExpected field for conditional error assertion.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[pablintino]

/test e2e-agent-compact-ipv4
/test e2e-agent-compact-ipv4-iso-no-registry

[coderabbitai]

🧹 Nitpick comments (1)

pkg/server/api.go (1)

295-302: Avoid a second source of truth for the max supported v3.

pkg/controller/common/ignition_converter.go already owns the supported-version set. Hard-coding ign3types.MaxVersion here can drift on the next v3 bump and make negotiation return an older version than the converter actually supports. Consider deriving the fallback target from the converter instead.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@pkg/server/api.go` around lines 295 - 302, The code currently returns
ign3types.MaxVersion as a fallback which duplicates the supported-version truth;
instead, derive the fallback from the ignition converter owned in
pkg/controller/common/ignition_converter.go—when detecting
ctrlcommon.ErrIgnitionConverterUnknownVersion and newerMinorRequested, call the
converter's exported API to obtain its max supported v3 version (rather than
using ign3types.MaxVersion) and return that value so negotiation always reflects
the converter's actual supported set.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@pkg/server/api.go`:
- Around line 295-302: The code currently returns ign3types.MaxVersion as a
fallback which duplicates the supported-version truth; instead, derive the
fallback from the ignition converter owned in
pkg/controller/common/ignition_converter.go—when detecting
ctrlcommon.ErrIgnitionConverterUnknownVersion and newerMinorRequested, call the
converter's exported API to obtain its max supported v3 version (rather than
using ign3types.MaxVersion) and return that value so negotiation always reflects
the converter's actual supported set.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 6f45236c-b330-4a44-abe0-0311e35f8278

📥 Commits

Reviewing files that changed from the base of the PR and between cf6b532 and 9a0ff42.

📒 Files selected for processing (4)

• pkg/controller/common/helpers_test.go
• pkg/controller/common/ignition_converter.go
• pkg/server/api.go
• pkg/server/api_test.go

[pablintino]

Note for the reviewer: I'll probably change the behavior of GetSupportedMinorVersion to handle this and return the lastest supported minor for the given major version. Any version less than 2.2 will still be rejected here.

[bfournie]

/lgtm
I have confirmed this works in my local setup. I now see the master-1 and master-2 pulling ignition normally and master-0 rebooting into the control plane

level=info msg=Host: master-2, reached installation stage Rebooting
level=info msg=Host: master-1, reached installation stage Rebooting
level=info msg=Host: master-0, reached installation stage Waiting for control plane: Waiting for masters to join bootstrap control plane
level=info msg=Host: master-2, reached installation stage Configuring
level=info msg=Host: master-2, reached installation stage Joined
level=info msg=Host: master-0, reached installation stage Waiting for bootkube
level=info msg=Host: master-2, reached installation stage Done
level=info msg=Node master-2 has been rebooted 1 times before completing installation
level=info msg=Node master-1 has been rebooted 1 times before completing installation

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bfournie, pablintino

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [pablintino]

Approvers can indicate their approval by writing /approve in a comment
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[pablintino]

/verified by @bfournie

[openshift-ci-robot]

@pablintino: This PR has been marked as verified by @bfournie.

Details

In response to this:

/verified by @bfournie

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.

[pablintino]

/retitle OCPBUGS-80970: MCS fallback to latest v3

[openshift-ci-robot]

@pablintino: This pull request references Jira Issue OCPBUGS-80970, which is valid.

3 validation(s) were run on this bug

• bug is open, matching expected state (open)
• bug target version (4.22.0) matches configured target version for branch (4.22.0)
• bug is in the state POST, which is one of the valid states (NEW, ASSIGNED, POST)

The bug has been updated to refer to the pull request using the external bug tracker.

Details

In response to this:

- What I did

When an Ignition client requests a v3 minor version higher than the one MCS supports (e.g. 3.6.0), return the latest supported v3 version (currently 3.5.0) instead of erroring. This is safe because Ignition v3 spec versions are backward-compatible within the same major. Requests for a different major (e.g. v4) still error as before.

- How to verify it

E2Es should suffice

- Description for the changelog

MCS now returns the latest supported Ignition v3 config instead of erroring when a client requests an unsupported v3.x minor version.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.

[pablintino]

/jira refresh

[openshift-ci-robot]

@pablintino: This pull request references Jira Issue OCPBUGS-80970, which is valid.

3 validation(s) were run on this bug

• bug is open, matching expected state (open)
• bug target version (4.22.0) matches configured target version for branch (4.22.0)
• bug is in the state POST, which is one of the valid states (NEW, ASSIGNED, POST)

Details

In response to this:

/jira refresh

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.

[pablintino]

/test e2e-gcp-op-ocl-part1

[openshift-ci-robot]

@pablintino: Jira Issue OCPBUGS-80970: Some pull requests linked via external trackers have merged:

The following pull request, linked via external tracker, has not merged:

• openshift/machine-config-operator#5815 is open

All associated pull requests must be merged or unlinked from the Jira bug in order for it to move to the next state. Once unlinked, request a bug refresh with /jira refresh.

Jira Issue OCPBUGS-80970 has not been moved to the MODIFIED state.

This PR is marked as verified. If the remaining PRs listed above are marked as verified before merging, the issue will automatically be moved to VERIFIED after all of the changes from the PRs are available in an accepted nightly payload.

Details

In response to this:

- What I did

When an Ignition client requests a v3 minor version higher than the one MCS supports (e.g. 3.6.0), return the latest supported v3 version (currently 3.5.0) instead of erroring. This is safe because Ignition v3 spec versions are backward-compatible within the same major. Requests for a different major (e.g. v4) still error as before.

- How to verify it

E2Es should suffice

- Description for the changelog

MCS now returns the latest supported Ignition v3 config instead of erroring when a client requests an unsupported v3.x minor version.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.

[openshift-ci]

@pablintino: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-agent-compact-ipv4	9a0ff42	link	false	/test e2e-agent-compact-ipv4

Full PR test history. Your PR dashboard.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.

[isabella-janssen]

/jira refresh

[openshift-merge-robot]

Fix included in accepted release 4.22.0-0.nightly-2026-03-29-055437

[pablintino]

/cherry-pick release-4.21

[openshift-cherrypick-robot]

@pablintino: new pull request created: #5883

Details

In response to this:

/cherry-pick release-4.21

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.