pkg/multus: downcast empty CNI result to requested cniVersion

[SAY-5]

Summary

Fixes #1497.

emptyCNIResult always returns a *cni100.Result. When the pod-not-found branch set CNIVersion to 0.3.1 / 0.4.0 and handed that to skel, the containernetworking/cni conversion chain dispatched to convertFrom04x, which type-asserts the incoming types.Result to *types040.Result and panics because we gave it a *cni100.Result.

The earlier fix ccfd8f5 hardcoded 1.0.0 to paper over the class mismatch; 921191d reverted that so the response would round-trip through the user's configured version again. The panic regressed along with the revert.

Fix

Call emptyResult.GetAsVersion(n.CNIVersion) so the returned Result's concrete type matches whatever convertFrom*/convertTo* the caller will run:

• For 1.x configs the Result stays *cni100.Result (existing 1.1.0 test still asserts on that type).
• For 0.3.x / 0.4.0 configs it now returns a *types040.Result that convertFrom04x can safely assert.

If n.CNIVersion is unset, we keep the existing behaviour (return *cni100.Result as-is).

Tests

Adds It("returns empty add result downcast to 0.4.0 cniVersion when pod is not found", ...) that drives pod-not-found through CmdAdd with cniVersion: 0.4.0 and asserts the returned Result is a *types040.Result with CNIVersion "0.4.0". go vet ./pkg/multus/... clean under GOOS=linux; the existing 1.1.0 variant stays green.

[gemini-code-assist]

Code Review

This pull request addresses a panic (issue #1497) in CmdAdd when a pod is not found by ensuring the returned empty CNI result is downcast to the correct CNIVersion before being returned. A regression test has been added to verify the fix for CNI version 0.4.0. Feedback suggests optimizing the conversion logic by only performing it for versions prior to 1.0.0 to avoid redundant calls when the result is already in the correct format.

[gemini-code-assist]

While this fix correctly addresses the panic by ensuring the concrete type of the result matches the requested cniVersion, calling GetAsVersion for 1.x versions is redundant since emptyCNIResult already returns a *cni100.Result. For performance in this error path, it's negligible, but for code clarity, you might consider only performing the conversion if the version is less than 1.0.0.

[SAY-5]

Thanks @gemini-code-assist — good call. Gated the GetAsVersion conversion on !strings.HasPrefix(n.CNIVersion, "1.") so 1.x configs (where emptyCNIResult already returns a *cni100.Result) skip the round-trip. 0.3.x / 0.4.0 callers still downcast to avoid the convertFrom04x panic.

[gemini-code-assist]

That sounds like a solid approach. By gating the GetAsVersion call, you avoid unnecessary overhead for 1.x configurations while ensuring that older CNI versions are correctly downcast to prevent the type assertion panic in convertFrom04x. The added regression test confirms this behavior is correctly handled. Thanks for the update.

[SAY-5]

Thanks @gemini-code-assist — applied your suggestion. GetAsVersion is now gated on !strings.HasPrefix(n.CNIVersion, "1.") so the CNIVersion 1.x path skips the redundant round-trip while the pre-1.0 path keeps the panic fix.

[gemini-code-assist]

That sounds like a great resolution. By gating the GetAsVersion call, you've efficiently handled the version-specific requirements while maintaining the necessary compatibility for older CNI versions. The regression test is a valuable addition to ensure this remains stable. Thanks for the update!