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Troubles with dependencies on openapi

If you see the error

cannot use api.Schema.SchemaProps.Properties

then you have a set of incompatible dependencies. This doc describes the problem and a fix.

tl;dr A mix of old and new is bad.

Anyone depending on k8s.io v0.20.x or older packages must avoid depending on anything that depends on k8s.io/kube-openapi newer than v0.0.0-20210323165736-1a6458611d18.

More context in https://github.com/kubernetes/cli-runtime/issues/19

This dir exists to test the problem.

Edit the main.go and go.mod in this dir to see what builds with various combinations of cli-runtime and kube-openapi.

A recent in change in kube-openapi

https://github.com/kubernetes/kube-openapi/pull/234

means that anyone depending on

k8s.io/cli-runtime@v0.20.4

and any other package that imports kube-openapi (e.g. kyaml) may see a build error like

~/go/pkg/mod/sigs.k8s.io/kustomize@v2.0.3+incompatible/pkg/transformers/config/factorycrd.go:71:47: cannot use api.Schema.SchemaProps.Properties (type map[string]"k8s.io/kube-openapi/pkg/validation/spec".Schema) as type myProperties in argument to looksLikeAk8sType

Why?

As it happens,

k8s.io/cli-runtime@v0.20.4

depends on

sigs.k8s.io/kustomize@v2.0.3+incompatible

Line 71 of factorycrd.go in kustomize v2.0.3 is:

if !looksLikeAk8sType(api.Schema.SchemaProps.Properties) {

The looksLikeAk8sType function accepts the argument

func looksLikeAk8sType(properties map[string]spec.Schema) bool {...}

At the call point in line 71 the argument is

common.OpenAPIDefinition.Schema.SchemaProps.Properties

The file factorycrd.go depends on

"github.com/go-openapi/spec"
"k8s.io/kube-openapi/pkg/common"

The module sigs.k8s.io/kustomize@v2.0.3 predates Go modules.

To pin its dependencies, it has a Gopkg.lock file and a vendor directory. Per the lock file:

sigs.k8s.io/kustomize@v2.0.3
depends on "k8s.io/kube-openapi"
revision = "b3f03f55328800731ce03a164b80973014ecd455"

Checking out this commit in the k8s.io/kube-openapi repo we see this

k8s.io/kube-openapi/pkg/common/common.go:
import  "github.com/go-openapi/spec"
...
type OpenAPIDefinition struct {
  Schema       spec.Schema 
  Dependencies []string
}

But per the imports in this file, spec.Schema lives in

github.com/go-openapi/spec

The aforementioned Gopkg.lock file pins that at

sigs.k8s.io/kustomize@v2.0.3
depends on "github.com/go-openapi/spec"
revision = "bcff419492eeeb01f76e77d2ebc714dc97b607f5"

The struct is

github.com/go-openapi/spec:
type Schema struct {
  VendorExtensible
  SchemaProps
  SwaggerSchemaProps
  ExtraProps map[string]interface{} `json:"-"`
}

type SchemaProps struct {
  Properties map[string]Schema
}

This is a recursive type; Schema holds a map[string]Schema.

All that is fine.

The problem arises when we build a binary that depends on both kustomize v2.0.3 and, say,

k8s.io/kube-openapi v0.0.0-20210421082810-95288971da7e

This particular version of kube-openapi has a 'go.mod' file.

kube-openapi/pkg/common/common.go at tag "95288..." contains:

import "k8s.io/kube-openapi/pkg/validation/spec"
...
type OpenAPIDefinition struct {
  Schema       spec.Schema 
  Dependencies []string
}

kube-openapi/pkg/validation/spec/schema.go at this tag contains:

type Schema struct {
  VendorExtensible
  SchemaProps
  SwaggerSchemaProps
  ExtraProps map[string]interface{} `json:"-"`

}

etc. etc. as above. The same layout as above, but in different files.

So adding this new dependency means that

factorycrd.go 

is going to include a version of "k8s.io/kube-openapi/pkg/common". that deals in the type

"k8s.io/kube-openapi/pkg/validation/spec".Schema

but will then attempt to pass this type to older code (the func looksLikeAk8sType) which is looking to accept a

"github.com/go-openapi/spec".Schema

It's the same type structure, but a different name so the 'linker' barfs.

To avoid this problem, one has to either

• roll forward on cli-runtime -- depend on v0.21.0 or higher.

• or stick with v0.20.4, which means retaining consistency with kustomize 2.0.3, which means depending on an older version of k8s.io/kube-openapi that still depends on go-openapi/spec.

  Fortunately you only have to go back to before PR https://github.com/kubernetes/kube-openapi/pull/234

  E.g. depend on k8s.io/kube-openapi v0.0.0-20210323165736-1a6458611d18