## Summary Valid `config.toml` files produce spurious validation errors at startup, and through `config lint`. The common one: ``` at '/runners/0/kubernetes/Volumes/EmptyDirs/0/MountPropagation': got null, want string ``` It fires for any `host_path`, `pvc`, or `empty_dir` volume that omits `mount_propagation`. Validation is best-effort so the runner continues, but the warning appears on every start and teaches operators to ignore validation output. The property in the path is the Go field name, not the documented TOML key, which is the tell. ## Root cause `commands/internal/configfile/validation.go` reflects `common.Config{}` with `invopop/jsonschema` to build the schema, then validates `json.Marshal(config)`. Both read the same tags. A nil-able field with a `toml` tag and no `json` tag gets typed concretely in the schema, while `json.Marshal` writes an explicit `null` for it when unset. Schema and instance contradict each other by construction. Independent of whatever wrote the `config.toml`. ## Affected fields Five in `common/config.go`, three distinct messages. Lines against `main` at `c61ef5392aad88145c38087f1857753378754324`. | Line | Struct | Field | Message when unset | | --- | --- | --- | --- | | 483 | `AutoscalerPolicyConfig` | `PreemptiveMode *bool` | `got null, want boolean` | | 879 | `KubernetesDNSConfigOption` | `Value *string` | `got null, want string` | | 906 | `KubernetesHostPath` | `MountPropagation *string` | `got null, want string` | | 914 | `KubernetesPVC` | `MountPropagation *string` | `got null, want string` | | 931 | `KubernetesEmptyDir` | `MountPropagation *string` | `got null, want string` | ## Reproduction Field declarations copied verbatim from `common/config.go`; flow mirrors `validation.go`. Uses the versions pinned in `go.mod` (`invopop/jsonschema v0.14.0`, `santhosh-tekuri/jsonschema/v6 v6.0.3`). ```go package main import ( "bytes" "encoding/json" "fmt" generator "github.com/invopop/jsonschema" validator "github.com/santhosh-tekuri/jsonschema/v6" ) type EmptyDir struct { Name string `toml:"name" json:"name"` MountPath string `toml:"mount_path"` MountPropagation *string `toml:"mount_propagation,omitempty"` } type DNSOption struct { Name string `toml:"name"` Value *string `toml:"value,omitempty"` } type AutoscalerPolicy struct { PreemptiveMode *bool `toml:"preemptive_mode,omitempty"` } type Config struct { EmptyDirs []EmptyDir `toml:"empty_dir" json:",omitempty"` DNSOptions []DNSOption `toml:"options" json:",omitempty"` Policies []AutoscalerPolicy `toml:"policy" json:",omitempty"` } func main() { r := &generator.Reflector{RequiredFromJSONSchemaTags: true, DoNotReference: true} schema, _ := json.Marshal(r.Reflect(&Config{})) doc, _ := validator.UnmarshalJSON(bytes.NewReader(schema)) c := validator.NewCompiler() _ = c.AddResource("s.json", doc) body, _ := json.Marshal(&Config{ EmptyDirs: []EmptyDir{{Name: "docker-certs", MountPath: "/certs/client"}}, DNSOptions: []DNSOption{{Name: "ndots"}}, Policies: []AutoscalerPolicy{{}}, }) val, _ := validator.UnmarshalJSON(bytes.NewReader(body)) fmt.Printf("instance: %s\n\n", body) fmt.Println(c.MustCompile("s.json").Validate(val)) } ``` ``` instance: {"EmptyDirs":[{"name":"docker-certs","MountPath":"/certs/client","MountPropagation":null}],"DNSOptions":[{"Name":"ndots","Value":null}],"Policies":[{"PreemptiveMode":null}]} - at '/EmptyDirs/0/MountPropagation': got null, want string - at '/DNSOptions/0/Value': got null, want string - at '/Policies/0/PreemptiveMode': got null, want boolean ``` ## Proposed fix Add a JSON tag with `omitempty` to each of the five: ```diff - MountPropagation *string `toml:"mount_propagation,omitempty" description:"Mount propagation mode for the volume"` + MountPropagation *string `toml:"mount_propagation,omitempty" json:"mount_propagation,omitempty" description:"Mount propagation mode for the volume"` ``` That makes the reproduction validate clean, and aligns schema property names with the documented TOML keys. Worth more than the tag fixes: a test that marshals a zero `common.Config{}` and asserts validation is clean, so the next field added this way is caught. ## Please do not "fix" this by emitting an empty string Setting `mount_propagation = ""` silences the warning and looks harmless. `executors/kubernetes/kubernetes.go` casts the pointer through unchecked at L1768, L1787 and L1805: ```go MountPropagation: (*api.MountPropagationMode)(mount.MountPropagation), ``` so the build pod is submitted with `mountPropagation: ""` and the API server rejects it: `Unsupported value: "": supported values: "Bidirectional", "HostToContainer", "None"`. Verified against Kubernetes 1.33. That turns a cosmetic log line into every build pod failing to create, so treating an empty string as unset in the executor, or rejecting it at config load, would be worth doing. ## Affected versions Present on `main` at `c61ef5392aad88145c38087f1857753378754324` (2026-08-15), on `v19.2.2` (latest release), and on `v19.1.0`. I found no existing issue; searches surfaced only the MRs that added `mount_propagation` (!4784, !5157, !6553), each carrying the same omission.