`json.Marshal` allocates a lot of unnecessary memory

[apelisse]

This is a follow-up of #315/#319. Now that Unmarshaling has been fixed, we need to worry about the next big thing, Marshaling.

As we can see from the pprof heap profile (alloc_space) below, json.Marshal is allocating a lot of memory, certainly because it serializes in buffers that are quickly discarded as these buffers are "concatenated" (e.g. https://github.com/kubernetes/kube-openapi/blob/master/pkg/validation/spec/header.go#L44-L63, https://github.com/kubernetes/kube-openapi/blob/master/pkg/validation/spec/schema.go#L435-L466)

There's clearly no great way to do that with json.Marshal v1, since when you have multiple embedded types (which we have everywhere in the spec), only one of the custom marshaller is called. That's also true for json.Marshal v2, I don't see how it could be different anyways.

I've been looking at go-json-experiment, but I can't find a good way to "embed" multiple values, all the methods require that you serialize exactly one json "value", but in most cases, we want to serialize a list of fields for each of these embedded sub-structures. I think we could give each of these types a "MarshalKeysAndValues", and construct the object itself, but then we will have to rewrite a lot of logic to extract the keys and values.

Anyways, I'm stuck. Suggestions welcome @dsnet @alexzielenski @liggitt

[dsnet]

Couldn't you do the opposite as UnmarshalNextJSON?

func (h *Header) MarshalNextJSON(opts jsonv2.MarshalOptions, enc *jsonv2.Encoder) error {
	// TODO: This mutates h.Extensions, which may be unacceptable.
	// Might need to clone the map if there are unsanitized keys.
	// or maybe we don't care for marshaling purposes?
	h.Extensions.sanitize()
	if len(h.Extensions) == 0 {
		h.Extensions = nil
	}

	var x struct {
		CommonValidations
		SimpleSchema
		Extensions
		HeaderProps
	} {
		CommonValidations: h.CommonValidations,
		SimpleSchema:      h.SimpleSchema,
		Extensions:        h.Extensions,
		HeaderProps:       h.HeaderProps,
	}
	return opts.MarshalNext(enc, &x)
}

[apelisse]

For Header it works because none of the types in X have a custom MarshalJSON/MarshalNextJSON function, but if they do, then only one will get called AFAICT.

[dsnet]

If I recall most of them were fine. Which type wasn't?

[alexzielenski]

@dsnet Operation embeds OperationProps which needs custom marshaling (so omit empty distinguishes nil from 0-length)

we did not see a way to get the inlining to work without somehow duplicating OperationProps' marshaling logic into Operation itself which is undesired.

[dsnet]

That's unfortunate. It seems that the only reason why OperationProps.MarshalJSON exists is because v1 doesn't support a omitzero tag that omits empty only nil slices.

How about?

func (o *Operation) MarshalNextJSON(opts jsonv2.MarshalOptions, enc *jsonv2.Encoder) error {
	// TODO: Should this be done?
	x.Extensions.sanitize()
	if len(x.Extensions) == 0 {
		x.Extensions = nil
	}

	// OperationPropsAlt is identical to OperationProps,
	// but lacks a MarshalJSON method and
	// uses `omitzero` instead of `omitempty` on the
	// Security and Deprecated fields.
	type OperationPropsAlt struct {
		Description  string                 `json:"description,omitempty"`
		Consumes     []string               `json:"consumes,omitempty"`
		Produces     []string               `json:"produces,omitempty"`
		Schemes      []string               `json:"schemes,omitempty"`
		Tags         []string               `json:"tags,omitempty"`
		Summary      string                 `json:"summary,omitempty"`
		ExternalDocs *ExternalDocumentation `json:"externalDocs,omitempty"`
		ID           string                 `json:"operationId,omitempty"`
		Deprecated   bool                   `json:"deprecated,omitzero"`
		Security     []map[string][]string  `json:"security,omitzero"`
		Parameters   []Parameter            `json:"parameters,omitempty"`
		Responses    *Responses             `json:"responses,omitempty"`
	}
	x := struct {
		Extensions
		*OperationPropsAlt
	}{
		Extension:         o.Extensions,
		OperationPropsAlt: (*OperationPropsAlt)(&o.OperationProps), // see https://go.dev/issue/16085
	}
	return opts.MarshalNext(enc, &x)
}

See https://pkg.go.dev/github.com/go-json-experiment/json#hdr-JSON_Representation_of_Go_structs for the semantics of omitzero vs omitempty.

[dsnet]

It's unfortunate that we need to declare the OperationPropsAlt type, but fortunately it's not a significant hit to maintainability since the language semantics of golang/go#16085 requires that OperationProps and OperationPropsAlt have the same field layout, so added/removed fields in OperationProps will result in a build failure until the equivalent change is made in OperationPropsAlt.

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/lifecycle stale

[apelisse]

/remove-lifecycle stale

[dsnet]

FYI, there's been some upstream activity that may help you.

We added jsonv1.DefaultOptionsV1 which can be passed to jsonv2.Marshal to marshal with v1 "encoding/json" semantics. The implementation is not 100% identical to v1 under this option as it is still a work-in-progress, but we will gradually get it as close to 100% as possible.

That said, there's been a few breaking changes at HEAD:

• Refactor API to use variadic options (#298)
• Delete jsontext declaration in json (#301)
• Remove time.Duration formatting support for format:nanos (#302)

Summary of breaking changes:

• We split the v2 "json" package apart into "json" and "jsontext", where the syntactic handling of JSON was moved over to the "jsontext" pacakge. The purpose of this change is to provide a package that does purely syntactic operations on JSON with a minimal dependency tree.
• We removed MarshalOptions, UnmarshalOptions, EncodeOptions, and DecodeOptions struct types and switched over to a variadic options API. The purpose of this to pave the way for implementing v1 "encoding/json" entirely in terms of v2.

This will remove the need for gross hacks like this:

kube-openapi/pkg/validation/spec/schema.go

Lines 201 to 237 in d090da1

	type schemaPropsOmitZero struct {
	ID string `json:"id,omitempty"`
	Ref Ref `json:"-"`
	Schema SchemaURL `json:"-"`
	Description string `json:"description,omitempty"`
	Type StringOrArray `json:"type,omitzero"`
	Nullable bool `json:"nullable,omitzero"`
	Format string `json:"format,omitempty"`
	Title string `json:"title,omitempty"`
	Default interface{} `json:"default,omitzero"`
	Maximum *float64 `json:"maximum,omitempty"`
	ExclusiveMaximum bool `json:"exclusiveMaximum,omitzero"`
	Minimum *float64 `json:"minimum,omitempty"`
	ExclusiveMinimum bool `json:"exclusiveMinimum,omitzero"`
	MaxLength *int64 `json:"maxLength,omitempty"`
	MinLength *int64 `json:"minLength,omitempty"`
	Pattern string `json:"pattern,omitempty"`
	MaxItems *int64 `json:"maxItems,omitempty"`
	MinItems *int64 `json:"minItems,omitempty"`
	UniqueItems bool `json:"uniqueItems,omitzero"`
	MultipleOf *float64 `json:"multipleOf,omitempty"`
	Enum []interface{} `json:"enum,omitempty"`
	MaxProperties *int64 `json:"maxProperties,omitempty"`
	MinProperties *int64 `json:"minProperties,omitempty"`
	Required []string `json:"required,omitempty"`
	Items *SchemaOrArray `json:"items,omitzero"`
	AllOf []Schema `json:"allOf,omitempty"`
	OneOf []Schema `json:"oneOf,omitempty"`
	AnyOf []Schema `json:"anyOf,omitempty"`
	Not *Schema `json:"not,omitzero"`
	Properties map[string]Schema `json:"properties,omitempty"`
	AdditionalProperties *SchemaOrBool `json:"additionalProperties,omitzero"`
	PatternProperties map[string]Schema `json:"patternProperties,omitempty"`
	Dependencies Dependencies `json:"dependencies,omitempty"`
	AdditionalItems *SchemaOrBool `json:"additionalItems,omitzero"`
	Definitions Definitions `json:"definitions,omitempty"`
	}