Implement -fixEmptyArrays to force empty array [] instead of null…

… in JSON (#32)

Golang issue: golang/go#27589

This option will enforce server to encode nil slices as empty arrays in JSON.

This is helpful mainly in JavaScript and TypeScript clients, where a response
field of type array should never be assigned to `null` value..

Example:

func (s *Server) GetItems(ctx context.Context) ([]*Items, error) {
	var items []*Item

	// Will send {"items": []} in JSON response instead of {"items": null}.
	return items, nil
}

README.md

@@ -22,14 +22,15 @@ As you can see, the `-target` supports default `golang`, any git URI, or a local
 ### Set custom template variables
 Change any of the following values by passing `-option="Value"` CLI flag to `webrpc-gen`.
 
-| webrpc-gen -option   | Description                             | Default value | Added in |
-|----------------------|-----------------------------------------|--------------|----------|
-| `-pkg=<name>`        | package name                            | `"proto"`    | v0.5.0   |
-| `-client`            | generate client code                    | `false`      | v0.5.0   |
-| `-server`            | generate server code                    | `false`      | v0.5.0   |
-| `-types=false`       | don't generate types                    | `true`       | v0.13.0  |
-| `-json=jsoniter`     | use alternative json encoding package   | `"stdlib"`   | v0.12.0  |
-| `-legacyErrors=true` | enable legacy errors (v0.10.0 or older) | `false`      | v0.11.0  |
+| webrpc-gen -option | Default    | Description                                                                 | Added in |
+|--------------------|------------|-----------------------------------------------------------------------------|----------|
+| `-pkg=<name>`      | `"proto"`  | package name                                                                | v0.5.0   |
+| `-client`          | `false`    | generate client code                                                        | v0.5.0   |
+| `-server`          | `false`    | generate server code                                                        | v0.5.0   |
+| `-types=false`     | `true`     | don't generate types                                                        | v0.13.0  |
+| `-json=jsoniter`   | `"stdlib"` | use alternative json encoding package                                       | v0.12.0  |
+| `-fixEmptyArrays`  | `false`    | force empty array `[]` instead of `null` in JSON (see Go [#27589][go27589]) | v0.13.0  |
+| `-legacyErrors`    | `false`    | enable legacy errors (v0.10.0 or older)                                     | v0.11.0  |
 
 Example:
 ```
@@ -86,3 +87,5 @@ See [_examples](./_examples)
 ## LICENSE
 
 [MIT LICENSE](./LICENSE)
+
+[go27589]: https://github.com/golang/go/issues/27589

_examples/golang-basics/example_test.go

@@ -42,7 +42,7 @@ func TestGetUser(t *testing.T) {
 	{
 		arg1 := map[string]string{"a": "1"}
 		user, err := client.GetUser(context.Background(), arg1, 12)
-		assert.Equal(t, &User{ID: 12, Username: "hihi"}, user)
+		assert.Equal(t, &User{ID: 12, Username: "hihi", Nicknames: []Nickname{}}, user)
 		assert.NoError(t, err)
 	}
 
@@ -81,7 +81,7 @@ func TestGetUser(t *testing.T) {
 	{
 		name, user, err := client.FindUser(context.Background(), &SearchFilter{Q: "joe"})
 		assert.Equal(t, "joe", name)
-		assert.Equal(t, &User{ID: 123, Username: "joe"}, user)
+		assert.Equal(t, &User{ID: 123, Username: "joe", Nicknames: []Nickname{}}, user)
 		assert.NoError(t, err)
 	}
 }

_examples/golang-basics/main.go

@@ -1,4 +1,4 @@
-//go:generate webrpc-gen -schema=example.ridl -target=../../../gen-golang -pkg=main -server -client -out=./example.gen.go -legacyErrors=true
+//go:generate webrpc-gen -schema=example.ridl -target=../../../gen-golang -pkg=main -server -client -legacyErrors -fixEmptyArrays -out=./example.gen.go
 package main
 
 import (

helpers.go.tmpl

@@ -1,4 +1,6 @@
 {{define "helpers"}}
+{{ $opts := .Opts -}}
+
 //
 // Helpers
 //
@@ -24,4 +26,45 @@ var (
 
 	MethodNameCtxKey = &contextKey{"MethodName"}
 )
+
+{{- if $opts.fixEmptyArrays }}
+
+// Copied from https://github.com/golang-cz/nilslice
+func initializeNilSlices(obj interface{}) interface{} {
+	v := reflect.ValueOf(obj)
+	initializeNils(v)
+
+	return obj
+}
+
+func initializeNils(v reflect.Value) {
+	// Dereference pointer(s).
+	for v.Kind() == reflect.Ptr && !v.IsNil() {
+		v = v.Elem()
+	}
+
+	if v.Kind() == reflect.Slice {
+		// Initialize a nil slice.
+		if v.IsNil() {
+			v.Set(reflect.MakeSlice(v.Type(), 0, 0))
+			return
+		}
+
+		// Recursively iterate over slice items.
+		for i := 0; i < v.Len(); i++ {
+			item := v.Index(i)
+			initializeNils(item)
+		}
+	}
+
+	// Recursively iterate over struct fields.
+	if v.Kind() == reflect.Struct {
+		for i := 0; i < v.NumField(); i++ {
+			field := v.Field(i)
+			initializeNils(field)
+		}
+	}
+}
 {{ end }}
+
+{{ end -}}

main.go.tmpl

@@ -8,6 +8,7 @@
 {{- set $opts "types" (ternary (eq (default .Opts.types "true") "false") false true) -}}
 {{- set $opts "json" (default .Opts.json "stdlib") -}}
 {{- set $opts "importTypesFrom" (default .Opts.importTypesFrom "" ) -}}
+{{- set $opts "fixEmptyArrays" (ternary (in .Opts.fixEmptyArrays "" "true") true false) -}}
 {{- set $opts "legacyErrors" (ternary (in .Opts.legacyErrors "" "true") true false) -}}
 
 {{- $typePrefix := (last (split "/" $opts.importTypesFrom)) -}}
@@ -92,14 +93,14 @@ func WebRPCSchemaHash() string {
 {{ end -}}
 
 {{- if $opts.server}}
-{{ template "server" dict "Services" .Services "TypeMap" $typeMap "TypePrefix" $typePrefix }}
+{{ template "server" dict "Services" .Services "TypeMap" $typeMap "TypePrefix" $typePrefix "Opts" $opts }}
 {{ end -}}
 
 {{ if $opts.client }}
 {{ template "client" dict "Services" .Services "TypeMap" $typeMap "TypePrefix" $typePrefix }}
 {{ end -}}
 
-{{ template "helpers" . }}
+{{ template "helpers" dict "Opts" $opts }}
 
 {{- template "errors" dict "WebrpcErrors" .WebrpcErrors "SchemaErrors" .Errors "Opts" $opts "TypePrefix" $typePrefix }}
 

server.go.tmpl

@@ -1,6 +1,8 @@
 {{- define "server"}}
 {{- $typeMap := .TypeMap -}}
 {{- $typePrefix := .TypePrefix -}}
+{{- $opts := .Opts -}}
+
 {{- if .Services -}}
 //
 // Server
@@ -120,7 +122,11 @@ func (s *{{$serviceName}}) serve{{ .Name | firstLetterToUpper }}JSON(ctx context
 	}
 
 	{{- if .Outputs | len}}
+	{{ if $opts.fixEmptyArrays -}}
+	respBody, err := json.Marshal(initializeNilSlices(respContent))
+	{{ else -}}
 	respBody, err := json.Marshal(respContent)
+	{{ end -}}
 	if err != nil {
 		err = ErrorWithCause(ErrWebrpcBadResponse, fmt.Errorf("failed to marshal json response: %w", err))
 		RespondWithError(w, err)