From 23b18faca8bbb0d46a5eb32d16f401172d37239d Mon Sep 17 00:00:00 2001
From: Denise Li <dli@squareup.com>
Date: Wed, 8 May 2024 13:42:59 -0400
Subject: [PATCH] feat: support omitempty in custom json de/encoder (#1441)

Fixes https://github.com/TBD54566975/ftl/issues/1262
---
 go-runtime/encoding/encoding.go      | 17 ++++++++++++++++-
 go-runtime/encoding/encoding_test.go |  7 +++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/go-runtime/encoding/encoding.go b/go-runtime/encoding/encoding.go
index f73040f9e3..4ddf175173 100644
--- a/go-runtime/encoding/encoding.go
+++ b/go-runtime/encoding/encoding.go
@@ -60,7 +60,8 @@ func encodeValue(ctx context.Context, v reflect.Value, w *bytes.Buffer) error {
 		enc := v.Interface().(OptionMarshaler) //nolint:forcetypeassert
 		return enc.Marshal(ctx, w, encodeValue)
 
-	//TODO: Remove once we support `omitempty` tag
+	// TODO(Issue #1439): remove this special case by removing all usage of
+	// json.RawMessage, which is not a type we support.
 	case t == reflect.TypeFor[json.RawMessage]():
 		data, err := json.Marshal(v.Interface())
 		if err != nil {
@@ -137,6 +138,9 @@ func encodeStruct(ctx context.Context, v reflect.Value, w *bytes.Buffer) error {
 		// 	(t == reflect.TypeOf((*any)(nil)).Elem() && fv.IsZero()) {
 		// 	continue
 		// }
+		if isTaggedOmitempty(v, i) && fv.IsZero() {
+			continue
+		}
 		if afterFirst {
 			w.WriteRune(',')
 		}
@@ -150,6 +154,17 @@ func encodeStruct(ctx context.Context, v reflect.Value, w *bytes.Buffer) error {
 	return nil
 }
 
+func isTaggedOmitempty(v reflect.Value, i int) bool {
+	tag := v.Type().Field(i).Tag
+	tagVals := strings.Split(tag.Get("json"), ",")
+	for _, tagVal := range tagVals {
+		if strings.TrimSpace(tagVal) == "omitempty" {
+			return true
+		}
+	}
+	return false
+}
+
 func encodeBytes(v reflect.Value, w *bytes.Buffer) error {
 	data := base64.StdEncoding.EncodeToString(v.Bytes())
 	fmt.Fprintf(w, "%q", data)
diff --git a/go-runtime/encoding/encoding_test.go b/go-runtime/encoding/encoding_test.go
index 063558647e..34975bb466 100644
--- a/go-runtime/encoding/encoding_test.go
+++ b/go-runtime/encoding/encoding_test.go
@@ -32,6 +32,11 @@ func TestMarshal(t *testing.T) {
 	type inner struct {
 		FooBar string
 	}
+	type validateOmitempty struct {
+		ShouldOmit   string `json:",omitempty"`
+		ShouldntOmit string `json:""`
+		NotTagged    string
+	}
 	tests := []struct {
 		name     string
 		input    any
@@ -58,6 +63,8 @@ func TestMarshal(t *testing.T) {
 		{name: "Pointer", input: &struct{ String string }{"foo"}, err: `pointer types are not supported: *struct { String string }`},
 		{name: "SumType", input: struct{ D discriminator }{variant{"hello"}}, expected: `{"d":{"name":"Variant","value":{"message":"hello"}}}`},
 		{name: "UnregisteredSumType", input: struct{ D unregistered }{variant{"hello"}}, err: `the only supported interface types are enums or any, not encoding_test.unregistered`},
+		{name: "OmitEmptyNotNull", input: validateOmitempty{"foo", "bar", "baz"}, expected: `{"shouldOmit":"foo","shouldntOmit":"bar","notTagged":"baz"}`},
+		{name: "OmitEmptyNull", input: validateOmitempty{}, expected: `{"shouldntOmit":"","notTagged":""}`},
 	}
 
 	tr := typeregistry.NewTypeRegistry()