Return user error when CCF encodes attachment field

encoding/ccf/ccf_test.go

@@ -35,9 +35,11 @@ import (
 	"github.com/onflow/cadence/encoding/ccf"
 	"github.com/onflow/cadence/runtime"
 	"github.com/onflow/cadence/runtime/common"
+	"github.com/onflow/cadence/runtime/errors"
 	"github.com/onflow/cadence/runtime/interpreter"
 	"github.com/onflow/cadence/runtime/sema"
 	"github.com/onflow/cadence/runtime/tests/checker"
+	"github.com/onflow/cadence/runtime/tests/runtime_utils"
 	"github.com/onflow/cadence/runtime/tests/utils"
 )
 
@@ -17108,3 +17110,67 @@ func TestDecodeFunctionTypeBackwardCompatibility(t *testing.T) {
 
 	testDecode(t, data, val)
 }
+
+func TestEncodeEventWithAttachement(t *testing.T) {
+	script := `
+		access(all) struct S {
+			access(all) let x: Int 
+			init(x: Int) {
+				self.x = x
+			}
+		}
+
+		access(all) attachment A for S {
+			access(all) let y: Int 
+			init(y: Int) {
+				self.y = y
+			}
+		}
+
+		access(all) event Foo(s: S)
+
+		access(all) 
+		fun main() {
+			let s = attach A(y: 3) to S(x: 4)
+			emit Foo(s: s)
+		}
+		`
+
+	v := exportEventFromScript(t, script)
+
+	_, err := ccf.Encode(v)
+
+	var attachmentFieldError ccf.AttachmentFieldNotSupportedEncodingError
+	require.ErrorAs(t, err, &attachmentFieldError)
+	require.Implements(t, (*errors.UserError)(nil), attachmentFieldError)
+}
+
+func exportEventFromScript(t *testing.T, script string) cadence.Event {
+	rt := runtime_utils.NewTestInterpreterRuntime()
+
+	var events []cadence.Event
+
+	inter := &runtime_utils.TestRuntimeInterface{
+		OnEmitEvent: func(event cadence.Event) error {
+			events = append(events, event)
+			return nil
+		},
+	}
+
+	_, err := rt.ExecuteScript(
+		runtime.Script{
+			Source: []byte(script),
+		},
+		runtime.Context{
+			Interface: inter,
+			Location:  common.ScriptLocation{},
+		},
+	)
+
+	require.NoError(t, err)
+	require.Len(t, events, 1)
+
+	event := events[0]
+
+	return event
+}

encoding/ccf/encode.go

@@ -176,6 +176,40 @@ func (em *encMode) NewEncoder(w io.Writer) *Encoder {
 
 var defaultEncMode = &encMode{}
 
+// AttachmentFieldNotSupportedEncodingError is a user error that is returned
+// when encoding composite value (such as Event) that has cadence.Attachment field.
+type AttachmentFieldNotSupportedEncodingError struct {
+	compositeType  string
+	fieldCount     int
+	fieldTypeCount int
+}
+
+func newAttachmentFieldNotSupportedError(
+	compositeType string,
+	fieldCount int,
+	fieldTypeCount int,
+) error {
+	return AttachmentFieldNotSupportedEncodingError{
+		compositeType:  compositeType,
+		fieldCount:     fieldCount,
+		fieldTypeCount: fieldTypeCount,
+	}
+}
+
+func (e AttachmentFieldNotSupportedEncodingError) Error() string {
+	return fmt.Sprintf(
+		"encoding attachment field in composite value isn't supported: %s field count %d doesn't match declared field type count %d",
+		e.compositeType,
+		e.fieldCount,
+		e.fieldTypeCount,
+	)
+}
+
+func (e AttachmentFieldNotSupportedEncodingError) IsUserError() {
+}
+
+var _ cadenceErrors.UserError = AttachmentFieldNotSupportedEncodingError{}
+
 // Encode returns the CCF-encoded representation of the given value
 // by using default CCF encoding options.  This function returns an
 // error if the Cadence value cannot be represented in CCF.
@@ -219,7 +253,7 @@ func (e *Encoder) Encode(value cadence.Value) (err error) {
 		// Add context to error if there is any.
 		if err != nil {
 			err = fmt.Errorf(
-				"ccf: failed to encode value (type %T, %q): %s",
+				"ccf: failed to encode value (type %T, %q): %w",
 				value,
 				value.Type().ID(),
 				err,
@@ -1102,6 +1136,22 @@ func (e *Encoder) encodeComposite(
 	staticFieldTypes := getCompositeTypeFields(typ)
 
 	if len(staticFieldTypes) != len(fields) {
+
+		// The CCF encoder requires field values and types to match. However,
+		// composite values that have attachment field don't satisfy this requirement because:
+		// - composite field values INCLUDE attachment field values
+		// - composite field types EXCLUDE attachment field types
+		// Given this, CCF encoder will return a user error if composite value has attachment field.
+		for _, f := range fields {
+			if _, ok := f.(cadence.Attachment); ok {
+				panic(newAttachmentFieldNotSupportedError(
+					typ.ID(),
+					len(fields),
+					len(staticFieldTypes),
+				))
+			}
+		}
+
 		panic(cadenceErrors.NewUnexpectedError(
 			"%s field count %d doesn't match declared field type count %d",
 			typ.ID(),