types: Add UnimplementedStub to have forward compatible implementations (#524)

* types: Implement UnimplementedStub to have forward compatible implementations

Signed-off-by: Xuanwo <github@xuanwo.io>

* Add mustEmbedUnimplementedXXX function

Signed-off-by: Xuanwo <github@xuanwo.io>

Author: Xuanwo

cmd/definitions/bindata.go

@@ -20,7 +20,33 @@ type {{ $i.DisplayName }} interface {
     {{ $op.Name | toPascal }}WithContext(ctx context.Context,{{ $op.FormatParams }}) ({{ $op.FormatResultsWithPackageName "storage" }})
     {{ end }}
     {{ end }}
+
+    mustEmbedUnimplemented{{ $i.DisplayName }}()
+}
+
+// Unimplemented{{ $i.DisplayName }} must be embedded to have forward compatible implementations.
+type Unimplemented{{ $i.DisplayName }} struct {}
+
+func (s Unimplemented{{ $i.DisplayName }}) mustEmbedUnimplemented{{ $i.DisplayName }}() {}
+
+func (s Unimplemented{{ $i.DisplayName }}) String() string {
+    return "Unimplemented{{ $i.DisplayName }}"
+}
+
+{{ range $_, $op := $i.Ops }}
+func (s Unimplemented{{ $i.DisplayName }})  {{ $op.Name | toPascal }}({{ $op.FormatParams }}) ({{ $op.FormatResultsWithPackageName "storage" }}) {
+    {{- if not $op.Local }}
+    err = NewOperationNotImplementedError("{{ $op.Name }}")
+    {{- end }}
+    return
+}
+{{- if not $op.Local }}
+func (s Unimplemented{{ $i.DisplayName }}) {{ $op.Name | toPascal }}WithContext(ctx context.Context,{{ $op.FormatParams }}) ({{ $op.FormatResultsWithPackageName "storage" }}) {
+    err = NewOperationNotImplementedError("{{ $op.Name }}")
+    return
 }
+{{ end }}
+{{ end }}
 {{- end }}
 
 type PairPolicy struct {

cmd/definitions/tmpl/operation.tmpl

@@ -0,0 +1,34 @@
+package types
+
+import (
+	"errors"
+	"fmt"
+)
+
+var (
+	// ErrNotImplemented will be returned while this operation is not
+	// implemented by services.
+	ErrNotImplemented = errors.New("not implemented")
+)
+
+// OperationError is the error for operation related errors.
+type OperationError struct {
+	op  string
+	err error
+}
+
+func (oe OperationError) Error() string {
+	return fmt.Sprintf("operation %s: %v", oe.op, oe.err)
+}
+
+func (oe OperationError) Unwrap() error {
+	return oe.err
+}
+
+// NewOperationNotImplementedError will create a new NotImplemented error.
+func NewOperationNotImplementedError(op string) error {
+	return OperationError{
+		op:  op,
+		err: ErrNotImplemented,
+	}
+}