feat: decouple billing correctness from usage log batching

backend/internal/service/usage_log_create_result.go

@@ -0,0 +1,60 @@
+package service
+
+import "errors"
+
+type usageLogCreateDisposition int
+
+const (
+	usageLogCreateDispositionUnknown usageLogCreateDisposition = iota
+	usageLogCreateDispositionNotPersisted
+)
+
+type UsageLogCreateError struct {
+	err         error
+	disposition usageLogCreateDisposition
+}
+
+func (e *UsageLogCreateError) Error() string {
+	if e == nil || e.err == nil {
+		return "usage log create error"
+	}
+	return e.err.Error()
+}
+
+func (e *UsageLogCreateError) Unwrap() error {
+	if e == nil {
+		return nil
+	}
+	return e.err
+}
+
+func MarkUsageLogCreateNotPersisted(err error) error {
+	if err == nil {
+		return nil
+	}
+	return &UsageLogCreateError{
+		err:         err,
+		disposition: usageLogCreateDispositionNotPersisted,
+	}
+}
+
+func IsUsageLogCreateNotPersisted(err error) bool {
+	if err == nil {
+		return false
+	}
+	var target *UsageLogCreateError
+	if !errors.As(err, &target) {
+		return false
+	}
+	return target.disposition == usageLogCreateDispositionNotPersisted
+}
+
+func ShouldBillAfterUsageLogCreate(inserted bool, err error) bool {
+	if inserted {
+		return true
+	}
+	if err == nil {
+		return false
+	}
+	return !IsUsageLogCreateNotPersisted(err)
+}