compute/backend_service: Remove TTLs when using cache mode USE_ORIGIN_HEADERS

mmv1/templates/terraform/encoders/backend_service.go.erb

@@ -47,4 +47,43 @@ for _, backendRaw := range backends {
 	}
 }
 
+// This custom encoding helps prevent sending 0 for clientTtl, defaultTtl and
+// maxTtl in API calls to update these values  when unset in the provider
+// (doing so results in an API level error)
+c, cdnPolicyOk := d.GetOk("cdn_policy")
+
+// Only apply during updates
+if !cdnPolicyOk || obj["cdnPolicy"] == nil {
+	return obj, nil
+}
+
+currentCdnPolicies := c.([]interface{})
+
+// state does not contain cdnPolicy, so we can return early here as well
+if len(currentCdnPolicies) == 0 {
+	return obj, nil
+}
+
+futureCdnPolicy := obj["cdnPolicy"].(map[string]interface{})
+currentCdnPolicy := currentCdnPolicies[0].(map[string]interface{})
+
+cacheMode, ok := futureCdnPolicy["cache_mode"].(string)
+// Fallback to state if doesn't exist in object
+if !ok {
+	cacheMode = currentCdnPolicy["cache_mode"].(string)
+}
+
+switch cacheMode {
+case "USE_ORIGIN_HEADERS":
+	if _, ok := futureCdnPolicy["clientTtl"]; ok {
+		delete(futureCdnPolicy, "clientTtl")
+	}
+	if _, ok := futureCdnPolicy["defaultTtl"]; ok {
+		delete(futureCdnPolicy, "defaultTtl")
+	}
+	if _, ok := futureCdnPolicy["maxTtl"]; ok {
+		delete(futureCdnPolicy, "maxTtl")
+	}
+}
+
 return obj, nil