service config: default service config (#2686)

dialoptions.go

@@ -55,14 +55,16 @@ type dialOptions struct {
 	// balancer, and also by WithBalancerName dial option.
 	balancerBuilder balancer.Builder
 	// This is to support grpclb.
-	resolverBuilder      resolver.Builder
-	reqHandshake         envconfig.RequireHandshakeSetting
-	channelzParentID     int64
-	disableServiceConfig bool
-	disableRetry         bool
-	disableHealthCheck   bool
-	healthCheckFunc      internal.HealthChecker
-	minConnectTimeout    func() time.Duration
+	resolverBuilder             resolver.Builder
+	reqHandshake                envconfig.RequireHandshakeSetting
+	channelzParentID            int64
+	disableServiceConfig        bool
+	disableRetry                bool
+	disableHealthCheck          bool
+	healthCheckFunc             internal.HealthChecker
+	minConnectTimeout           func() time.Duration
+	defaultServiceConfig        *ServiceConfig // defaultServiceConfig is parsed from defaultServiceConfigRawJSON.
+	defaultServiceConfigRawJSON *string
 }
 
 // DialOption configures how we set up the connection.
@@ -441,12 +443,27 @@ func WithChannelzParentID(id int64) DialOption {
 // WithDisableServiceConfig returns a DialOption that causes grpc to ignore any
 // service config provided by the resolver and provides a hint to the resolver
 // to not fetch service configs.
+//
+// Note that, this dial option only disables service config from resolver. If
+// default service config is provided, grpc will use the default service config.
 func WithDisableServiceConfig() DialOption {
 	return newFuncDialOption(func(o *dialOptions) {
 		o.disableServiceConfig = true
 	})
 }
 
+// WithDefaultServiceConfig returns a DialOption that configures the default
+// service config, which will be used in cases where:
+// 1. WithDisableServiceConfig is called.
+// 2. Resolver does not return service config or if the resolver gets and invalid config.
+//
+// This API is EXPERIMENTAL.
+func WithDefaultServiceConfig(s string) DialOption {
+	return newFuncDialOption(func(o *dialOptions) {
+		o.defaultServiceConfigRawJSON = &s
+	})
+}
+
 // WithDisableRetry returns a DialOption that disables retries, even if the
 // service config enables them.  This does not impact transparent retries, which
 // will happen automatically if no data is written to the wire or if the RPC is