internal/serviceconfig: Fix typo and improve docs. (#3802)

internal/serviceconfig/serviceconfig.go

@@ -30,8 +30,11 @@ import (
 
 var logger = grpclog.Component("core")
 
-// BalancerConfig is the balancer config part that service config's
-// loadBalancingConfig fields can be unmarshalled to. It's a json unmarshaller.
+// BalancerConfig wraps the name and config associated with one load balancing
+// policy. It corresponds to a single entry of the loadBalancingConfig field
+// from ServiceConfig.
+//
+// It implements the json.Unmarshaler interface.
 //
 // https://github.com/grpc/grpc-proto/blob/54713b1e8bc6ed2d4f25fb4dff527842150b91b2/grpc/service_config/service_config.proto#L247
 type BalancerConfig struct {
@@ -41,7 +44,15 @@ type BalancerConfig struct {
 
 type intermediateBalancerConfig []map[string]json.RawMessage
 
-// UnmarshalJSON implements json unmarshaller.
+// UnmarshalJSON implements the json.Unmarshaler interface.
+//
+// ServiceConfig contains a list of loadBalancingConfigs, each with a name and
+// config. This method iterates through that list in order, and stops at the
+// first policy that is supported.
+// - If the config for the first supported policy is invalid, the whole service
+//   config is invalid.
+// - If the list doesn't contain any supported policy, the whole service config
+//   is invalid.
 func (bc *BalancerConfig) UnmarshalJSON(b []byte) error {
 	var ir intermediateBalancerConfig
 	err := json.Unmarshal(b, &ir)
@@ -53,13 +64,16 @@ func (bc *BalancerConfig) UnmarshalJSON(b []byte) error {
 		if len(lbcfg) != 1 {
 			return fmt.Errorf("invalid loadBalancingConfig: entry %v does not contain exactly 1 policy/config pair: %q", i, lbcfg)
 		}
+
 		var (
 			name    string
 			jsonCfg json.RawMessage
 		)
-		// Get the key:value pair from the map.
+		// Get the key:value pair from the map. We have already made sure that
+		// the map contains a single entry.
 		for name, jsonCfg = range lbcfg {
 		}
+
 		builder := balancer.Get(name)
 		if builder == nil {
 			// If the balancer is not registered, move on to the next config.