core: Validate load-balancing policy name early in ManagedChannelBuilder (#12695)

Author: themechbro

api/src/main/java/io/grpc/ManagedChannelBuilder.java

@@ -282,6 +282,10 @@ public T useTransportSecurity() {
    */
   @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1771")
   public T defaultLoadBalancingPolicy(String policy) {
+    Preconditions.checkArgument(
+      LoadBalancerRegistry.getDefaultRegistry().getProvider(policy) != null,
+      "invalid load balancing policy %s", policy
+    );
     throw new UnsupportedOperationException();
   }
 

core/src/main/java/io/grpc/internal/ManagedChannelImplBuilder.java

@@ -39,6 +39,7 @@
 import io.grpc.InternalChannelz;
 import io.grpc.InternalConfiguratorRegistry;
 import io.grpc.InternalFeatureFlags;
+import io.grpc.LoadBalancerRegistry;
 import io.grpc.ManagedChannel;
 import io.grpc.ManagedChannelBuilder;
 import io.grpc.MethodDescriptor;
@@ -447,6 +448,12 @@ public ManagedChannelImplBuilder defaultLoadBalancingPolicy(String policy) {
         "directServerAddress is set (%s), which forbids the use of load-balancing policy",
         directServerAddress);
     Preconditions.checkArgument(policy != null, "policy cannot be null");
+
+    Preconditions.checkArgument(
+      LoadBalancerRegistry.getDefaultRegistry().getProvider(policy) != null,
+      "invalid load-balancing policy %s", policy
+    );
+
     this.defaultLbPolicy = policy;
     return this;
   }