HDFS-15623. Respect configured values of rpc.engine (#2403) Contributed by Hector Chaverri.

(cherry picked from commit 6eacaffeea21e7e9286497ee17f89fd939d2eead)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/RPC.java

@@ -195,14 +195,18 @@ public class RPC {
   private static final String ENGINE_PROP = "rpc.engine";
 
   /**
-   * Set a protocol to use a non-default RpcEngine.
+   * Set a protocol to use a non-default RpcEngine if one
+   * is not specified in the configuration.
    * @param conf configuration to use
    * @param protocol the protocol interface
    * @param engine the RpcEngine impl
    */
   public static void setProtocolEngine(Configuration conf,
                                 Class<?> protocol, Class<?> engine) {
-    conf.setClass(ENGINE_PROP+"."+protocol.getName(), engine, RpcEngine.class);
+    if (conf.get(ENGINE_PROP+"."+protocol.getName()) == null) {
+      conf.setClass(ENGINE_PROP+"."+protocol.getName(), engine,
+                    RpcEngine.class);
+    }
   }
 
   // return the RpcEngine configured to handle a protocol

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/ipc/TestRPC.java

@@ -1554,6 +1554,18 @@ public class TestRPC extends TestRpcBase {
     }
   }
 
+  @Test
+  public void testSetProtocolEngine() {
+    Configuration conf = new Configuration();
+    RPC.setProtocolEngine(conf, StoppedProtocol.class, StoppedRpcEngine.class);
+    RpcEngine rpcEngine = RPC.getProtocolEngine(StoppedProtocol.class, conf);
+    assertTrue(rpcEngine instanceof StoppedRpcEngine);
+
+    RPC.setProtocolEngine(conf, StoppedProtocol.class, ProtobufRpcEngine.class);
+    rpcEngine = RPC.getProtocolEngine(StoppedProtocol.class, conf);
+    assertTrue(rpcEngine instanceof StoppedRpcEngine);
+  }
+
   public static void main(String[] args) throws Exception {
     new TestRPC().testCallsInternal(conf);
   }