[SPARK-3788] [yarn] Fix compareFs to do the right thing for HDFS namespaces (1.1 version).

HA and viewfs use namespaces instead of host names, so you can't
resolve them since that will fail. So be smarter to avoid doing
unnecessary work.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #2650 from vanzin/SPARK-3788-1.1 and squashes the following commits:

174bf71 [Marcelo Vanzin] Update comment.
0e36be7 [Marcelo Vanzin] Use Objects.equal() instead of ==.
772aead [Marcelo Vanzin] [SPARK-3788] [yarn] Fix compareFs to do the right thing for HA, federation (1.1 version).

yarn/common/src/main/scala/org/apache/spark/deploy/yarn/ClientBase.scala

@@ -24,6 +24,7 @@ import scala.collection.JavaConversions._
 import scala.collection.mutable.{HashMap, ListBuffer, Map}
 import scala.util.{Try, Success, Failure}
 
+import com.google.common.base.Objects
 import org.apache.hadoop.conf.Configuration
 import org.apache.hadoop.fs._
 import org.apache.hadoop.fs.permission.FsPermission
@@ -122,15 +123,17 @@ trait ClientBase extends Logging {
   private def compareFs(srcFs: FileSystem, destFs: FileSystem): Boolean = {
     val srcUri = srcFs.getUri()
     val dstUri = destFs.getUri()
-    if (srcUri.getScheme() == null) {
-      return false
-    }
-    if (!srcUri.getScheme().equals(dstUri.getScheme())) {
+    if (srcUri.getScheme() == null || srcUri.getScheme() != dstUri.getScheme()) {
       return false
     }
+
     var srcHost = srcUri.getHost()
     var dstHost = dstUri.getHost()
-    if ((srcHost != null) && (dstHost != null)) {
+
+    // In HA or when using viewfs, the host part of the URI may not actually be a host, but the
+    // name of the HDFS namespace. Those names won't resolve, so avoid even trying if they
+    // match.
+    if (srcHost != null && dstHost != null && srcHost != dstHost) {
       try {
         srcHost = InetAddress.getByName(srcHost).getCanonicalHostName()
         dstHost = InetAddress.getByName(dstHost).getCanonicalHostName()
@@ -138,19 +141,9 @@ trait ClientBase extends Logging {
         case e: UnknownHostException =>
           return false
       }
-      if (!srcHost.equals(dstHost)) {
-        return false
-      }
-    } else if (srcHost == null && dstHost != null) {
-      return false
-    } else if (srcHost != null && dstHost == null) {
-      return false
-    }
-    if (srcUri.getPort() != dstUri.getPort()) {
-      false
-    } else {
-      true
     }
+
+    Objects.equal(srcHost, dstHost) && srcUri.getPort() == dstUri.getPort()
   }
 
   /** Copy the file into HDFS if needed. */
@@ -621,7 +614,7 @@ object ClientBase extends Logging {
     YarnSparkHadoopUtil.addToEnvironment(env, Environment.CLASSPATH.name, path,
             File.pathSeparator)
 
-  /** 
+  /**
    * Get the list of namenodes the user may access.
    */
   private[yarn] def getNameNodesToAccess(sparkConf: SparkConf): Set[Path] = {