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

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 #2649 from vanzin/SPARK-3788 and squashes the following commits:

fedbc73 [Marcelo Vanzin] Update comment.
c938845 [Marcelo Vanzin] Use Objects.equal() to avoid issues with ==.
9f7b571 [Marcelo Vanzin] [SPARK-3788] [yarn] Fix compareFs to do the right thing for HA, federation.

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

@@ -23,6 +23,7 @@ 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
@@ -64,12 +65,12 @@ private[spark] trait ClientBase extends Logging {
       s"memory capability of the cluster ($maxMem MB per container)")
     val executorMem = args.executorMemory + executorMemoryOverhead
     if (executorMem > maxMem) {
-      throw new IllegalArgumentException(s"Required executor memory (${args.executorMemory}" + 
+      throw new IllegalArgumentException(s"Required executor memory (${args.executorMemory}" +
         s"+$executorMemoryOverhead MB) is above the max threshold ($maxMem MB) of this cluster!")
     }
     val amMem = args.amMemory + amMemoryOverhead
     if (amMem > maxMem) {
-      throw new IllegalArgumentException(s"Required AM memory (${args.amMemory}" + 
+      throw new IllegalArgumentException(s"Required AM memory (${args.amMemory}" +
         s"+$amMemoryOverhead MB) is above the max threshold ($maxMem MB) of this cluster!")
     }
     logInfo("Will allocate AM container, with %d MB memory including %d MB overhead".format(
@@ -771,15 +772,17 @@ private[spark] object 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()
@@ -787,19 +790,9 @@ private[spark] object 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()
   }
 
 }