AMBARI-20811. Trying to upgrade HDF 2.0.2.0 to HDF 3.0.0.0 fails with NPE (dlysnichenko)

ambari-server/src/main/java/org/apache/ambari/server/stack/StackDirectory.java

@@ -20,6 +20,7 @@ package org.apache.ambari.server.stack;
 
 import java.io.File;
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashMap;
@@ -438,6 +439,9 @@ public class StackDirectory extends StackDefinitionDirectory {
     if (configUpgradePack != null) {
       this.configUpgradePack = configUpgradePack;
     } else {
+      ConfigUpgradePack emptyConfigUpgradePack = new ConfigUpgradePack();
+      emptyConfigUpgradePack.services = new ArrayList<>();
+      this.configUpgradePack = emptyConfigUpgradePack;
       LOG.info("Stack '{}' doesn't contain config upgrade pack file", getPath());
     }
   }

ambari-server/src/test/java/org/apache/ambari/server/stack/StackManagerTest.java

@@ -58,6 +58,7 @@ import org.apache.ambari.server.state.PropertyInfo;
 import org.apache.ambari.server.state.ServiceInfo;
 import org.apache.ambari.server.state.ServiceOsSpecific;
 import org.apache.ambari.server.state.StackInfo;
+import org.apache.ambari.server.state.stack.ConfigUpgradePack;
 import org.apache.ambari.server.state.stack.MetricDefinition;
 import org.apache.ambari.server.state.stack.OsFamily;
 import org.apache.ambari.server.state.stack.UpgradePack;
@@ -712,7 +713,7 @@ public class StackManagerTest {
   }
 
   /**
-   * Tests that {@link UpgradePack} instances are correctly initialized
+   * Tests that {@link UpgradePack} and {@link ConfigUpgradePack} instances are correctly initialized
    * post-unmarshalling.
    *
    * @throws Exception
@@ -729,6 +730,9 @@ public class StackManagerTest {
       // reference equality (make sure it's the same list)
       assertTrue(upgradePack.getTasks() == upgradePack.getTasks());
     }
+    ConfigUpgradePack configUpgradePack = stack.getConfigUpgradePack();
+    assertNotNull(configUpgradePack);
+    assertNotNull(configUpgradePack.services);
   }
 
   @Test