HDFS-15290. NPE in HttpServer during NameNode startup. Contributed by Simbarashe Dzinamarira.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/ImageServlet.java

@@ -112,12 +112,25 @@ public class ImageServlet extends HttpServlet {
     recentImageCheckTimePrecision = ratio;
   }
 
+  private FSImage getAndValidateFSImage(ServletContext context,
+      final HttpServletResponse response)
+      throws IOException {
+    final FSImage nnImage = NameNodeHttpServer.getFsImageFromContext(context);
+    if (nnImage == null) {
+      String errorMsg = "NameNode initialization not yet complete. "
+          + "FSImage has not been set in the NameNode.";
+      response.sendError(HttpServletResponse.SC_FORBIDDEN, errorMsg);
+      throw new IOException(errorMsg);
+    }
+    return nnImage;
+  }
+
   @Override
   public void doGet(final HttpServletRequest request,
       final HttpServletResponse response) throws ServletException, IOException {
     try {
       final ServletContext context = getServletContext();
-      final FSImage nnImage = NameNodeHttpServer.getFsImageFromContext(context);
+      final FSImage nnImage = getAndValidateFSImage(context, response);
       final GetImageParams parsedParams = new GetImageParams(request, response);
       final Configuration conf = (Configuration) context
           .getAttribute(JspHelper.CURRENT_CONF);
@@ -524,7 +537,7 @@ public class ImageServlet extends HttpServlet {
       final HttpServletResponse response) throws ServletException, IOException {
     try {
       ServletContext context = getServletContext();
-      final FSImage nnImage = NameNodeHttpServer.getFsImageFromContext(context);
+      final FSImage nnImage = getAndValidateFSImage(context, response);
       final Configuration conf = (Configuration) getServletContext()
           .getAttribute(JspHelper.CURRENT_CONF);
       final PutImageParams parsedParams = new PutImageParams(request, response,

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/NameNodeAdapter.java

@@ -56,6 +56,7 @@ import org.apache.hadoop.ipc.StandbyException;
 import org.apache.hadoop.security.AccessControlException;
 import org.apache.hadoop.test.Whitebox;
 import org.mockito.Mockito;
+import static org.apache.hadoop.hdfs.server.namenode.NameNodeHttpServer.FSIMAGE_ATTRIBUTE_KEY;
 
 /**
  * This is a utility class to expose NameNode functionality for unit tests.
@@ -129,6 +130,17 @@ public class NameNodeAdapter {
     return ((NameNodeRpcServer)namenode.getRpcServer()).clientRpcServer;
   }
 
+  /**
+   * Sets the FSImage used in the NameNodeHttpServer and returns the old value.
+   */
+  public static FSImage getAndSetFSImageInHttpServer(NameNode namenode,
+      FSImage fsImage) {
+    FSImage previous = (FSImage) namenode.httpServer.getHttpServer()
+        .getAttribute(FSIMAGE_ATTRIBUTE_KEY);
+    namenode.httpServer.setFSImage(fsImage);
+    return  previous;
+  }
+
   public static DelegationTokenSecretManager getDtSecretManager(
       final FSNamesystem ns) {
     return ns.getDelegationTokenSecretManager();

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/ha/TestStandbyCheckpoints.java

@@ -21,6 +21,8 @@ import java.util.function.Supplier;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Lists;
+import org.apache.hadoop.hdfs.LogVerificationAppender;
+import org.apache.log4j.spi.LoggingEvent;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.apache.hadoop.conf.Configuration;
@@ -287,6 +289,49 @@ public class TestStandbyCheckpoints {
     cluster.transitionToStandby(2);
   }
 
+  /**
+   * Tests that a null FSImage is handled gracefully by the ImageServlet.
+   * If putImage is called while a NameNode is still starting up, the FSImage
+   * may not have been initialized yet. See HDFS-15290.
+   */
+  @Test(timeout = 30000)
+  public void testCheckpointBeforeNameNodeInitializationIsComplete()
+      throws Exception {
+    final LogVerificationAppender appender = new LogVerificationAppender();
+    final org.apache.log4j.Logger logger = org.apache.log4j.Logger
+        .getRootLogger();
+    logger.addAppender(appender);
+
+    // Transition 2 to observer
+    cluster.transitionToObserver(2);
+    doEdits(0, 10);
+    // After a rollEditLog, Standby(nn1)'s next checkpoint would be
+    // ahead of observer(nn2).
+    nns[0].getRpcServer().rollEditLog();
+
+    NameNode nn2 = nns[2];
+    FSImage nnFSImage = NameNodeAdapter.getAndSetFSImageInHttpServer(nn2, null);
+
+    // After standby creating a checkpoint, it will try to push the image to
+    // active and all observer, updating it's own txid to the most recent.
+    HATestUtil.waitForCheckpoint(cluster, 1, ImmutableList.of(12));
+    HATestUtil.waitForCheckpoint(cluster, 0, ImmutableList.of(12));
+
+    NameNodeAdapter.getAndSetFSImageInHttpServer(nn2, nnFSImage);
+    cluster.transitionToStandby(2);
+    logger.removeAppender(appender);
+
+    for (LoggingEvent event : appender.getLog()) {
+      String message = event.getRenderedMessage();
+      if (message.contains("PutImage failed") &&
+          message.contains("FSImage has not been set in the NameNode.")) {
+        //Logs have the expected exception.
+        return;
+      }
+    }
+    fail("Expected exception not present in logs.");
+  }
+
   /**
    * Test for the case when the SBN is configured to checkpoint based
    * on a time period, but no transactions are happening on the