HBASE-30346: Fix NPE in AbstractFSWAL WAL-roll debug logging aborting…

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/FSHLog.java

@@ -591,13 +591,21 @@ boolean isLowReplicationRollEnabled() {
       + (3 * Bytes.SIZEOF_INT) + (4 * Bytes.SIZEOF_LONG));
 
   /**
-   * This method gets the pipeline for the current WAL.
+   * This method gets the pipeline for the current WAL. Note that DFSOutputStream#getPipeline() can
+   * legitimately return null (e.g. the underlying streamer is closed, or no block pipeline is
+   * currently established, such as between blocks) -- see HDFS-826. We normalize that to an empty
+   * array here so that callers (in particular debug-log formatting in AbstractFSWAL) never have to
+   * null-check, matching the contract already honored by AsyncFSWAL#getPipeline().
    */
   @Override
   DatanodeInfo[] getPipeline() {
     if (this.hdfs_out != null) {
       if (this.hdfs_out.getWrappedStream() instanceof DFSOutputStream) {
-        return ((DFSOutputStream) this.hdfs_out.getWrappedStream()).getPipeline();
+        DatanodeInfo[] pipeline =
+          ((DFSOutputStream) this.hdfs_out.getWrappedStream()).getPipeline();
+        if (pipeline != null) {
+          return pipeline;
+        }
       }
     }
     return new DatanodeInfo[0];

hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/wal/TestFSHLog.java

@@ -18,6 +18,9 @@
 package org.apache.hadoop.hbase.regionserver.wal;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 import java.io.IOException;
 import java.lang.reflect.Field;
@@ -29,6 +32,7 @@
 import java.util.concurrent.Executors;
 import java.util.concurrent.atomic.AtomicBoolean;
 import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.fs.FSDataOutputStream;
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.hbase.HConstants;
@@ -50,6 +54,9 @@
 import org.apache.hadoop.hbase.util.Threads;
 import org.apache.hadoop.hbase.wal.WALEdit;
 import org.apache.hadoop.hbase.wal.WALKey;
+import org.apache.hadoop.hdfs.DFSOutputStream;
+import org.apache.hadoop.hdfs.client.HdfsDataOutputStream;
+import org.apache.hadoop.hdfs.protocol.DatanodeInfo;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Tag;
 import org.junit.jupiter.api.Test;
@@ -215,4 +222,37 @@ public void run() {
       region.close();
     }
   }
+
+  /**
+   * Regression test for HBASE-30346: FSHLog#getPipeline() must never return null, even when the
+   * underlying DFSOutputStream#getPipeline() legitimately returns null (e.g. the DFS streamer is
+   * closed, or no block pipeline is currently established -- see HDFS-826 and the
+   * DFSOutputStream#getPipeline() javadoc: "returns the list of targets, if any"). Prior to this
+   * fix, AbstractFSWAL#rollWriterInternal's debug-log statement called
+   * Arrays.stream(getPipeline()), which threw a NullPointerException whenever this happened during
+   * a WAL roll, aborting the RegionServer on what was otherwise a successful roll.
+   */
+  @Test
+  public void testGetPipelineDoesNotReturnNullWhenUnderlyingStreamerHasNone() throws Exception {
+    FS.mkdirs(new Path(CommonFSUtils.getRootDir(CONF), this.name));
+    try (FSHLog log = new FSHLog(FS, CommonFSUtils.getRootDir(CONF), this.name,
+      HConstants.HREGION_OLDLOGDIR_NAME, CONF, null, true, null, null)) {
+      log.init();
+
+      // Simulate the legitimate HDFS contract: the wrapped DFSOutputStream currently has no
+      // established pipeline (e.g. streamer closed, or between blocks) and returns null.
+      DFSOutputStream mockDfsOut = mock(DFSOutputStream.class);
+      when(mockDfsOut.getPipeline()).thenReturn(null);
+      FSDataOutputStream wrappedOut = new HdfsDataOutputStream(mockDfsOut, null);
+
+      Field hdfsOutField = FSHLog.class.getDeclaredField("hdfs_out");
+      hdfsOutField.setAccessible(true);
+      hdfsOutField.set(log, wrappedOut);
+
+      DatanodeInfo[] pipeline = log.getPipeline();
+      assertNotNull(pipeline, "getPipeline() must never return null");
+      assertEquals(0, pipeline.length,
+        "Should normalize a null underlying pipeline to an empty array");
+    }
+  }
 }