HDFS-17368. HA: Standby should exit safemode when resources are available. (#6518). Contributed by Zilong Zhu.

Signed-off-by: He Xiaoqiao <hexiaoqiao@apache.org>

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

@@ -1578,6 +1578,10 @@ void startStandbyServices(final Configuration conf, boolean isObserver)
       standbyCheckpointer = new StandbyCheckpointer(conf, this);
       standbyCheckpointer.start();
     }
+    if (isNoManualAndResourceLowSafeMode()) {
+      LOG.info("Standby should not enter safe mode when resources are low, exiting safe mode.");
+      leaveSafeMode(false);
+    }
   }
 
   /**
@@ -5253,10 +5257,9 @@ String getSafeModeTip() {
     String cmd = "Use \"hdfs dfsadmin -safemode leave\" to turn safe mode off.";
     synchronized (this) {
       if (resourceLowSafeMode) {
-        return "Resources are low on NN. Please add or free up more resources"
-            + "then turn off safe mode manually. NOTE:  If you turn off safe "
-            + "mode before adding resources, the NN will immediately return to "
-            + "safe mode. " + cmd;
+        return "Resources are low on NN. Please add or free up more resources. "
+            + "NOTE:  If you turn off safe mode before adding resources, the "
+            + "NN will immediately return to safe mode. ";
       } else if (manualSafeMode) {
         return "It was turned on manually. " + cmd;
       }

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

@@ -999,4 +999,11 @@ public void testTransitionToObserverWhenSafeMode() throws Exception {
           () -> miniCluster.transitionToObserver(0));
     }
   }
+
+  @Test
+  public void testTransitionToStandbyWhenSafeModeWithResourcesLow() throws Exception {
+    NameNodeAdapter.enterSafeMode(nn0, true);
+    cluster.transitionToStandby(0);
+    assertFalse("SNN should not enter safe mode when resources low", nn0.isInSafeMode());
+  }
 }