HBASE-24940: runCatalogJanitor() API should return -1 to indicate already running status

Closes #2331

Co-authored-by: Viraj Jasani <vjasani@apache.org>

Signed-off-by: Viraj Jasani <vjasani@apache.org>

hbase-client/src/main/java/org/apache/hadoop/hbase/client/Admin.java

@@ -779,7 +779,7 @@ public interface Admin extends Abortable, Closeable {
   /**
    * Ask for a scan of the catalog table
    *
-   * @return the number of entries cleaned
+   * @return the number of entries cleaned. Returns -1 if previous run is in progress.
    * @throws IOException if a remote or network exception occurs
    */
   int runCatalogScan() throws IOException;

hbase-server/src/main/java/org/apache/hadoop/hbase/master/CatalogJanitor.java

@@ -238,7 +238,9 @@ public class CatalogJanitor extends ScheduledChore {
   int scan() throws IOException {
     try {
       if (!alreadyRunning.compareAndSet(false, true)) {
-        return 0;
+        LOG.debug("CatalogJanitor already running");
+        // -1 indicates previous scan is in progress
+        return -1;
       }
       Triple<Integer, Map<HRegionInfo, Result>, Map<HRegionInfo, Result>> scanTriple =
         getMergedRegionsAndSplitParents();

hbase-server/src/test/java/org/apache/hadoop/hbase/master/TestCatalogJanitor.java

@@ -26,6 +26,7 @@ import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.spy;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 import java.util.SortedMap;
@@ -1068,6 +1069,37 @@ public class TestCatalogJanitor {
     janitor.cancel(true);
   }
 
+  @Test
+  public void testAlreadyRunningStatus() throws Exception {
+    int numberOfThreads = 2;
+    final List<Integer> gcValues = new ArrayList<>();
+    Thread[] threads = new Thread[numberOfThreads];
+    HBaseTestingUtility hBaseTestingUtility = new HBaseTestingUtility();
+    hBaseTestingUtility.getConfiguration().setInt("hbase.client.retries.number", 5);
+    Server server = new MockServer(hBaseTestingUtility);
+    MasterServices services = new MockMasterServices(server);
+    final CatalogJanitor catalogJanitor = new CatalogJanitor(server, services);
+    for (int i = 0; i < numberOfThreads; i++) {
+      threads[i] = new Thread(new Runnable() {
+        @Override
+        public void run() {
+          try {
+            gcValues.add(catalogJanitor.scan());
+          } catch (IOException e) {
+            throw new RuntimeException(e);
+          }
+        }
+      });
+    }
+    for (int i = 0; i < numberOfThreads; i++) {
+      threads[i].start();
+    }
+    for (int i = 0; i < numberOfThreads; i++) {
+      threads[i].join();
+    }
+    assertTrue("One janitor.scan() call should have returned -1", gcValues.contains(-1));
+  }
+
   private FileStatus[] addMockStoreFiles(int count, MasterServices services, Path storedir)
       throws IOException {
     // get the existing store files