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

Closes #2331

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

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

@@ -903,7 +903,7 @@ 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 runCatalogJanitor() throws IOException;

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

@@ -162,6 +162,7 @@ public class CatalogJanitor extends ScheduledChore {
    * Run janitorial scan of catalog <code>hbase:meta</code> table looking for
    * garbage to collect.
    * @return How many items gc'd whether for merge or split.
+   *   Returns -1 if previous scan is in progress.
    */
   @VisibleForTesting
   public int scan() throws IOException {
@@ -169,7 +170,8 @@ public class CatalogJanitor extends ScheduledChore {
     try {
       if (!alreadyRunning.compareAndSet(false, true)) {
         LOG.debug("CatalogJanitor already running");
-        return gcs;
+        // -1 indicates previous scan is in progress
+        return -1;
       }
       this.lastReport = scanForReport();
       if (!this.lastReport.isEmpty()) {

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

@@ -25,6 +25,8 @@ 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.NavigableMap;
 import java.util.Objects;
@@ -551,6 +553,29 @@ public class TestCatalogJanitor {
     assertArchiveEqualToOriginal(storeFiles, archivedStoreFiles, fs, true);
   }
 
+  @Test
+  public void testAlreadyRunningStatus() throws Exception {
+    int numberOfThreads = 2;
+    List<Integer> gcValues = new ArrayList<>();
+    Thread[] threads = new Thread[numberOfThreads];
+    for (int i = 0; i < numberOfThreads; i++) {
+      threads[i] = new Thread(() -> {
+        try {
+          gcValues.add(janitor.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