NIFI-4167: StandardResourceClaimManager should not synchronize on a ResourceClaim in order to determine the claim count. This closes #1996

nifi-nar-bundles/nifi-framework-bundle/nifi-framework/nifi-framework-core/src/test/java/org/apache/nifi/controller/repository/claim/TestStandardResourceClaimManager.java

@@ -18,8 +18,13 @@
 package org.apache.nifi.controller.repository.claim;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
 
+import java.util.ArrayList;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 import org.junit.Assert;
@@ -28,6 +33,34 @@ import org.junit.Test;
 
 public class TestStandardResourceClaimManager {
 
+
+    @Test(timeout = 10000)
+    public void testGetClaimantCountWhileMarkingDestructable() throws InterruptedException, ExecutionException {
+        final StandardResourceClaimManager manager = new StandardResourceClaimManager();
+
+        for (int i = 0; i < 50000; i++) {
+            final ResourceClaim rc = manager.newResourceClaim("container", "section", String.valueOf(i), false, false);
+            manager.markDestructable(rc);
+        }
+
+        final Object completedObject = new Object();
+        final CompletableFuture<Object> future = new CompletableFuture<>();
+        final ResourceClaim lastClaim = manager.newResourceClaim("container", "section", "lastOne", false, false);
+        final Thread backgroundThread = new Thread(() -> {
+            manager.markDestructable(lastClaim);
+            future.complete(completedObject);
+        });
+
+        backgroundThread.start();
+
+        Thread.sleep(10);
+        assertEquals(0, manager.getClaimantCount(lastClaim));
+        assertNull(future.getNow(null));
+        manager.drainDestructableClaims(new ArrayList<>(), 1);
+        assertTrue(completedObject == future.get());
+    }
+
+
     @Test
     @Ignore("Unit test was created to repeat a concurrency bug in StandardResourceClaimManager. "
         + "However, now that the concurrency bug has been fixed, the test will deadlock. Leaving here for now in case it's valuable before the commit is pushed")

nifi-nar-bundles/nifi-framework-bundle/nifi-framework/nifi-repository-models/src/main/java/org/apache/nifi/controller/repository/claim/StandardResourceClaimManager.java

@@ -70,10 +70,15 @@ public class StandardResourceClaimManager implements ResourceClaimManager {
             return 0;
         }
 
-        synchronized (claim) {
-            final ClaimCount counter = claimantCounts.get(claim);
-            return counter == null ? 0 : counter.getCount().get();
-        }
+        // No need to synchronize on the Resource Claim here, since this is simply obtaining a value.
+        // We synchronize elsewhere because we want to atomically perform multiple operations, such as
+        // getting the claimant count and then updating a queue. However, the operation of obtaining
+        // the ClaimCount and getting its count value has no side effect and therefore can be performed
+        // without synchronization (since the claimantCounts map and the ClaimCount are also both thread-safe
+        // and there is no need for the two actions of obtaining the ClaimCount and getting its Count value
+        // to be performed atomically).
+        final ClaimCount counter = claimantCounts.get(claim);
+        return counter == null ? 0 : counter.getCount().get();
     }
 
     @Override