GH-7: Avoid NPE when performing concurrent destroys.

Author: jdeppe-pivotal

src/main/java/dev/gemfire/dtype/DCountDownLatch.java

@@ -6,12 +6,16 @@ public interface DCountDownLatch {
   /**
    * Causes the current thread to wait until the latch has counted down to zero, unless the thread
    * is interrupted.
+   * <p>
+   * If another thread destroys the latch, the awaiting thread will throw a {@code DTypeException}.
    */
   void await();
 
   /**
    * Causes the current thread to wait until the latch has counted down to zero, unless the thread
    * is interrupted, or the specified waiting time has elapsed.
+   * <p>
+   * If another thread destroys the latch, the awaiting thread will throw a {@code DTypeException}.
    *
    * @param timeout the maximum time to wait
    * @param unit the time unit of the timeout argument

src/main/java/dev/gemfire/dtype/internal/CollectionsBackendFunction.java

@@ -32,6 +32,12 @@ public void execute(FunctionContext<Object> context) {
     Region<String, AbstractDType> region = ((RegionFunctionContext<?>) context).getDataSet();
     AbstractDType entry = region.get(name);
 
+    // This may be the case if multiple clients are destroying an entry concurrently.
+    if (entry == null) {
+      context.getResultSender().lastResult(null);
+      return;
+    }
+
     Callable<Object> wrappingFn = () -> {
       Object innerResult;
       if (operationType == QUERY) {

src/test/java/dev/gemfire/dtype/AbstractDCountDownLatchTest.java

@@ -1,6 +1,7 @@
 package dev.gemfire.dtype;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
 import static org.assertj.core.api.Assertions.assertThatNoException;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
@@ -133,7 +134,7 @@ public void testCountDownConcurrency() {
   }
 
   @Test
-  public void testDestroyReleasesAwaitingClient() throws Exception {
+  public void testDestroyReleasesAwaitingClient() {
     DCountDownLatch ref1 = getFactory().createDCountDownLatch(testName.getMethodName(), 1);
 
     Future<?> future = executor.submit(() -> ref1.await());
@@ -146,4 +147,17 @@ public void testDestroyReleasesAwaitingClient() throws Exception {
         .withFailMessage("countdown latch is destroyed");
   }
 
+  @Test
+  public void testDestroyFromMultipleThreads() {
+    assertThatCode(() -> new ConcurrentLoopingThreads(2_000,
+        i -> createDestroyLatch(),
+        i -> createDestroyLatch()).runInLockstep())
+        .doesNotThrowAnyException();
+  }
+
+  private void createDestroyLatch() {
+    DCountDownLatch latch = getFactory().createDCountDownLatch(testName.getMethodName(), 1);
+    latch.destroy();
+  }
+
 }