Fix for issue#2742 : NullPointerException in disk store under high load on clear()

ehcache-impl/src/main/java/org/ehcache/impl/internal/store/tiering/TieredStore.java

@@ -572,7 +572,10 @@ public NoopCachingTier(final AuthoritativeTier<K, V> authoritativeTier) {
     @Override
     public ValueHolder<V> getOrComputeIfAbsent(final K key, final Function<K, ValueHolder<V>> source) {
       final ValueHolder<V> apply = source.apply(key);
-      authoritativeTier.flush(key, apply);
+      if (apply != null) {
+        //immediately flushes any entries faulted from authority as this tier has no capacity
+        authoritativeTier.flush(key, apply);
+      }
       return apply;
     }
 

ehcache-impl/src/test/java/org/ehcache/impl/internal/store/tiering/TieredStoreTest.java

@@ -560,6 +560,33 @@ public void CachingTierDoesNotSeeAnyOperationDuringClear() throws StoreAccessExc
       ArgumentMatchers.any(), ArgumentMatchers.any());
   }
 
+  @Test
+  public void AuthoritativeTierNullCheckDuringFlush() throws StoreAccessException, BrokenBarrierException, InterruptedException {
+    final TieredStore<String, String> tieredStore = new TieredStore<>(stringCachingTier, stringAuthoritativeTier);
+
+    final CyclicBarrier barrier = new CyclicBarrier(2);
+
+    doAnswer((Answer<Void>) invocation -> {
+      barrier.await();
+      barrier.await();
+      return null;
+    }).when(stringAuthoritativeTier).clear();
+    Thread t = new Thread(() -> {
+      try {
+        tieredStore.clear();
+      } catch (Exception e) {
+        throw new RuntimeException(e);
+      }
+    });
+
+    t.start();
+    barrier.await();
+    tieredStore.get("foo");
+    barrier.await();
+    t.join();
+    verify(stringAuthoritativeTier, never()).flush("foo", null);
+  }
+
   @Test
   @SuppressWarnings("unchecked")
   public void testReleaseStoreFlushes() throws Exception {