GenericScope can delegate actual bean creation to lower level

GenericScope has some requirements to synchronize the bean creation
and destruction phases, which exposes it to potential deadlocks (e.g.
as reported in #383). The fact that the bean factory is also taking
locks is taken advantage of to ensure that the same lock is taken on
bean creation in all cases. So all threads have to wait for the same
lock, and there is no danger of interleaving.

spring-cloud-context/src/main/java/org/springframework/cloud/context/scope/GenericScope.java

@@ -49,6 +49,7 @@ import org.springframework.beans.factory.config.Scope;
 import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;
 import org.springframework.beans.factory.support.DefaultListableBeanFactory;
+import org.springframework.beans.factory.support.DefaultSingletonBeanRegistry;
 import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.expression.Expression;
 import org.springframework.expression.ExpressionParser;
@@ -360,7 +361,7 @@ public class GenericScope implements Scope, BeanFactoryPostProcessor,
 	 * @author Dave Syer
 	 *
 	 */
-	private static class BeanLifecycleWrapper {
+	private class BeanLifecycleWrapper {
 
 		private Object bean;
 
@@ -385,9 +386,23 @@ public class GenericScope implements Scope, BeanFactoryPostProcessor,
 
 		public Object getBean() {
 			if (this.bean == null) {
-				synchronized (this.name) {
-					if (this.bean == null) {
-						this.bean = this.objectFactory.getObject();
+				if (beanFactory instanceof DefaultSingletonBeanRegistry
+						&& this.callback == null) {
+					// DefaultSingletonBeanRegistry takes a lock in here that we need to
+					// have in case two threads want to create a bean at the same time.
+					this.bean = ((DefaultSingletonBeanRegistry) beanFactory)
+							.getSingleton(this.name, this.objectFactory);
+					// The callback is null at this point so nothing will happen, but we
+					// don't want the bean registered as a singleton, otherwise it won't
+					// be scoped.
+					((DefaultSingletonBeanRegistry) beanFactory)
+							.destroySingleton(this.name);
+				}
+				else {
+					synchronized (this.name) {
+						if (this.bean == null) {
+							this.bean = this.objectFactory.getObject();
+						}
 					}
 				}
 			}
@@ -497,7 +512,8 @@ public class GenericScope implements Scope, BeanFactoryPostProcessor,
 				}
 				return invocation.proceed();
 			}
-			// see gh-349. Throw the original exception rather than the UndeclaredThrowableException
+			// see gh-349. Throw the original exception rather than the
+			// UndeclaredThrowableException
 			catch (UndeclaredThrowableException e) {
 				throw e.getUndeclaredThrowable();
 			}