prevent NPE when discriminator is not set

logback-core/src/main/java/ch/qos/logback/core/sift/SiftingAppenderBase.java

@@ -76,8 +76,7 @@ public void start() {
         if (discriminator == null) {
             addError("Missing discriminator. Aborting");
             errors++;
-        }
-        if (!discriminator.isStarted()) {
+        } else if (!discriminator.isStarted()) {
             addError("Discriminator has not started successfully. Aborting");
             errors++;
         }

logback-core/src/test/java/ch/qos/logback/core/appender/SiftingAppenderTest.java

@@ -0,0 +1,88 @@
+/**
+ * Logback: the reliable, generic, fast and flexible logging framework.
+ * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
+ *
+ * This program and the accompanying materials are dual-licensed under
+ * either the terms of the Eclipse Public License v1.0 as published by
+ * the Eclipse Foundation
+ *
+ *   or (per the licensee's choosing)
+ *
+ * under the terms of the GNU Lesser General Public License version 2.1
+ * as published by the Free Software Foundation.
+ */
+package ch.qos.logback.core.appender;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+import ch.qos.logback.core.Appender;
+import ch.qos.logback.core.ContextBase;
+import ch.qos.logback.core.sift.DefaultDiscriminator;
+import ch.qos.logback.core.sift.SiftingAppenderBase;
+import ch.qos.logback.core.status.Status;
+
+/**
+ * Basic tests of SiftingAppender.
+ */
+public class SiftingAppenderTest extends AbstractAppenderTest<Object> {
+
+    @Test
+    public void testNoAppenderConfiuration() {
+        final ContextBase context = new ContextBase();
+        final SiftingAppenderBase<Object> appender = getAppender();
+        appender.setContext(context);
+
+        appender.start();
+
+        Assertions.assertEquals(2, context.getStatusManager().getCount());
+        final List<Status> statuses = context.getStatusManager().getCopyOfStatusList();
+        Assertions.assertEquals("Missing discriminator. Aborting", statuses.get(0).getMessage());
+        Assertions.assertEquals("AppenderFactory has not been set. Aborting", statuses.get(1).getMessage());
+    }
+
+    @Test
+    public void testAppenderConfiurationWhenDiscriminatorIsNotStarted() {
+        final ContextBase context = new ContextBase();
+        final SiftingAppenderBase<Object> appender = getAppender();
+        appender.setContext(context);
+        appender.setDiscriminator(new DefaultDiscriminator<>());
+
+        appender.start();
+
+        Assertions.assertEquals(2, context.getStatusManager().getCount());
+        final List<Status> statuses = context.getStatusManager().getCopyOfStatusList();
+        Assertions.assertEquals("Discriminator has not started successfully. Aborting", statuses.get(0).getMessage());
+        Assertions.assertEquals("AppenderFactory has not been set. Aborting", statuses.get(1).getMessage());
+    }
+
+    @Override
+    protected SiftingAppenderBase<Object> getAppender() {
+        return new SiftingAppenderBase<Object>() {
+
+            @Override
+            protected long getTimestamp(Object event) {
+                return 0;
+            }
+
+            @Override
+            protected boolean eventMarksEndOfLife(Object event) {
+                return false;
+            }
+        };
+    }
+
+    @Disabled("SiftingAppender does not call 'super.stop()', and I don't know why")
+    @Test
+    public void testConfiguredAppender() {
+
+    }
+
+    @Override
+    protected Appender<Object> getConfiguredAppender() {
+        throw new UnsupportedOperationException("Implement it while fixing 'testConfiguredAppender'");
+    }
+}