Make DelegatingAsyncDisruptorAppender more resilient to exceptions + flush Flushable appenders at end of batch (#457)

* Make DelegatingAsyncDisruptorAppender more resilient to exceptions thrown by child appenders

- Wrap calls to appenders within a try/catch block and make sure exceptions thrown by one does not prevent other appenders from receiving the event (gh456).
- Flush appenders at end of batch only if they are started (gh456).
- OutputStreamAppender may occasionally return a null OutputStream (gh455)
- Flush appenders implementing java.io.Flushable (gh454)

Author: brenuart

src/main/java/net/logstash/logback/appender/DelegatingAsyncDisruptorAppender.java

@@ -13,21 +13,24 @@
  */
 package net.logstash.logback.appender;
 
+import java.io.Flushable;
+import java.io.IOException;
+import java.io.OutputStream;
 import java.util.Iterator;
 import java.util.concurrent.BlockingQueue;
 
-import ch.qos.logback.core.OutputStreamAppender;
-import net.logstash.logback.appender.listener.AppenderListener;
+import com.lmax.disruptor.EventHandler;
+import com.lmax.disruptor.RingBuffer;
+
 import ch.qos.logback.access.spi.IAccessEvent;
 import ch.qos.logback.classic.spi.ILoggingEvent;
 import ch.qos.logback.core.Appender;
 import ch.qos.logback.core.AsyncAppenderBase;
+import ch.qos.logback.core.OutputStreamAppender;
 import ch.qos.logback.core.spi.AppenderAttachable;
 import ch.qos.logback.core.spi.AppenderAttachableImpl;
 import ch.qos.logback.core.spi.DeferredProcessingAware;
-
-import com.lmax.disruptor.EventHandler;
-import com.lmax.disruptor.RingBuffer;
+import net.logstash.logback.appender.listener.AppenderListener;
 
 /**
  * An {@link AsyncDisruptorAppender} that delegates appending of an event
@@ -37,6 +40,8 @@
  * <ul>
  * <li>it uses a {@link RingBuffer} instead of a {@link BlockingQueue}</li>
  * <li>it allows any number of delegate appenders, instead of just one</li>
+ * <li>it flushes appenders of type {@link OutputStreamAppender} or {@link Flushable} at the end of a batch</li>
+ * <li>it is resilient to exceptions and guarantees that all appenders are invoked</li>
  * </ul>
  *
  * @param <Event> type of event ({@link ILoggingEvent} or {@link IAccessEvent}).
@@ -49,28 +54,68 @@ public abstract class DelegatingAsyncDisruptorAppender<Event extends DeferredPro
     private final AppenderAttachableImpl<Event> appenders = new AppenderAttachableImpl<Event>();
 
     private class DelegatingEventHandler implements EventHandler<LogEvent<Event>> {
+        /**
+         * Whether exceptions should be reported with a error status or not.
+         */
+        private boolean silentError;
 
         @Override
         public void onEvent(LogEvent<Event> logEvent, long sequence, boolean endOfBatch) throws Exception {
-            appenders.appendLoopOnAppenders(logEvent.event);
-
-            /*
-             * Optimization:
-             *
-             * If any of the delegate appenders are instances of OutputStreamAppender,
-             * then flush the OutputStreams at the end of the batch.
-             */
-            if (endOfBatch) {
-                for (Iterator<Appender<Event>> iter = appenders.iteratorForAppenders(); iter.hasNext(); ) {
-                    Appender<Event> appender = iter.next();
-                    if (appender instanceof OutputStreamAppender) {
-                        OutputStreamAppender outputStreamAppender = (OutputStreamAppender) appender;
-                        if (!outputStreamAppender.isImmediateFlush()) {
-                            outputStreamAppender.getOutputStream().flush();
-                        }
+            
+            boolean exceptionThrown = false;
+            for(Iterator<Appender<Event>> it=appenders.iteratorForAppenders(); it.hasNext(); ) {
+                Appender<Event> appender = it.next();
+                
+                try {
+                    appender.doAppend(logEvent.event);
+                    
+                    /*
+                     * Optimization:
+                     *
+                     * If any of the delegate appenders are instances of OutputStreamAppender or Flushable,
+                     * then flush them at the end of the batch.
+                     */
+                    if (endOfBatch) {
+                        flushAppender(appender);
+                    }
+                }
+                catch(Exception e) {
+                    exceptionThrown = true;
+                    if (!this.silentError) {
+                        addError(String.format("Unable to forward event to appender [%s]: %s", appender.getName(), e.getMessage()), e);
                     }
                 }
             }
+            
+            this.silentError = exceptionThrown;
+        }
+        
+        
+        private void flushAppender(Appender<Event> appender) throws IOException {
+            // Similar to #doAppend() - don't flush if appender is stopped
+            if (!appender.isStarted()) {
+                return;
+            }
+            if (appender instanceof Flushable) {
+                flushAppender((Flushable)appender);
+            }
+            else
+            if (appender instanceof OutputStreamAppender) {
+                flushAppender((OutputStreamAppender<Event>)appender);
+            }
+        }
+        
+        private void flushAppender(OutputStreamAppender<Event> appender) throws IOException {
+            if (!appender.isImmediateFlush()) {
+                OutputStream os = appender.getOutputStream();
+                if (os != null) {
+                    os.flush();
+                }
+            }
+        }
+        
+        private void flushAppender(Flushable appender) throws IOException {
+            appender.flush();
         }
     }