L-849 Add maxQueueSize parameter (default 100k) to avoid memory overf…

…low on network issues (#17)

* LogtailAppender: add maxQueueSize parameter (default 100k)

* Prevent potential dead-lock, when a blocking logger is configured

src/main/java/com/logtail/logback/LogtailAppender.java

@@ -37,6 +37,7 @@ public class LogtailAppender extends UnsynchronizedAppenderBase<ILoggingEvent> {
     protected List<String> mdcFields = new ArrayList<>();
     protected List<String> mdcTypes = new ArrayList<>();
 
+    protected int maxQueueSize = 100000;
     protected int batchSize = 1000;
     protected int batchInterval = 3000;
     protected int connectTimeout = 5000;
@@ -48,6 +49,7 @@ public class LogtailAppender extends UnsynchronizedAppenderBase<ILoggingEvent> {
     protected Vector<ILoggingEvent> batch = new Vector<>();
     protected AtomicBoolean isFlushing = new AtomicBoolean(false);
     protected boolean mustReflush = false;
+    protected boolean warnAboutMaxQueueSize = true;
 
     // Utils
     protected ScheduledExecutorService scheduledExecutorService;
@@ -83,23 +85,40 @@ protected void append(ILoggingEvent event) {
             return;
 
         if (this.ingestUrl.isEmpty() || this.sourceToken == null || this.sourceToken.isEmpty()) {
-            errorLog.warn("Missing Source token for Better Stack - disabling LogtailAppender. Find out how to fix this at: https://betterstack.com/docs/logs/java ");
+            // Prevent potential dead-lock, when a blocking logger is configured - avoid using errorLog directly in append
+            startThread("logtail-warning-logger", () -> {
+                errorLog.warn("Missing Source token for Better Stack - disabling LogtailAppender. Find out how to fix this at: https://betterstack.com/docs/logs/java ");
+            });
             this.disabled = true;
             return;
         }
 
-        batch.add(event);
+        if (batch.size() < maxQueueSize) {
+            batch.add(event);
+        }
+
+        if (warnAboutMaxQueueSize && batch.size() == maxQueueSize) {
+            this.warnAboutMaxQueueSize = false;
+            // Prevent potential dead-lock, when a blocking logger is configured - avoid using errorLog directly in append
+            startThread("logtail-error-logger", () -> {
+                errorLog.error("Maximum number of messages in queue reached ({}). New messages will be dropped.", maxQueueSize);
+            });
+        }
 
         if (batch.size() >= batchSize) {
             if (isFlushing.get())
                 return;
 
-            Thread thread = Executors.defaultThreadFactory().newThread(new LogtailSender());
-            thread.setName("logtail-appender-flush");
-            thread.start();
+            startThread("logtail-appender-flush", new LogtailSender());
         }
     }
 
+    protected void startThread(String threadName, Runnable runnable) {
+        Thread thread = Executors.defaultThreadFactory().newThread(runnable);
+        thread.setName(threadName);
+        thread.start();
+    }
+
     protected void flush() {
         if (batch.isEmpty())
             return;
@@ -119,6 +138,7 @@ protected void flush() {
             }
 
             batch.subList(0, flushedSize).clear();
+            this.warnAboutMaxQueueSize = true;
         } catch (JsonProcessingException e) {
             errorLog.error("Error processing JSON data : {}", e.getMessage(), e);
 
@@ -341,6 +361,16 @@ public void setMdcTypes(String mdcTypes) {
         this.mdcTypes = Arrays.asList(mdcTypes.split(","));
     }
 
+    /**
+     * Sets the maximum number of messages in the queue. Messages over the limit will be dropped.
+     *
+     * @param maxQueueSize
+     *            max size of the message queue
+     */
+    public void setMaxQueueSize(int maxQueueSize) {
+        this.maxQueueSize = maxQueueSize;
+    }
+
     /**
      * Sets the batch size for the number of messages to be sent via the API
      *