T-11913 Avoid ConcurrentModificationException under heavy loads (#24)

Author: PetrHeinz

examples/gradle/build.gradle

@@ -7,7 +7,7 @@ repositories {
 }
 
 dependencies {
-    implementation 'com.logtail:logback-logtail:0.3.4'
+    implementation 'com.logtail:logback-logtail:0.3.5'
     implementation 'ch.qos.logback:logback-classic:1.2.11'
     implementation 'ch.qos.logback:logback-core:1.2.11'
     implementation 'com.fasterxml.jackson.core:jackson-databind:2.13.5'

examples/gradle/src/main/resources/logback.xml

@@ -2,6 +2,7 @@
     <appender name="Logtail" class="com.logtail.logback.LogtailAppender">
         <appName>Better Stack Gradle example app</appName>
         <sourceToken><!-- YOUR SOURCE TOKEN --></sourceToken>
+        <ingestUrl>https://<!-- YOUR INGESTING HOST --></ingestUrl>
         <mdcFields>requestId,requestTime</mdcFields>
         <mdcTypes>string,int</mdcTypes>
         <objectMapperModule>com.fasterxml.jackson.datatype.jsr310.JavaTimeModule</objectMapperModule>

examples/maven/pom.xml

@@ -26,7 +26,7 @@
         <dependency>
             <groupId>com.logtail</groupId>
             <artifactId>logback-logtail</artifactId>
-            <version>0.3.4</version>
+            <version>0.3.5</version>
         </dependency>
         <dependency>
             <groupId>ch.qos.logback</groupId>

examples/maven/src/main/resources/logback.xml

@@ -2,6 +2,7 @@
     <appender name="Logtail" class="com.logtail.logback.LogtailAppender">
         <appName>Better Stack Maven example app</appName>
         <sourceToken><!-- YOUR SOURCE TOKEN --></sourceToken>
+        <ingestUrl>https://<!-- YOUR INGESTING HOST --></ingestUrl>
         <mdcFields>requestId,requestTime</mdcFields>
         <mdcTypes>string,int</mdcTypes>
         <objectMapperModule>com.fasterxml.jackson.datatype.jsr310.JavaTimeModule</objectMapperModule>

pom.xml

@@ -11,7 +11,7 @@
     <groupId>com.logtail</groupId>
     <artifactId>logback-logtail</artifactId>
     <packaging>jar</packaging>
-    <version>0.3.4</version>
+    <version>0.3.5</version>
 
     <name>${project.groupId}:${project.artifactId}</name>
     <description>Logback Java appender for sending logs to BetterStack.com</description>

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

@@ -161,7 +161,9 @@ protected boolean flushLogs(int flushedSize) {
 
         try {
             if (retries > maxRetries) {
-                batch.subList(0, flushedSize).clear();
+                synchronized (batch) {
+                    batch.subList(0, flushedSize).clear();
+                }
                 logger.error("Dropped batch of {} logs.", flushedSize);
                 warnAboutMaxQueueSize = true;
                 retries = 0;
@@ -187,7 +189,9 @@ protected boolean flushLogs(int flushedSize) {
                 return false;
             }
 
-            batch.subList(0, flushedSize).clear();
+            synchronized (batch) {
+                batch.subList(0, flushedSize).clear();
+            }
             warnAboutMaxQueueSize = true;
             retries = 0;
 
@@ -246,9 +250,12 @@ protected HttpURLConnection getHttpURLConnection() throws IOException {
     }
 
     protected String batchToJson(int flushedSize) throws JsonProcessingException {
+        List<ILoggingEvent> snapshot;
+        synchronized (batch) {
+            snapshot = new ArrayList<>(batch.subList(0, flushedSize));
+        }
         return this.dataMapper.writeValueAsString(
-            new ArrayList<>(batch.subList(0, flushedSize))
-                .stream()
+            snapshot.stream()
                 .map(this::buildPostData)
                 .collect(Collectors.toList())
         );