Capture the MDC and thread name before queueing the event

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

@@ -101,6 +101,10 @@ protected void append(ILoggingEvent event) {
             return;
         }
 
+        // The batch is serialized later on another thread: take the formatted message, the thread name and the
+        // MDC from the logging thread now, as logback's own AsyncAppender does
+        event.prepareForDeferredProcessing();
+
         if (batch.size() < maxQueueSize) {
             batch.add(event);
         }

src/test/java/com/logtail/logback/LogtailAppenderMdcTest.java

@@ -0,0 +1,61 @@
+package com.logtail.logback;
+
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.LoggerContext;
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.Test;
+import org.slf4j.MDC;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Events are queued in append() and serialized later on the sender thread, so everything taken from the logging
+ * thread (its MDC, its name) has to be captured while the event is still on that thread.
+ */
+public class LogtailAppenderMdcTest {
+
+    @Test
+    public void testMdcAndThreadNameAreCapturedWhenTheEventIsQueued() throws Exception {
+        LoggerContext context = new LoggerContext();
+        LogtailAppender appender = new LogtailAppender();
+        appender.setContext(context);
+        appender.setSourceToken("source-token");
+        appender.setMdcFields("requestId");
+        appender.setMdcTypes("string");
+        appender.start();
+        Logger logger = context.getLogger("mdc-test");
+        logger.addAppender(appender);
+
+        MDC.put("requestId", "req-42");
+        try {
+            logger.info("Queued while the MDC was set");
+        } finally {
+            MDC.remove("requestId");
+        }
+
+        AtomicReference<String> json = new AtomicReference<>();
+        Thread sender = new Thread(() -> {
+            try {
+                json.set(appender.batchToJson(1));
+            } catch (Exception e) {
+                throw new RuntimeException(e);
+            }
+        }, "logtail-appender");
+        sender.start();
+        sender.join();
+
+        Map<String, Object> line = new ObjectMapper().readValue(json.get(), new TypeReference<List<Map<String, Object>>>() {}).get(0);
+        Map<String, Object> meta = new HashMap<>();
+        meta.put("logger", "mdc-test");
+        meta.put("requestId", "req-42");
+        assertEquals(meta, line.get("meta"));
+        assertEquals(Collections.singletonMap("thread", "main"), line.get("runtime"));
+    }
+}