Slf4j MDC context propagation with null MDC map fixed

logging/slf4j/src/main/java/io/helidon/logging/slf4j/Slf4jMdcPropagator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020 Oracle and/or its affiliates.
+ * Copyright (c) 2020, 2021 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 package io.helidon.logging.slf4j;
 
 import java.util.Map;
+import java.util.Optional;
 
 import io.helidon.common.context.spi.DataPropagationProvider;
 
@@ -29,7 +30,7 @@ public class Slf4jMdcPropagator implements DataPropagationProvider<Map<String, S
 
     @Override
     public Map<String, String> data() {
-        return MDC.getCopyOfContextMap();
+        return Optional.ofNullable(MDC.getCopyOfContextMap()).orElseGet(Map::of);
     }
 
     @Override

logging/slf4j/src/test/java/Slf4jMdcTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020 Oracle and/or its affiliates.
+ * Copyright (c) 2020, 2021 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,7 @@
 import io.helidon.common.context.ExecutorException;
 import io.helidon.logging.common.HelidonMdc;
 
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
 import org.slf4j.MDC;
 
@@ -38,6 +39,11 @@ public class Slf4jMdcTest {
     private static final String TEST_KEY = "test";
     private static final String TEST_VALUE = "value";
 
+    @AfterEach
+    public void clearMdc() {
+        HelidonMdc.clear();
+    }
+
     @Test
     public void testMdc() {
         HelidonMdc.set(TEST_KEY, TEST_VALUE);
@@ -65,6 +71,21 @@ public void testThreadPropagation() {
         });
     }
 
+    @Test
+    public void testThreadPropagationWithEmptyMdc() {
+        Context context = Context.create();
+        ExecutorService executor = Contexts.wrap(Executors.newFixedThreadPool(1));
+
+        Contexts.runInContext(context, () -> {
+            try {
+                Boolean value = executor.submit(new TestEmptyMdc()).get();
+                assertThat(value, is(true));
+            } catch (Exception e) {
+                throw new ExecutorException("failed to execute", e);
+            }
+        });
+    }
+
     private static final class TestCallable implements Callable<String> {
 
         @Override
@@ -73,4 +94,12 @@ public String call() {
         }
     }
 
+    private static final class TestEmptyMdc implements Callable<Boolean> {
+
+        @Override
+        public Boolean call() {
+            return MDC.getCopyOfContextMap().isEmpty();
+        }
+    }
+
 }