Fix merging of structured data elements with the same SD-ID in Rfc5424Layout

log4j-core-test/src/test/java/org/apache/logging/log4j/core/layout/Rfc5424LayoutTest.java

@@ -313,6 +313,36 @@ void testCollection() {
         }
     }
 
+    @ParameterizedTest
+    @ValueSource(strings = {"Transfer", "Transfer@32473"})
+    void testCollectionMergesElementsWithSameId(final String id) {
+        final Rfc5424Layout layout = Rfc5424Layout.newBuilder().build();
+        final StructuredDataMessage msg = new StructuredDataMessage(id, "Transfer Complete", "Audit");
+        msg.put("Amount", "200.00");
+        final StructuredDataMessage msg2 = new StructuredDataMessage(id, null, "Audit");
+        msg2.put("ToAccount", "123456");
+        final LogEvent event = Log4jLogEvent.newBuilder()
+                .setMessage(new StructuredDataCollectionMessage(Arrays.asList(msg, msg2)))
+                .build();
+
+        assertThat(layout.toSerializable(event)).contains("[Transfer@32473 Amount=\"200.00\" ToAccount=\"123456\"]");
+    }
+
+    @Test
+    void testMdcElementMergesStructuredDataWithSameId() {
+        final Rfc5424Layout layout = Rfc5424Layout.newBuilder().build();
+        final StringMap contextData = ContextDataFactory.createContextData();
+        contextData.putValue("loginId", "JohnDoe");
+        final StructuredDataMessage msg = new StructuredDataMessage("mdc", "Transfer Complete", "Audit");
+        msg.put("Amount", "200.00");
+        final LogEvent event = Log4jLogEvent.newBuilder()
+                .setMessage(msg)
+                .setContextData(contextData)
+                .build();
+
+        assertThat(layout.toSerializable(event)).contains("[mdc@32473 Amount=\"200.00\" loginId=\"JohnDoe\"]");
+    }
+
     /**
      * Test case for escaping newlines and other SD PARAM-NAME special characters.
      */

log4j-core/src/main/java/org/apache/logging/log4j/core/layout/Rfc5424Layout.java

@@ -442,7 +442,7 @@ private void addStructuredData(
         final String sdId = getId(id);
 
         if (sdElements.containsKey(sdId)) {
-            final StructuredDataElement union = sdElements.get(id.toString());
+            final StructuredDataElement union = sdElements.get(sdId);
             union.union(map);
             sdElements.put(sdId, union);
         } else {
@@ -1214,7 +1214,7 @@ public StructuredDataElement format(final LogEvent event) {
 
     private class StructuredDataElement {
 
-        private final Map<String, String> fields;
+        private Map<String, String> fields;
         private final boolean discardIfEmpty;
         private final String prefix;
 
@@ -1240,7 +1240,10 @@ boolean discard() {
         }
 
         void union(final Map<String, String> addFields) {
-            this.fields.putAll(addFields);
+            // The current fields might be unmodifiable, e.g. those of a `StructuredDataMessage`
+            final Map<String, String> union = new HashMap<>(fields);
+            union.putAll(addFields);
+            this.fields = union;
         }
 
         Map<String, String> getFields() {

src/changelog/.2.x.x/4420_fix_rfc5424_layout_merge_structured_data.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xmlns="https://logging.apache.org/xml/ns"
+       xsi:schemaLocation="https://logging.apache.org/xml/ns https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+  <issue id="4420" link="https://github.com/apache/logging-log4j2/pull/4420"/>
+  <description format="asciidoc">
+    Fix `Rfc5424Layout` failing on two SD elements with one SD-ID.
+  </description>
+</entry>