Avoid potentially mutating item metadata collection

Closes gh-13027

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/metadata/ConfigurationMetadata.java

@@ -146,8 +146,8 @@ public class ConfigurationMetadata {
 	}
 
 	private ItemMetadata findMatchingItemMetadata(ItemMetadata metadata) {
-		List<ItemMetadata> candidates = this.items.get(metadata.getName());
-		if (candidates == null || candidates.isEmpty()) {
+		List<ItemMetadata> candidates = getCandidates(metadata.getName());
+		if (candidates.isEmpty()) {
 			return null;
 		}
 		ListIterator<ItemMetadata> it = candidates.listIterator();
@@ -167,6 +167,12 @@ public class ConfigurationMetadata {
 		return null;
 	}
 
+	private List<ItemMetadata> getCandidates(String name) {
+		List<ItemMetadata> candidates = this.items.get(name);
+		return (candidates != null ? new ArrayList<ItemMetadata>(candidates)
+				: new ArrayList<ItemMetadata>());
+	}
+
 	private boolean nullSafeEquals(Object o1, Object o2) {
 		if (o1 == o2) {
 			return true;

spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessorTests.java

@@ -543,6 +543,17 @@ public class ConfigurationMetadataAnnotationProcessorTests {
 				.fromSource(AdditionalMetadata.class));
 	}
 
+	@Test
+	public void mergingOfAdditionalPropertyMatchingGroup() throws Exception {
+		ItemMetadata property = ItemMetadata.newProperty(null, "simple",
+				"java.lang.String", null, null, null, null, null);
+		writeAdditionalMetadata(property);
+		ConfigurationMetadata metadata = compile(SimpleProperties.class);
+		assertThat(metadata).has(Metadata.withGroup("simple")
+				.fromSource(SimpleProperties.class));
+		assertThat(metadata).has(Metadata.withProperty("simple", String.class));
+	}
+
 	@Test
 	public void mergeExistingPropertyDefaultValue() throws Exception {
 		ItemMetadata property = ItemMetadata.newProperty("simple", "flag", null, null,