MessageHeaderAccessor handle self-copy correctly

1. Revert changes in setHeader from 5.2.9 that caused regression on self-copy.
2. Update copyHeaders/IfAbsent to ensure a copy of native headers.
3. Exit if source and target are the same instance, as an optimization.

Closes gh-26155

spring-messaging/src/main/java/org/springframework/messaging/support/MessageHeaderAccessor.java

@@ -378,13 +378,14 @@ public class MessageHeaderAccessor {
 	 * {@link #copyHeadersIfAbsent(Map)} to avoid overwriting values.
 	 */
 	public void copyHeaders(@Nullable Map<String, ?> headersToCopy) {
-		if (headersToCopy != null) {
-			headersToCopy.forEach((key, value) -> {
-				if (!isReadOnly(key)) {
-					setHeader(key, value);
-				}
-			});
+		if (headersToCopy == null || this.headers == headersToCopy) {
+			return;
 		}
+		headersToCopy.forEach((key, value) -> {
+			if (!isReadOnly(key)) {
+				setHeader(key, value);
+			}
+		});
 	}
 
 	/**
@@ -392,13 +393,14 @@ public class MessageHeaderAccessor {
 	 * <p>This operation will <em>not</em> overwrite any existing values.
 	 */
 	public void copyHeadersIfAbsent(@Nullable Map<String, ?> headersToCopy) {
-		if (headersToCopy != null) {
-			headersToCopy.forEach((key, value) -> {
-				if (!isReadOnly(key)) {
-					setHeaderIfAbsent(key, value);
-				}
-			});
+		if (headersToCopy == null || this.headers == headersToCopy) {
+			return;
 		}
+		headersToCopy.forEach((key, value) -> {
+			if (!isReadOnly(key)) {
+				setHeaderIfAbsent(key, value);
+			}
+		});
 	}
 
 	protected boolean isReadOnly(String headerName) {

spring-messaging/src/main/java/org/springframework/messaging/support/NativeMessageHeaderAccessor.java

@@ -16,6 +16,7 @@
 
 package org.springframework.messaging.support;
 
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
@@ -75,6 +76,8 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 			@SuppressWarnings("unchecked")
 			Map<String, List<String>> map = (Map<String, List<String>>) getHeader(NATIVE_HEADERS);
 			if (map != null) {
+				// setHeader checks for equality but we need copy of native headers
+				setHeader(NATIVE_HEADERS, null);
 				setHeader(NATIVE_HEADERS, new LinkedMultiValueMap<>(map));
 			}
 		}
@@ -103,6 +106,8 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 		if (isMutable()) {
 			Map<String, List<String>> map = getNativeHeaders();
 			if (map != null) {
+				// setHeader checks for equality but we need immutable wrapper
+				setHeader(NATIVE_HEADERS, null);
 				setHeader(NATIVE_HEADERS, Collections.unmodifiableMap(map));
 			}
 			super.setImmutable();
@@ -110,31 +115,34 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 	}
 
 	@Override
-	public void setHeader(String name, @Nullable Object value) {
-		if (name.equalsIgnoreCase(NATIVE_HEADERS)) {
-			// Force removal since setHeader checks for equality
-			super.setHeader(NATIVE_HEADERS, null);
+	public void copyHeaders(@Nullable Map<String, ?> headersToCopy) {
+		if (headersToCopy == null) {
+			return;
+		}
+
+		@SuppressWarnings("unchecked")
+		Map<String, List<String>> map = (Map<String, List<String>>) headersToCopy.get(NATIVE_HEADERS);
+		if (map != null && map != getNativeHeaders()) {
+			map.forEach(this::setNativeHeaderValues);
 		}
-		super.setHeader(name, value);
+
+		// setHeader checks for equality, native headers should be equal by now
+		super.copyHeaders(headersToCopy);
 	}
 
 	@Override
-	@SuppressWarnings("unchecked")
-	public void copyHeaders(@Nullable Map<String, ?> headersToCopy) {
-		if (headersToCopy != null) {
-			Map<String, List<String>> nativeHeaders = getNativeHeaders();
-			Map<String, List<String>> map = (Map<String, List<String>>) headersToCopy.get(NATIVE_HEADERS);
-			if (map != null) {
-				if (nativeHeaders != null) {
-					nativeHeaders.putAll(map);
-				}
-				else {
-					nativeHeaders = new LinkedMultiValueMap<>(map);
-				}
-			}
-			super.copyHeaders(headersToCopy);
-			setHeader(NATIVE_HEADERS, nativeHeaders);
+	public void copyHeadersIfAbsent(@Nullable Map<String, ?> headersToCopy) {
+		if (headersToCopy == null) {
+			return;
+		}
+
+		@SuppressWarnings("unchecked")
+		Map<String, List<String>> map = (Map<String, List<String>>) headersToCopy.get(NATIVE_HEADERS);
+		if (map != null && getNativeHeaders() == null) {
+			map.forEach(this::setNativeHeaderValues);
 		}
+
+		super.copyHeadersIfAbsent(headersToCopy);
 	}
 
 	/**
@@ -201,6 +209,30 @@ public class NativeMessageHeaderAccessor extends MessageHeaderAccessor {
 		}
 	}
 
+	/**
+	 * Variant of {@link #addNativeHeader(String, String)} for all values.
+	 * @since 5.2.12
+	 */
+	public void setNativeHeaderValues(String name, @Nullable List<String> values) {
+		Assert.state(isMutable(), "Already immutable");
+		Map<String, List<String>> map = getNativeHeaders();
+		if (values == null) {
+			if (map != null && map.get(name) != null) {
+				setModified(true);
+				map.remove(name);
+			}
+			return;
+		}
+		if (map == null) {
+			map = new LinkedMultiValueMap<>(3);
+			setHeader(NATIVE_HEADERS, map);
+		}
+		if (!ObjectUtils.nullSafeEquals(values, getHeader(name))) {
+			setModified(true);
+			map.put(name, new ArrayList<>(values));
+		}
+	}
+
 	/**
 	 * Add the specified native header value to existing values.
 	 * <p>In order for this to work, the accessor must be {@link #isMutable()

spring-messaging/src/test/java/org/springframework/messaging/support/NativeMessageHeaderAccessorTests.java

@@ -226,19 +226,46 @@ public class NativeMessageHeaderAccessorTests {
 
 	@Test // gh-25821
 	void copyImmutableToMutable() {
-		NativeMessageHeaderAccessor source = new NativeMessageHeaderAccessor();
-		source.addNativeHeader("foo", "bar");
-		Message<String> message = MessageBuilder.createMessage("payload", source.getMessageHeaders());
+		NativeMessageHeaderAccessor sourceAccessor = new NativeMessageHeaderAccessor();
+		sourceAccessor.addNativeHeader("foo", "bar");
+		Message<String> source = MessageBuilder.createMessage("payload", sourceAccessor.getMessageHeaders());
 
-		NativeMessageHeaderAccessor target = new NativeMessageHeaderAccessor();
-		target.copyHeaders(message.getHeaders());
-		target.setLeaveMutable(true);
-		message = MessageBuilder.createMessage(message.getPayload(), target.getMessageHeaders());
+		NativeMessageHeaderAccessor targetAccessor = new NativeMessageHeaderAccessor();
+		targetAccessor.copyHeaders(source.getHeaders());
+		targetAccessor.setLeaveMutable(true);
+		Message<?> target = MessageBuilder.createMessage(source.getPayload(), targetAccessor.getMessageHeaders());
 
-		MessageHeaderAccessor accessor = MessageHeaderAccessor.getMutableAccessor(message);
+		MessageHeaderAccessor accessor = MessageHeaderAccessor.getMutableAccessor(target);
 		assertThat(accessor.isMutable());
 		((NativeMessageHeaderAccessor) accessor).addNativeHeader("foo", "baz");
 		assertThat(((NativeMessageHeaderAccessor) accessor).getNativeHeader("foo")).containsExactly("bar", "baz");
 	}
 
+	@Test // gh-25821
+	void copyIfAbsentImmutableToMutable() {
+		NativeMessageHeaderAccessor sourceAccessor = new NativeMessageHeaderAccessor();
+		sourceAccessor.addNativeHeader("foo", "bar");
+		Message<String> source = MessageBuilder.createMessage("payload", sourceAccessor.getMessageHeaders());
+
+		MessageHeaderAccessor targetAccessor = new NativeMessageHeaderAccessor();
+		targetAccessor.copyHeadersIfAbsent(source.getHeaders());
+		targetAccessor.setLeaveMutable(true);
+		Message<?> target = MessageBuilder.createMessage(source.getPayload(), targetAccessor.getMessageHeaders());
+
+		MessageHeaderAccessor accessor = MessageHeaderAccessor.getMutableAccessor(target);
+		assertThat(accessor.isMutable());
+		((NativeMessageHeaderAccessor) accessor).addNativeHeader("foo", "baz");
+		assertThat(((NativeMessageHeaderAccessor) accessor).getNativeHeader("foo")).containsExactly("bar", "baz");
+	}
+
+	@Test // gh-26155
+	void copySelf() {
+		NativeMessageHeaderAccessor accessor = new NativeMessageHeaderAccessor();
+		accessor.addNativeHeader("foo", "bar");
+		accessor.setHeader("otherHeader", "otherHeaderValue");
+		accessor.setLeaveMutable(true);
+
+		// Does not fail with ConcurrentModificationException
+		accessor.copyHeaders(accessor.getMessageHeaders());
+	}
 }