From a603779f33d47cd9ac87919ccc26f8c2d279fa40 Mon Sep 17 00:00:00 2001
From: Sam Brannen <sbrannen@vmware.com>
Date: Fri, 22 Oct 2021 14:57:12 +0200
Subject: [PATCH] Return previous value in UndertowHeadersAdapter's remove()
 method

Prior to this commit, UndertowHeadersAdapter's remove() method violated
the java.util.Map contract by always returning null.

This commit fixes this by returning the previous list stored under the
specified key, and otherwise returning null if no previous value was
present.

Closes gh-27592
---
 .../http/server/reactive/UndertowHeadersAdapter.java       | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java b/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java
index fa197aa0c2..8b7c6aed14 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java
@@ -17,6 +17,7 @@
 package org.springframework.http.server.reactive;
 
 import java.util.AbstractSet;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
@@ -36,6 +37,7 @@ import org.springframework.util.MultiValueMap;
  * {@code MultiValueMap} implementation for wrapping Undertow HTTP headers.
  *
  * @author Brian Clozel
+ * @author Sam Brannen
  * @since 5.1.1
  */
 class UndertowHeadersAdapter implements MultiValueMap<String, String> {
@@ -131,7 +133,10 @@ class UndertowHeadersAdapter implements MultiValueMap<String, String> {
 	@Nullable
 	public List<String> remove(Object key) {
 		if (key instanceof String) {
-			this.headers.remove((String) key);
+			Collection<String> removed = this.headers.remove((String) key);
+			if (removed != null) {
+				return new ArrayList<>(removed);
+			}
 		}
 		return null;
 	}