Allow update of existing WebSession after max sessions limit is reached

Previously, when saving a WebSession, the system did not check whether
the session ID already existed. As a result, even if the session being
saved was an update to an existing one, it was incorrectly treated as a
new session, and a "maximum sessions exceeded" error was triggered.

This fix ensures that if a WebSession with the same ID already exists,
it will be updated rather than counted as a new session, thereby
preventing unnecessary session limit violations.

See gh-35013
Closes gh-35018

Signed-off-by: Mohammad Saeed Nouri <msnsaeed71@gmail.com>
(cherry picked from commit c04902fefb)

spring-web/src/main/java/org/springframework/web/server/session/InMemoryWebSessionStore.java

@@ -280,7 +280,7 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 		private void checkMaxSessionsLimit() {
 			if (sessions.size() >= maxSessions) {
 				expiredSessionChecker.removeExpiredSessions(clock.instant());
-				if (sessions.size() >= maxSessions) {
+				if (sessions.size() >= maxSessions && !sessions.containsKey(this.getId())) {
 					throw new IllegalStateException("Max sessions limit reached: " + sessions.size());
 				}
 			}

spring-web/src/test/java/org/springframework/web/server/session/InMemoryWebSessionStoreTests.java

@@ -23,6 +23,7 @@ import java.util.stream.IntStream;
 
 import org.junit.jupiter.api.Test;
 import reactor.core.scheduler.Schedulers;
+import reactor.test.StepVerifier;
 
 import org.springframework.beans.DirectFieldAccessor;
 import org.springframework.web.server.WebSession;
@@ -157,6 +158,25 @@ class InMemoryWebSessionStoreTests {
 				.withMessage("Max sessions limit reached: 10");
 	}
 
+	@Test
+	void updateSession() {
+		WebSession oneWebSession = insertSession();
+
+		StepVerifier.create(oneWebSession.save())
+				.expectComplete()
+				.verify();
+	}
+
+	@Test
+	void updateSession_whenMaxSessionsReached() {
+		WebSession onceWebSession = insertSession();
+		IntStream.range(1, 10000).forEach(i -> insertSession());
+
+		StepVerifier.create(onceWebSession.save())
+				.expectComplete()
+				.verify();
+	}
+
 
 	private WebSession insertSession() {
 		WebSession session = this.store.createWebSession().block();