Fix bug in JDBC SaveMode.ON_GET_ATTRIBUTE

Closes gh-2040

Author: felixscheinost

spring-session-jdbc/src/main/java/org/springframework/session/jdbc/JdbcIndexedSessionRepository.java

@@ -728,7 +728,8 @@ public <T> T getAttribute(String attributeName) {
 			T attributeValue = supplier.get();
 			if (attributeValue != null
 					&& JdbcIndexedSessionRepository.this.saveMode.equals(SaveMode.ON_GET_ATTRIBUTE)) {
-				this.delta.put(attributeName, DeltaValue.UPDATED);
+				this.delta.merge(attributeName, DeltaValue.UPDATED, (oldDeltaValue,
+						deltaValue) -> (oldDeltaValue == DeltaValue.ADDED) ? oldDeltaValue : deltaValue);
 			}
 			return attributeValue;
 		}

spring-session-jdbc/src/test/java/org/springframework/session/jdbc/JdbcIndexedSessionRepositoryTests.java

@@ -655,6 +655,20 @@ void saveWithSaveModeOnGetAttribute() {
 		verifyNoMoreInteractions(this.jdbcOperations);
 	}
 
+	@Test
+	void saveWithSaveModeOnGetAttributeAndNewAttributeSetAndGet() {
+		this.repository.setSaveMode(SaveMode.ON_GET_ATTRIBUTE);
+		MapSession delegate = new MapSession();
+		delegate.setAttribute("attribute1", (Supplier<String>) () -> "value1");
+		JdbcSession session = this.repository.new JdbcSession(delegate, UUID.randomUUID().toString(), false);
+		session.setAttribute("attribute2", "value2");
+		session.getAttribute("attribute2");
+		this.repository.save(session);
+		verify(this.jdbcOperations, times(1)).update(startsWith("INSERT INTO SPRING_SESSION_ATTRIBUTES ("),
+				isA(PreparedStatementSetter.class));
+		verifyNoMoreInteractions(this.jdbcOperations);
+	}
+
 	@Test
 	void saveWithSaveModeAlways() {
 		this.repository.setSaveMode(SaveMode.ALWAYS);