Add caching headers to If-Unmodified-Since responses

Conditional requests using "If-Unmodified-Since" headers are generally
used as precondition checks for state-changing methods (POST, PUT,
DELETE). See https://datatracker.ietf.org/doc/html/rfc7232#section-3.4
The spec also allows for idempotent methods like GET and HEAD.

Prior to this commit, the "If-Unmodified-Since" processing done in
`checkNotModified` (see `ServletWebRequest` and
`DefaultServerWebExchange`) would only focus on the state changing
methods and not take into account the safe methods. For those cases, the
"ETag" and "Last-Modified" would be missing from the response.

This commit ensures that such headers are added as expected in these
cases.

Fixes gh-29362

spring-web/src/main/java/org/springframework/web/context/request/ServletWebRequest.java

@@ -220,6 +220,12 @@ public boolean checkNotModified(@Nullable String etag, long lastModifiedTimestam
 			if (this.notModified && response != null) {
 				response.setStatus(HttpStatus.PRECONDITION_FAILED.value());
 			}
+			if (SAFE_METHODS.contains(getRequest().getMethod())) {
+				if (StringUtils.hasLength(etag) && response.getHeader(HttpHeaders.ETAG) == null) {
+					response.setHeader(HttpHeaders.ETAG, padEtagIfNecessary(etag));
+				}
+				response.setDateHeader(HttpHeaders.LAST_MODIFIED, lastModifiedTimestamp);
+			}
 			return this.notModified;
 		}
 

spring-web/src/main/java/org/springframework/web/server/adapter/DefaultServerWebExchange.java

@@ -56,6 +56,7 @@
  * Default implementation of {@link ServerWebExchange}.
  *
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  * @since 5.0
  */
 public class DefaultServerWebExchange implements ServerWebExchange {
@@ -261,6 +262,12 @@ public boolean checkNotModified(@Nullable String etag, Instant lastModified) {
 			if (this.notModified) {
 				getResponse().setStatusCode(HttpStatus.PRECONDITION_FAILED);
 			}
+			if (SAFE_METHODS.contains(getRequest().getMethod())) {
+				if (StringUtils.hasLength(etag) && getResponseHeaders().getETag() == null) {
+					getResponseHeaders().setETag(padEtagIfNecessary(etag));
+				}
+				getResponseHeaders().setLastModified(lastModified.toEpochMilli());
+			}
 			return this.notModified;
 		}
 

spring-web/src/test/java/org/springframework/web/context/request/ServletWebRequestHttpMethodsTests.java

@@ -23,6 +23,7 @@
 import java.time.ZonedDateTime;
 import java.util.Date;
 
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
 
@@ -286,9 +287,9 @@ void checkNotModifiedMultipleETags(String method) {
 		assertThat(servletResponse.getHeader("ETag")).isEqualTo(etag);
 	}
 
-	@ParameterizedHttpMethodTest
-	void checkNotModifiedTimestampWithLengthPart(String method) {
-		setUpRequest(method);
+	@Test
+	void checkNotModifiedTimestampWithLengthPart() {
+		setUpRequest("GET");
 
 		long epochTime = ZonedDateTime.parse(CURRENT_TIME, RFC_1123_DATE_TIME).toInstant().toEpochMilli();
 		servletRequest.setMethod("GET");
@@ -299,47 +300,57 @@ void checkNotModifiedTimestampWithLengthPart(String method) {
 		assertThat(servletResponse.getDateHeader("Last-Modified") / 1000).isEqualTo(epochTime / 1000);
 	}
 
-	@ParameterizedHttpMethodTest
-	void checkModifiedTimestampWithLengthPart(String method) {
-		setUpRequest(method);
+	@Test
+	void checkModifiedTimestampWithLengthPart() {
+		setUpRequest("GET");
 
 		long epochTime = ZonedDateTime.parse(CURRENT_TIME, RFC_1123_DATE_TIME).toInstant().toEpochMilli();
-		servletRequest.setMethod("GET");
 		servletRequest.addHeader("If-Modified-Since", "Wed, 08 Apr 2014 09:57:42 GMT; length=13774");
 
 		assertThat(request.checkNotModified(epochTime)).isFalse();
 		assertThat(servletResponse.getStatus()).isEqualTo(200);
 		assertThat(servletResponse.getDateHeader("Last-Modified") / 1000).isEqualTo(epochTime / 1000);
 	}
 
-	@ParameterizedHttpMethodTest
-	void checkNotModifiedTimestampConditionalPut(String method) {
-		setUpRequest(method);
+	@Test
+	void checkNotModifiedTimestampConditionalPut() {
+		setUpRequest("PUT");
 
 		long currentEpoch = currentDate.getTime();
 		long oneMinuteAgo = currentEpoch - (1000 * 60);
-		servletRequest.setMethod("PUT");
 		servletRequest.addHeader("If-UnModified-Since", currentEpoch);
 
 		assertThat(request.checkNotModified(oneMinuteAgo)).isFalse();
 		assertThat(servletResponse.getStatus()).isEqualTo(200);
 		assertThat(servletResponse.getHeader("Last-Modified")).isNull();
 	}
 
-	@ParameterizedHttpMethodTest
-	void checkNotModifiedTimestampConditionalPutConflict(String method) {
-		setUpRequest(method);
+	@Test
+	void checkNotModifiedTimestampConditionalPutConflict() {
+		setUpRequest("PUT");
 
 		long currentEpoch = currentDate.getTime();
 		long oneMinuteAgo = currentEpoch - (1000 * 60);
-		servletRequest.setMethod("PUT");
 		servletRequest.addHeader("If-UnModified-Since", oneMinuteAgo);
 
 		assertThat(request.checkNotModified(currentEpoch)).isTrue();
 		assertThat(servletResponse.getStatus()).isEqualTo(412);
 		assertThat(servletResponse.getHeader("Last-Modified")).isNull();
 	}
 
+	@ParameterizedHttpMethodTest
+	void checkNotModifiedConditionalGet(String method) {
+		setUpRequest(method);
+		long currentEpoch = currentDate.getTime();
+		long oneMinuteAgo = currentEpoch - (1000 * 60);
+		servletRequest.addHeader("If-UnModified-Since", currentEpoch);
+
+		assertThat(request.checkNotModified(oneMinuteAgo)).isFalse();
+		assertThat(servletResponse.getStatus()).isEqualTo(200);
+		assertThat(servletResponse.getDateHeader("Last-Modified") / 1000).isEqualTo(oneMinuteAgo / 1000);
+	}
+
+
 	private void setUpRequest(String method) {
 		this.servletRequest.setMethod(method);
 		this.servletRequest.setRequestURI("https://example.org");

spring-web/src/test/java/org/springframework/web/server/adapter/DefaultServerWebExchangeCheckNotModifiedTests.java

@@ -307,4 +307,17 @@ void checkNotModifiedTimestampConditionalPutConflict() throws Exception {
 		assertThat(exchange.getResponse().getHeaders().getLastModified()).isEqualTo(-1);
 	}
 
+	@Test
+	void checkNotModifiedTimestampConditionalGet() throws Exception {
+		String eTag = "\"Test\"";
+		Instant oneMinuteAgo = currentDate.minusSeconds(60);
+		MockServerHttpRequest request = MockServerHttpRequest.get("/").ifUnmodifiedSince(currentDate.toEpochMilli()).build();
+		MockServerWebExchange exchange = MockServerWebExchange.from(request);
+
+		assertThat(exchange.checkNotModified(eTag, oneMinuteAgo)).isFalse();
+		assertThat(exchange.getResponse().getStatusCode()).isNull();
+		assertThat(exchange.getResponse().getHeaders().getLastModified()).isEqualTo(oneMinuteAgo.toEpochMilli());
+		assertThat(exchange.getResponse().getHeaders().getETag()).isEqualTo(eTag);
+	}
+
 }