Add Last-Modified header to 304 response when ETag is not present

.

This commit adds the Last-Modified header to the 304 Not Modified response when the ETag header is not present in the cache entry. This aligns the behavior with the recommendations in RFC 7232 and helps clients that rely on the Last-Modified header for cache updates when

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/CachedHttpResponseGenerator.java

@@ -139,6 +139,18 @@ SimpleHttpResponse generateNotModifiedResponse(final HttpCacheEntry entry) {
             response.addHeader(varyHeader);
         }
 
+        //Since the goal of a 304 response is to minimize information transfer
+        //when the recipient already has one or more cached representations, a
+        //sender SHOULD NOT generate representation metadata other than the
+        //above listed fields unless said metadata exists for the purpose of
+        //guiding cache updates (e.g., Last-Modified might be useful if the
+        //response does not have an ETag field).
+        if (etagHeader == null) {
+            final Header lastModifiedHeader = entry.getFirstHeader(HttpHeaders.LAST_MODIFIED);
+            if (lastModifiedHeader != null) {
+                response.addHeader(lastModifiedHeader);
+            }
+        }
         return response;
     }
 

httpclient5-cache/src/test/java/org/apache/hc/client5/http/impl/cache/TestCachingExecChain.java

@@ -391,6 +391,7 @@ public void testReturns304ForIfModifiedSinceHeaderIf304ResponseInCache() throws
         resp1.setHeader("Date", DateUtils.formatStandardDate(now));
         resp1.setHeader("Cache-control", "max-age=600");
         resp1.setHeader("Expires", DateUtils.formatStandardDate(inTenMinutes));
+        resp1.setHeader("ETag", "\"etag\"");
 
         Mockito.when(mockExecChain.proceed(Mockito.any(), Mockito.any())).thenReturn(resp1);
 
@@ -403,6 +404,32 @@ public void testReturns304ForIfModifiedSinceHeaderIf304ResponseInCache() throws
         Mockito.verify(mockExecChain).proceed(Mockito.any(), Mockito.any());
     }
 
+    @Test
+    public void testReturns304ForIfModifiedSinceHeaderIf304ResponseInCacheWithLastModified() throws Exception {
+        final Instant now = Instant.now();
+        final Instant oneHourAgo = now.minus(1, ChronoUnit.HOURS);
+        final Instant inTenMinutes = now.plus(10, ChronoUnit.MINUTES);
+        final ClassicHttpRequest req1 = new HttpGet("http://foo.example.com/");
+        req1.addHeader("If-Modified-Since", DateUtils.formatStandardDate(oneHourAgo));
+        final ClassicHttpRequest req2 = new HttpGet("http://foo.example.com/");
+        req2.addHeader("If-Modified-Since", DateUtils.formatStandardDate(oneHourAgo));
+
+        final ClassicHttpResponse resp1 = HttpTestUtils.make304Response();
+        resp1.setHeader("Date", DateUtils.formatStandardDate(now));
+        resp1.setHeader("Cache-control", "max-age=600");
+        resp1.setHeader("Expires", DateUtils.formatStandardDate(inTenMinutes));
+
+        Mockito.when(mockExecChain.proceed(Mockito.any(), Mockito.any())).thenReturn(resp1);
+
+        execute(req1);
+
+        final ClassicHttpResponse result = execute(req2);
+        Assertions.assertEquals(HttpStatus.SC_NOT_MODIFIED, result.getCode());
+        Assertions.assertTrue(result.containsHeader("Last-Modified"));
+
+        Mockito.verify(mockExecChain).proceed(Mockito.any(), Mockito.any());
+    }
+
     @Test
     public void testReturns200ForIfModifiedSinceDateIsLess() throws Exception {
         final Instant now = Instant.now();

httpclient5-cache/src/test/java/org/apache/hc/client5/http/impl/cache/TestProtocolRecommendations.java

@@ -175,6 +175,7 @@ private void cacheGenerated304ForValidatorShouldNotContainEntityHeader(
         resp1.setHeader("Cache-Control","max-age=3600");
         resp1.setHeader(validatorHeader, validator);
         resp1.setHeader(headerName, headerValue);
+        resp1.setHeader("ETag", "\"etag\"");
 
         Mockito.when(mockExecChain.proceed(Mockito.any(), Mockito.any())).thenReturn(resp1);