[http-cache] Don't bypass the cache for if-unmodified-since or if-mat…

…ch (envoyproxy#33080)

* [cache] Don't bypass the cache for if-unmodified-since or if-match

---------

Signed-off-by: Raven Black <ravenblack@dropbox.com>

source/extensions/filters/http/cache/cacheability_utils.cc

@@ -26,21 +26,23 @@ const absl::flat_hash_set<absl::string_view>& cacheableStatusCodes() {
 const std::vector<const Http::LowerCaseString*>& conditionalHeaders() {
  // As defined by: https://httpwg.org/specs/rfc7232.html#preconditions.
  CONSTRUCT_ON_FIRST_USE(
- std::vector<const Http::LowerCaseString*>, &Http::CustomHeaders::get().IfMatch,
- &Http::CustomHeaders::get().IfNoneMatch, &Http::CustomHeaders::get().IfModifiedSince,
- &Http::CustomHeaders::get().IfUnmodifiedSince, &Http::CustomHeaders::get().IfRange);
+ std::vector<const Http::LowerCaseString*>, &Http::CustomHeaders::get().IfNoneMatch,
+ &Http::CustomHeaders::get().IfModifiedSince, &Http::CustomHeaders::get().IfRange);
 }
 } // namespace
 
 bool CacheabilityUtils::canServeRequestFromCache(const Http::RequestHeaderMap& headers) {
  const absl::string_view method = headers.getMethodValue();
  const Http::HeaderValues& header_values = Http::Headers::get();
 
- // Check if the request contains any conditional headers.
+ // Check if the request contains any conditional headers other than if-unmodified-since
+ // or if-match.
  // For now, requests with conditional headers bypass the CacheFilter.
  // This behavior does not cause any incorrect results, but may reduce the cache effectiveness.
  // If needed to be handled properly refer to:
  // https://httpwg.org/specs/rfc7234.html#validation.received
+ // if-unmodified-since and if-match are ignored, as the spec explicitly says these
+ // header fields can be ignored by caches and intermediaries.
  for (auto conditional_header : conditionalHeaders()) {
  if (!headers.get(*conditional_header).empty()) {
  return false;

test/extensions/filters/http/cache/cacheability_utils_test.cc

@@ -94,8 +94,7 @@ TEST_F(CanServeRequestFromCacheTest, AuthorizationHeader) {
 }
 
 INSTANTIATE_TEST_SUITE_P(ConditionalHeaders, RequestConditionalHeadersTest,
- testing::Values("if-match", "if-none-match", "if-modified-since",
- "if-unmodified-since", "if-range"),
+ testing::Values("if-none-match", "if-modified-since", "if-range"),
  [](const auto& info) {
  std::string test_name = info.param;
  absl::c_replace_if(