Fix http2 content-length header check (#12747)

Fix issue where in HTTP2 response defined to have no payload like HEAD
or 304 reponse to conditional GET were failing with 502 status when
having non-zero Content-Length header.
According to the RFC they may have Content-length representing the
size the payload would have in the case of a 200 regular GET response.

See https://datatracker.ietf.org/doc/html/rfc7540#section-8.1.2.6

Author: floom4

include/proxy/http2/Http2Stream.h

@@ -31,6 +31,7 @@
 #include "proxy/http2/Http2DependencyTree.h"
 #include "tscore/History.h"
 #include "proxy/Milestones.h"
+#include "proxy/http/HttpSM.h"
 
 class Http2Stream;
 class Http2ConnectionState;
@@ -390,11 +391,22 @@ inline bool
 Http2Stream::payload_length_is_valid() const
 {
   uint32_t content_length = _receive_header.get_content_length();
-  if (content_length != 0 && content_length != data_length) {
+  HTTPHdr *server_request = &this->get_sm()->t_state.hdr_info.server_request;
+  uint64_t mask           = (MIME_PRESENCE_IF_UNMODIFIED_SINCE | MIME_PRESENCE_IF_MODIFIED_SINCE | MIME_PRESENCE_IF_RANGE |
+                   MIME_PRESENCE_IF_MATCH | MIME_PRESENCE_IF_NONE_MATCH);
+
+  // Skip Content-Length check on [RFC 7230] 3.3.2 conditions
+  bool is_payload_precluded =
+    this->is_outbound_connection() && (server_request->method_get_wksidx() == HTTP_WKSIDX_HEAD ||
+                                       (server_request->method_get_wksidx() == HTTP_WKSIDX_GET && server_request->presence(mask) &&
+                                        _receive_header.status_get() == HTTPStatus::NOT_MODIFIED));
+
+  if (content_length != 0 && !is_payload_precluded && content_length != data_length) {
     Warning("Bad payload length content_length=%d data_legnth=%d session_id=%" PRId64, content_length,
             static_cast<int>(data_length), _proxy_ssn->connection_id());
+    return false;
   }
-  return content_length == 0 || content_length == data_length;
+  return true;
 }
 
 inline bool