Add a priority header to HTTP/3 response if it doesn't already have one

Summary:
This is to notify the client the priority that server uses when it
sends out the response headers

Reviewed By: afrind

Differential Revision: D26753465

fbshipit-source-id: 4a50e91b6f2fd8fbcc06ed6eb07cf608577da55c

Author: Yang Chi

proxygen/lib/http/HTTPMessage.cpp

@@ -27,16 +27,16 @@ namespace {
  * approximately 1% of our total CPU time on temporary locale objects.)
  */
 std::locale defaultLocale;
+} // namespace
+
+namespace proxygen {
 
 std::string httpPriorityToString(uint8_t urgency, bool incremental) {
   return folly::to<std::string>(
       "u=",
       std::min(static_cast<uint8_t>(proxygen::kMaxPriority), urgency),
       incremental ? ",i" : "");
 }
-} // namespace
-
-namespace proxygen {
 
 std::mutex HTTPMessage::mutexDump_;
 

proxygen/lib/http/HTTPMessage.h

@@ -52,6 +52,9 @@ struct HTTPPriority {
   virtual ~HTTPPriority() = default;
 };
 
+// Convert Priority to a string representation in the form of "u=urgency[,i]"
+std::string httpPriorityToString(uint8_t urgency, bool incremental);
+
 class HTTPMessage;
 
 folly::Optional<HTTPPriority> httpPriorityFromHTTPMessage(

proxygen/lib/http/session/HQDownstreamSession.cpp

@@ -258,4 +258,23 @@ hq::PushId HQDownstreamSession::createNewPushId() {
   return newPushId;
 }
 
+folly::Optional<HTTPHeaders> HQDownstreamSession::getExtraHeaders(
+    const HTTPMessage& headers, quic::StreamId streamId) {
+  if (!sock_) {
+    return folly::none;
+  }
+  if (headers.getHeaders().exists(HTTP_HEADER_PRIORITY)) {
+    return folly::none;
+  }
+  auto priority = sock_->getStreamPriority(streamId);
+  if (!priority) {
+    return folly::none;
+  }
+  HTTPHeaders extraHeaders;
+  extraHeaders.add(
+      HTTP_HEADER_PRIORITY,
+      httpPriorityToString(priority->level, priority->incremental));
+  return extraHeaders;
+}
+
 } // namespace proxygen

proxygen/lib/http/session/HQDownstreamSession.h

@@ -75,6 +75,9 @@ class HQDownstreamSession : public HQSession {
     return static_cast<uint32_t>(streams_.size());
   }
 
+  folly::Optional<HTTPHeaders> getExtraHeaders(
+      const HTTPMessage& haeders, quic::StreamId streamId) override;
+
  private:
   ~HQDownstreamSession() override {
     CHECK_EQ(getNumStreams(), 0);

proxygen/lib/http/session/HQSession.cpp

@@ -2872,8 +2872,12 @@ void HQSession::HQStreamTransportBase::sendHeaders(HTTPTransaction* txn,
       << headers.isRequest()
       << "; assocTxnId=" << txn->getAssocTxnId().value_or(-1)
       << "; txn=" << txn->getID();
-  codecFilterChain->generateHeader(
-      writeBuf_, *codecStreamId_, headers, includeEOM, size);
+  codecFilterChain->generateHeader(writeBuf_,
+                                   *codecStreamId_,
+                                   headers,
+                                   includeEOM,
+                                   size,
+                                   session_.getExtraHeaders(headers, streamId));
 
   const uint64_t newOffset = streamWriteByteOffset();
   egressHeadersStreamOffset_ = newOffset;

proxygen/lib/http/session/HQSession.h

@@ -773,6 +773,12 @@ class HQSession
                    quic::StreamId id,
                    HTTP3::ErrorCode err);
 
+  // Get extra HTTP headers we want to add to the HTTPMessage in sendHeaders.
+  virtual folly::Optional<HTTPHeaders> getExtraHeaders(const HTTPMessage&,
+                                                       quic::StreamId) {
+    return folly::none;
+  }
+
   proxygen::TransportDirection direction_;
   std::chrono::milliseconds transactionsTimeout_;
   TimePoint transportStart_;

proxygen/lib/http/session/test/HQDownstreamSessionTest.cpp

@@ -356,6 +356,29 @@ TEST_P(HQDownstreamSessionTest, PriorityUpdateIntoTransport) {
   hqSession_->closeWhenIdle();
 }
 
+TEST_P(HQDownstreamSessionTest, ReplyResponsePriority) {
+  if (!IS_HQ) { // H1Q tests do not support priority
+    hqSession_->closeWhenIdle();
+    return;
+  }
+  auto request = getProgressiveGetRequest();
+  sendRequest(request);
+  auto handler = addSimpleStrictHandler();
+  EXPECT_CALL(*socketDriver_->getSocket(), setStreamPriority(_, 1, true))
+      .Times(1);
+  handler->expectHeaders();
+  handler->expectEOM([&]() {
+    auto resp = makeResponse(200, 0);
+    EXPECT_CALL(*socketDriver_->getSocket(), getStreamPriority(_))
+        .Times(1)
+        .WillOnce(Return(quic::Priority(1, true)));
+    handler->sendRequest(*std::get<0>(resp));
+  });
+  handler->expectDetachTransaction();
+  flushRequestsAndLoop();
+  hqSession_->closeWhenIdle();
+}
+
 TEST_P(HQDownstreamSessionTestHQPush, PushPriority) {
   sendRequest("/", 1);
   HTTPMessage promiseReq, parentResp;