public relnote: Add comment of quic async signing.

net/quic/core/crypto/quic_crypto_server_config.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/core/crypto/quic_crypto_server_config.h"
 
 #include <stdlib.h>
 
 #include <algorithm>
 #include <memory>
@@ skipping 726 matching lines @@
     return;
   }
 
   if (!ClientDemandsX509Proof(client_hello)) {
     helper.Fail(QUIC_UNSUPPORTED_PROOF_DEMAND, "Missing or invalid PDMD");
     return;
   }
   DCHECK(proof_source_.get());
   string chlo_hash;
   CryptoUtils::HashHandshakeMessage(client_hello, &chlo_hash);
+
   // No need to get a new proof if one was already generated.
   if (!crypto_proof->chain) {
     const QuicTag* tag_ptr;
     size_t num_tags;
     QuicTagVector connection_options;
     if (client_hello.GetTaglist(kCOPT, &tag_ptr, &num_tags) == QUIC_NO_ERROR) {
       connection_options.assign(tag_ptr, tag_ptr + num_tags);
     }
     if (FLAGS_enable_async_get_proof) {
       std::unique_ptr<ProcessClientHelloCallback> cb(
@@ skipping 812 matching lines @@
   CryptoHandshakeMessage message;
   message.set_tag(kSCUP);
   message.SetStringPiece(kSCFG, serialized);
   message.SetStringPiece(kSourceAddressTokenTag, source_address_token);
 
   std::unique_ptr<BuildServerConfigUpdateMessageProofSourceCallback>
       proof_source_cb(new BuildServerConfigUpdateMessageProofSourceCallback(
           this, version, compressed_certs_cache, common_cert_sets, params,
           std::move(message), std::move(cb)));
 
+  // Note: We unconditionally use the async variant of GetProof here, unlike
+  // elsewhere in this file where we check for the kSYNC tag in the CHLO for the
+  // connection before deciding.  This call is not in the critical serving path,
+  // and so should not have much impact on the experiments associated with that
+  // tag (plus it would be a chore to plumb information about the tag down to
+  // here).
   proof_source_->GetProof(server_ip, params.sni, serialized, version, chlo_hash,
                           connection_options, std::move(proof_source_cb));
 }
 
 QuicCryptoServerConfig::BuildServerConfigUpdateMessageProofSourceCallback::
     ~BuildServerConfigUpdateMessageProofSourceCallback() {}
 
 QuicCryptoServerConfig::BuildServerConfigUpdateMessageProofSourceCallback::
     BuildServerConfigUpdateMessageProofSourceCallback(
         const QuicCryptoServerConfig* config,
@@ skipping 678 matching lines @@
       priority(0),
       source_address_token_boxer(nullptr) {}
 
 QuicCryptoServerConfig::Config::~Config() {
 }
 
 QuicCryptoProof::QuicCryptoProof() {}
 QuicCryptoProof::~QuicCryptoProof() {}
 
 }  // namespace net