Allow LogDnsClient queries to be rate-limited

components/certificate_transparency/log_dns_client.h

 // Copyright 2016 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.
 
 #ifndef COMPONENTS_CERTIFICATE_TRANSPARENCY_LOG_DNS_CLIENT_H_
 #define COMPONENTS_CERTIFICATE_TRANSPARENCY_LOG_DNS_CLIENT_H_
 
 #include <stdint.h>
 
 #include <list>
@@ skipping 34 matching lines @@
   // The log ID of |proof| will not be set, as that is not known by this class,
   // but the leaf index will be set.
   using AuditProofCallback =
       base::Callback<void(int net_error,
                           std::unique_ptr<net::ct::MerkleAuditProof> proof)>;
 
   // Creates a log client that will take ownership of |dns_client| and use it
   // to perform DNS queries. Queries will be logged to |net_log|.
   // The |dns_client| does not need to be configured first - this will be done
   // automatically as needed.
+  // A limit can be set on the number of concurrent DNS queries by providing a
+  // positive value for |max_concurrent_queries|. Queries that would exceed this
+  // limit will fail with net::TEMPORARILY_THROTTLED. Setting this to 0 will
+  // disable this limit.
   LogDnsClient(std::unique_ptr<net::DnsClient> dns_client,
-               const net::NetLogWithSource& net_log);
+               const net::NetLogWithSource& net_log,
+               size_t max_concurrent_queries);
   // Must be deleted on the same thread that it was created on.
   ~LogDnsClient() override;
 
   // Called by NetworkChangeNotifier when the DNS config changes.
   // The DnsClient's config will be updated in response.
   void OnDNSChanged() override;
 
   // Called by NetworkChangeNotifier when the DNS config is first read.
   // The DnsClient's config will be updated in response.
   void OnInitialDNSConfigRead() override;
@@ skipping 34 matching lines @@
                             const AuditProofCallback& callback);
 
   void QueryAuditProofNodesComplete(
       std::unique_ptr<net::ct::MerkleAuditProof> proof,
       base::StringPiece domain_for_log,
       uint64_t tree_size,
       net::DnsTransaction* transaction,
       int net_error,
       const net::DnsResponse* response);
 
+  // Returns true if the maximum number of queries are currently in flight.
+  // If the maximum number of concurrency queries is set to 0, this will always
+  // return false.
+  bool HasMaxConcurrentQueriesInProgress() const;
+
   // Updates the |dns_client_| config using NetworkChangeNotifier.
   void UpdateDnsConfig();
 
   // A DNS query that is in flight.
   template <typename CallbackType>
   struct Query {
     std::unique_ptr<net::DnsTransaction> transaction;
     CallbackType callback;
   };
 
   // Used to perform DNS queries.
   std::unique_ptr<net::DnsClient> dns_client_;
   // Passed to the DNS client for logging.
   net::NetLogWithSource net_log_;
   // Leaf index queries that haven't completed yet.
   std::list<Query<LeafIndexCallback>> leaf_index_queries_;
   // Audit proof queries that haven't completed yet.
   std::list<Query<AuditProofCallback>> audit_proof_queries_;
+  // The maximum number of queries that can be in flight at one time.
+  size_t max_concurrent_queries_;
   // Creates weak_ptrs to this, for callback purposes.
   base::WeakPtrFactory<LogDnsClient> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(LogDnsClient);
 };
 
 }  // namespace certificate_transparency
 #endif  // COMPONENTS_CERTIFICATE_TRANSPARENCY_LOG_DNS_CLIENT_H_