Allow LogDnsClient queries to be rate-limited

components/certificate_transparency/log_dns_client.h

Index: components/certificate_transparency/log_dns_client.h
diff --git a/components/certificate_transparency/log_dns_client.h b/components/certificate_transparency/log_dns_client.h
index 366702925c619ec1809ded8d3931ab9f806e5c36..e36cb7a0dadbfe5a716aefa154038f490502cf8e 100644
--- a/components/certificate_transparency/log_dns_client.h
+++ b/components/certificate_transparency/log_dns_client.h
@@ -57,6 +57,18 @@ class LogDnsClient : public net::NetworkChangeNotifier::DNSObserver {
   // Must be deleted on the same thread that it was created on.
   ~LogDnsClient() override;
 
+  // Gets the number of queries that can be in flight at any one time.
+  // If this returns 0, there is no limit.
+  size_t GetMaxConcurrentQueries() const { return max_concurrent_queries_; }

[Eran Messeri, 2016/09/12 13:26:48]

Nit: What use do clients have for this method? 

[Rob Percival, 2016/09/12 17:26:24]

Probably none, so I'm happy to remove it.

+
+  // A limit can be set on the number of concurrent 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.
+  void SetMaxConcurrentQueries(size_t max_queries) {

[Eran Messeri, 2016/09/12 13:26:48]

Nit: I'd make the value a c'tor parameter so clients that get a pointer to this
instance can't set the number.

[Rob Percival, 2016/09/12 17:26:24]

Done.

+    max_concurrent_queries_ = max_queries;
+  }
+
   // Called by NetworkChangeNotifier when the DNS config changes.
   // The DnsClient's config will be updated in response.
   void OnDNSChanged() override;
@@ -108,6 +120,11 @@ class LogDnsClient : public net::NetworkChangeNotifier::DNSObserver {
       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();
 
@@ -126,6 +143,8 @@ class LogDnsClient : public net::NetworkChangeNotifier::DNSObserver {
   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_;