Add in-memory caching of network quality estimates across network changes.

net/base/network_quality_estimator.h

Index: net/base/network_quality_estimator.h
diff --git a/net/base/network_quality_estimator.h b/net/base/network_quality_estimator.h
index 6f3a4674cfcf3509d3b0004eac010fcb5988f4ee..61a441e8b88062ecda35015f8cacf53d2554aca8 100644
--- a/net/base/network_quality_estimator.h
+++ b/net/base/network_quality_estimator.h
@@ -8,6 +8,8 @@
 #include <stdint.h>
 
 #include <deque>
+#include <map>
+#include <string>
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
@@ -15,11 +17,10 @@
 #include "base/time/time.h"
 #include "net/base/net_export.h"
 #include "net/base/network_change_notifier.h"
+#include "net/base/network_quality.h"
 
 namespace net {
 
-class NetworkQuality;
-
 // NetworkQualityEstimator provides network quality estimates (quality of the
 // full paths to all origins that have been connected to).
 // The estimates are based on the observed organic traffic.
@@ -51,13 +52,110 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
                           int64_t cumulative_prefilter_bytes_read,
                           int64_t prefiltered_bytes_read);
 
+ protected:
+  // NetworkID is used to uniquely identify a network.
+  // For the purpose of network quality estimation and caching, a network is
+  // uniquely identified by a combination of |type| and
+  // |id|. This approach is unable to distinguish networks with
+  // same name (e.g., different Wi-Fi networks with same SSID).
+  // This is a protected member to expose it to tests.
+  struct NetworkID {
+    NetworkID(NetworkChangeNotifier::ConnectionType type, const std::string& id)
+        : type(type), id(id) {}
+
+    NetworkID(const NetworkID& other) : type(other.type), id(other.id) {}
+
+    ~NetworkID() {}
+
+    NetworkID& operator=(const NetworkID& other) {
+      type = other.type;
+      id = other.id;
+      return *this;
+    }
+
+    // Overloaded because NetworkID is used as key in a map.
+    bool operator<(const NetworkID& other) const {
+      return type < other.type || (type == other.type && id < other.id);
+    }
+
+    // Connection type of the network.
+    NetworkChangeNotifier::ConnectionType type;
+
+    // Name of this network. This is set to:
+    // - Wi-Fi SSID (if the user is connected to a Wi-Fi access point and the
+    //   SSID name is available), or
+    // - MCC/MNC code of the cellular carrier if the device is connected to a
+    //   cellular network, or
+    // - An empty string in all other cases or if the network name is not
+    //   exposed by platform APIs.
+    std::string id;
+  };
+
+  // Construct a NetworkQualityEstimator instance allowing for test
+  // configuration.
+  // Registers for network type change notifications so estimates can be kept
+  // network specific.
+  // |allow_local_host_requests_for_tests| should only be true when testing
+  // against local HTTP server and allows the requests to local host to be
+  // used for network quality estimation.
+  // |allow_smaller_responses_for_tests| should only be true when testing
+  // against local HTTP server and allows the responses smaller than
+  // |kMinTransferSizeInBytes| or shorter than |kMinRequestDurationMicroseconds|
+  // to be used for network quality estimation.
+  NetworkQualityEstimator(bool allow_local_host_requests_for_tests,
+                          bool allow_smaller_responses_for_tests);
+
+  // Returns true if the cached network quality estimate was successfully read.
+  bool ReadCachedNetworkQualityEstimate();
+
+  // NetworkChangeNotifier::ConnectionTypeObserver implementation.
+  // |type| is ignored.
+  void OnConnectionTypeChanged(
+      NetworkChangeNotifier::ConnectionType type) override;
+
+  // Returns the number of entries in the network quality cache.
+  // Used only for testing.
+  size_t GetNetworkQualityCacheSizeForTests() const;
+
  private:
   FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest, StoreObservations);
   FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest,
                            TestPeakKbpsFastestRTTUpdates);
   FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest, TestAddObservation);
+  FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest, TestCaching);
+  FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest,
+                           TestLRUCacheMaximumSize);
   FRIEND_TEST_ALL_PREFIXES(URLRequestTestHTTP, NetworkQualityEstimator);
 
+  // CachedNetworkQuality stores the quality of a previously seen network.
+  class CachedNetworkQuality {
+   public:
+    explicit CachedNetworkQuality(const NetworkQuality& network_quality);
+
+    ~CachedNetworkQuality();
+
+    // Returns the network quality associated with this cached entry.
+    const NetworkQuality network_quality() const { return network_quality_; }
+
+    // Updates the network quality to the specified |median_kbps| and
+    // |median_rtt|.
+    void UpdateNetworkQuality(int32_t median_kbps,
+                              const base::TimeDelta& median_rtt);
+
+    // Returns true if this cache entry was updated before
+    // |cached_network_quality|.
+    bool OlderThan(const CachedNetworkQuality& cached_network_quality) const;
+
+   private:
+    // Time when this cache entry was last updated.
+    base::TimeTicks last_update_time_;
+
+    // Quality of this cached network.
+    NetworkQuality network_quality_;
+
+    DISALLOW_COPY_AND_ASSIGN(CachedNetworkQuality);
+  };
+
   // Records the round trip time or throughput observation, along with the time
   // the observation was made.
   struct Observation {
@@ -99,6 +197,12 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
     DISALLOW_COPY_AND_ASSIGN(ObservationBuffer);
   };
 
+  // This does not use a unordered_map or hash_map for code simplicity (key just
+  // implements operator<, rather than hash and equality) and because the map is
+  // tiny.
+  typedef std::map<NetworkID, scoped_ptr<CachedNetworkQuality>>
+      CachedNetworkQualities;
+
   // Tiny transfer sizes may give inaccurate throughput results.
   // Minimum size of the transfer over which the throughput is computed.
   static const int kMinTransferSizeInBytes = 10000;
@@ -107,31 +211,26 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
   // throughput is computed.
   static const int kMinRequestDurationMicroseconds = 1000;
 
-  // Construct a NetworkQualityEstimator instance allowing for test
-  // configuration.
-  // Registers for network type change notifications so estimates can be kept
-  // network specific.
-  // |allow_local_host_requests_for_tests| should only be true when testing
-  // against local HTTP server and allows the requests to local host to be
-  // used for network quality estimation.
-  // |allow_smaller_responses_for_tests| should only be true when testing
-  // against local HTTP server and allows the responses smaller than
-  // |kMinTransferSizeInBytes| or shorter than |kMinRequestDurationMicroseconds|
-  // to be used for network quality estimation.
-  NetworkQualityEstimator(bool allow_local_host_requests_for_tests,
-                          bool allow_smaller_responses_for_tests);
+  // Maximum size of the cache that holds network quality estimates.
+  // Smaller size may reduce the cache hit rate due to frequent evictions.
+  // Larger size may affect performance.
+  static const size_t kMaximumNetworkQualityCacheSize;
 
-  // Returns the maximum size of the observation buffer.
-  // Used for testing.
+  // Returns the maximum size of the observation buffers. Used for testing.
   size_t GetMaximumObservationBufferSizeForTests() const;
 
-  // Returns true if the size of all observation buffers is equal to the
-  // |expected_size|. Used for testing.
-  bool VerifyBufferSizeForTests(size_t expected_size) const;
+  // Returns the current size of the Kbps observation buffer. Used for testing.
+  size_t GetKbpsObservationBufferSizeForTests() const;
 
-  // NetworkChangeNotifier::ConnectionTypeObserver implementation.
-  void OnConnectionTypeChanged(
-      NetworkChangeNotifier::ConnectionType type) override;
+  // Returns the current size of the RTT observation buffer. Used for testing.
+  size_t GetRTTObservationBufferSizeForTests() const;
+
+  // Returns the current network ID checking by calling the platform APIs.
+  // Virtualized for testing.
+  virtual NetworkID GetCurrentNetworkID() const;
+
+  // Writes the estimated quality of the current network to the cache.
+  void CacheNetworkQualityEstimate();
 
   // Determines if the requests to local host can be used in estimating the
   // network quality. Set to true only for tests.
@@ -145,14 +244,16 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
   // Time when last connection change was observed.
   base::TimeTicks last_connection_change_;
 
-  // Last value passed to |OnConnectionTypeChanged|. This indicates the
-  // current connection type.
-  NetworkChangeNotifier::ConnectionType current_connection_type_;
+  // ID of the current network.
+  NetworkID current_network_id_;
 
   // Fastest round-trip-time (RTT) since last connectivity change. RTT measured
   // from URLRequest creation until first byte received.
   base::TimeDelta fastest_rtt_since_last_connection_change_;
 
+  // Cache that stores quality of previously seen networks.
+  CachedNetworkQualities cached_network_qualities_;
+
   // Rough measurement of downstream peak Kbps witnessed since last connectivity
   // change. The accuracy is decreased by ignoring these factors:
   // 1) Multiple URLRequests can occur concurrently.