Introduce a request throttler for limiting requests in mobile NTP.

components/ntp_snippets/request_throttler.h

Index: components/ntp_snippets/request_throttler.h
diff --git a/components/ntp_snippets/request_throttler.h b/components/ntp_snippets/request_throttler.h
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+// 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_NTP_SNIPPETS_REQUEST_THROTTLER_H_
+#define COMPONENTS_NTP_SNIPPETS_REQUEST_THROTTLER_H_
+
+#include <string>
+
+#include "base/macros.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace base {
+class HistogramBase;
+}  // namespace base
+
+namespace ntp_snippets {
+
+struct RequestTypeInfo;

[Bernhard Bauer, 2016/07/22 13:58:24]

This type could be made a private member of the class.

[jkrcal, 2016/07/25 10:02:36]

I would keep it there as I did already move it back and forth. 

Out of curiosity: what is dividing line in your opinion between the situations
where one of the following is more appropriate?
 - defining such a struct as a private member 
 - defining it in the .cc file (potentially in an anonymous namespace)

[Marc Treib, 2016/07/25 10:06:16]

Generally, keep the scope as small as possible. That means, prefer the anonymous
namespace, unless you need it in the header. If you do, make it a private
member, but keep the definition in the .cc file if possible (that's the case
here). So, you can just move this one line into the class.

[jkrcal, 2016/07/25 10:14:45]

Thanks! 
If it is a private member, how should I define the kRequestTypeInfo constant
list in the .cc file. Do I need to make kRequestTypeInfo a member of
RequestThrottler? Is it desirable?

[Marc Treib, 2016/07/25 10:22:15]

Ah, right - that would need to become a static member then. That's kinda
awkward. I'm not sure if the actual definition could stay in the .cc file then?

[Bernhard Bauer, 2016/07/25 11:15:03]

Yeah, you would still declare it in the header as a private static member, then
define it in the .cc file.  (And I think that's still better than what this does
right now, which is exporting the type declaration to files that include this
header, without making the definition available).

+
+// Counts requests to external services, compares them to a daily quota, reports
+// them to UMA. In the application code, create one local instance for each type
+// of requests, identified by the RequestType. The request counter is based on:
+//  - daily quota from a variation param "quota_|type|" in the NTPSnippets trial
+//  - pref "ntp.request_throttler.|type|.count" to store the current counter,
+//  - pref "ntp.request_throttler.|type|.day" to store current day to which the
+//    current counter value applies.
+// Furthermore the counter reports to histograms:
+//  - "NewTabPage.RequestThrottler.RequestStatus_|type|" - status of each
+//  request;
+//  - "NewTabPage.RequestThrottler.PerDay_|type|" - the daily count of requests.
+//
+// Implementation notes: When extending this class for a new RequestType, please
+//  1) define in request_counter.cc in kRequestTypeInfo
+//     a) the string value for your |type| and
+//     b) constants for day/count prefs;
+//  2) define a new RequestThrottlerTypes histogram suffix in histogram.xml
+//     (with the same string value as in 1a)).
+class RequestThrottler {
+ public:
+  // Enumeration listing all current applications of the request counter.
+  enum class RequestType {
+    CONTENT_SUGGESTION_FETCHER
+  };
+
+  RequestThrottler(PrefService* pref_service,
+                 RequestType type,

[Bernhard Bauer, 2016/07/22 13:58:24]

Alignment

[jkrcal, 2016/07/25 10:02:36]

Done (in the follow-up CL).

+                 int default_quota);
+
+  // Registers profile prefs for all RequestTypes. Called from browser_prefs.cc.
+  static void RegisterProfilePrefs(PrefRegistrySimple* registry);
+
+  // Returns whether quota is available for another request and reports this
+  // information to UMA. Forced requests always return true -- should be only
+  // used for requests initiated by the user (if it is safe to assume that all
+  // users cannot generate an amount of requests we cannot handle).
+  bool DemandQuotaForRequest(bool force_request);
+
+ private:
+  // Also emits the PerDay histogram if the day changed.
+  void ResetCounterIfDayChanged();
+
+  const char* GetRequestTypeAsString() const;
+
+  int GetCount() const;
+  void SetCount(int count);
+  int GetDay() const;
+  void SetDay(int day);
+  bool HasDay() const;
+
+  PrefService* pref_service_;
+  const RequestTypeInfo& type_info_;
+
+  // It comes from a variation parameter or |default_quota| as a fallback.
+  int quota_;
+
+  // The histograms for reporting the requests of the given |type_|.
+  base::HistogramBase* histogram_request_status_;
+  base::HistogramBase* histogram_per_day_;
+
+  DISALLOW_COPY_AND_ASSIGN(RequestThrottler);
+};
+
+}  // namespace ntp_snippets
+
+#endif  // COMPONENTS_NTP_SNIPPETS_REQUEST_THROTTLER_H_