[translate] Add translate ranker model loader.

components/translate/core/browser/ranker_model_loader.h

Index: components/translate/core/browser/ranker_model_loader.h
diff --git a/components/translate/core/browser/ranker_model_loader.h b/components/translate/core/browser/ranker_model_loader.h
new file mode 100644
index 0000000000000000000000000000000000000000..f804eabd541ad16b807fc4dcb33a81ce7c63c913
--- /dev/null
+++ b/components/translate/core/browser/ranker_model_loader.h
@@ -0,0 +1,152 @@
+// 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_TRANSLATE_CORE_BROWSER_RANKER_MODEL_LOADER_H_
+#define COMPONENTS_TRANSLATE_CORE_BROWSER_RANKER_MODEL_LOADER_H_
+
+#include <memory>
+#include <string>
+
+#include "base/files/file_path.h"
+#include "base/memory/ref_counted.h"
+#include "base/sequence_checker.h"
+#include "base/threading/thread.h"
+#include "components/translate/core/browser/proto/ranker_model.pb.h"
+#include "url/gurl.h"
+
+namespace chrome_intelligence {
+class RankerModel;
+}  // namespace chrome_intelligence
+
+namespace translate {
+
+class TranslateURLFetcher;
+
+// Enumeration denoting the outcome of an attempt to download the model. This
+// must be kept in sync with the TranslateRankerModelStatus enum in
+// histograms.xml
+// TODO(rogerm): rename the enum in histograms.xml to be more generic
+enum class RankerModelStatus {
+  OK = 0,
+  DOWNLOAD_THROTTLED = 1,
+  DOWNLOAD_FAILED = 2,
+  PARSE_FAILED = 3,
+  VALIDATION_FAILED = 4,
+  INCOMPATIBLE = 5,
+
+  // Insert new values above this line.
+  MAX
+};
+
+// Interface exposed by a Model Loader's Observer. The observer must outlive
+// the model loader.
+class RankerModelObserver {
+ public:
+  virtual ~RankerModelObserver() = default;
+
+  // The type of model the observer is willing/able to accept. Must be
+  // thread-safe.
+  virtual RankerModelStatus Validate(
+      const chrome_intelligence::RankerModel& model) const = 0;
+
+  // Called when a model having the expected model type and compatibility level
+  // is loaded the model loader. This call will be executed by the task_runner
+  // used by this observer.

[groby-ooo-7-16, 2017/01/23 21:41:57]

Wait.. isn't that called by the task_runner associated with the
RankerModelLoader?

[Roger McFarlane (Chromium), 2017/02/08 23:08:09]

Sorry, a previous iteration had a task_runner() getter and the loader would
PostTask to the task_runner to run OnModelAvailable. I neglected to update the
comment when I hid the task_runner behind the interface.

Done.

+  virtual void OnModelAvailable(
+      std::unique_ptr<chrome_intelligence::RankerModel> model) = 0;
+
+ protected:
+  RankerModelObserver() = default;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(RankerModelObserver);
+};
+
+// If enabled, downloads a translate ranker model and uses it to determine
+// whether the user should be given a translation prompt or not.
+class RankerModelLoader {
+ public:
+  RankerModelLoader(RankerModelObserver* observer,
+                    const base::FilePath& model_path,
+                    const GURL& model_url,
+                    const std::string& uma_prefix);
+
+  ~RankerModelLoader();
+
+  // Asynchronously initiates loading the model from the cache file path and URL
+  // previously configured.
+  void Start();
+
+  // Call this method periodically to notify the model loader the network might
+  // be available. If a model download is pending, this will trigger (subject to
+  // retry and frequency limits) a download attempt.
+  void NotifyOfNetworkAvailability();
+
+ private:
+  // The maximum number of model download attempts to make. Download may fail
+  // due to server error or network availability issues.
+  const int kMaxRetryOn5xx = 8;
+
+  // The minimum duration, in minutes, between download attempts.
+  const int kDownloadRefractoryPeriodMin = 3;
+
+  // Log the result of loading a model to UMA.
+  void ReportModelStatus(RankerModelStatus model_status);
+
+  // Called to construct a model from the given |data|.
+  std::unique_ptr<chrome_intelligence::RankerModel> Parse(
+      const std::string& data);
+
+  // Attempt to read the model from the path returns by T::GerModelPath()
+  void LoadFromFile();
+
+  // Attempt to read the model from the URL returns by T::GetModelURL()
+  void LoadFromURL();
+
+  // Called when the background task to download the model from |model_url_|
+  // has completed.
+  void OnDownloadComplete(int id, bool success, const std::string& data);
+
+  // Transfer owhersnip of |model| to the |observer_|.
+  void TransferModelToObserver(
+      std::unique_ptr<chrome_intelligence::RankerModel> model);
+
+  base::SequenceChecker sequence_checker_;
+
+  // The thread on which to perform background IO to read, download and
+  // cache the model.
+  base::Thread thread_;
+
+  // Used to download model data from |model_url_|.
+  // TODO(rogerm): Use net::URLFetcher directly?

[groby-ooo-7-16, 2017/01/23 21:41:56]

There's a bunch of useful setup code in TranslateURLFetcher, amongst others the
"never do cookies" part. Maybe it's worth hoisting that out into its own file?

[Roger McFarlane (Chromium), 2017/02/08 23:08:09]

I can look at creating a CookieLessURLFetcher or something, that might be
reusable.

The main reason I reused the TranslateURLFetcher is that it also defers the
retry handling to the caller, which I used to implement the retry if and when
the browser navigates, because that may mean it has network connectivity.

+  std::unique_ptr<TranslateURLFetcher> url_fetcher_;
+
+  // The next time before which no new attempts to download the model should be
+  // attempted.
+  base::TimeTicks next_earliest_download_time_;
+
+  // Tracks the last time of the last attempt to download a model. Used for UMA
+  // reporting of download duration.
+  base::TimeTicks download_start_time_;
+
+  // A pointer to the observer for this model loader. The observer must outlive
+  // the loader.
+  RankerModelObserver* const observer_;  // Weak.
+
+  // The path at which the model is (or should be) cached.
+  const base::FilePath model_path_;
+
+  // The URL from which to download the model if the model is not in the cache
+  // or the cached model is invalid/expired.
+  const GURL model_url_;
+
+  // This will prefix all UMA metrics generated by the model loader.
+  const std::string uma_prefix_;
+
+  DISALLOW_COPY_AND_ASSIGN(RankerModelLoader);
+};
+
+}  // namespace translate
+
+#endif  // COMPONENTS_TRANSLATE_CORE_BROWSER_RANKER_MODEL_LOADER_H_