Add a SiteEngagementEvictionPolicy.

chrome/browser/engagement/site_engagement_eviction_policy.cc

+// Copyright 2015 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.
+
+#include "base/barrier_closure.h"
+#include "chrome/browser/browser_process.h"
+#include "chrome/browser/engagement/site_engagement_eviction_policy.h"
+#include "chrome/browser/engagement/site_engagement_service.h"
+#include "chrome/browser/profiles/profile.h"
+#include "chrome/browser/profiles/profile_manager.h"
+#include "content/public/browser/browser_thread.h"
+
+namespace {
+
+const int kExpectedEngagementSites = 200;
+
+// Gets the quota that an origin deserves based on its site engagement.
+int64 GetSoftQuotaForOrigin(const GURL& origin,
+                            int score,
+                            int total_engagement_points,
+                            int64 global_quota) {
+  double quota_per_point =
+      global_quota /
+      std::max(kExpectedEngagementSites * SiteEngagementScore::kMaxPoints,
+               static_cast<double>(total_engagement_points));
+
+  return score * quota_per_point;
+}
+
+
+}  // namespace
+
+SiteEngagementEvictionPolicy::GetEvictionOriginTask::GetEvictionOriginTask(
+    content::BrowserContext* browser_context,
+    const std::map<GURL, int64>& usage_map,
+    int64 global_quota,
+    const storage::GetEvictionOriginCallback& result_callback)
+    : usage_map_(usage_map),
+      global_quota_(global_quota),
+      browser_context_(browser_context),
+      result_callback_(result_callback) {
+}
+
+SiteEngagementEvictionPolicy::GetEvictionOriginTask::~GetEvictionOriginTask() {
+}
+
+void SiteEngagementEvictionPolicy::GetEvictionOriginTask::Start() {
+  // The callback keeps a reference to this task, keeping it alive.
+  content::BrowserThread::PostTaskAndReplyWithResult(
+      content::BrowserThread::UI, FROM_HERE,
+      base::Bind(
+          &GetEvictionOriginTask::GetSiteEngagementEvictionOriginOnUIThread,
+          browser_context_, usage_map_, global_quota_),
+      base::Bind(&GetEvictionOriginTask::ReplyWithEvictionOrigin,
+                 base::Unretained(this)));

[raymes, 2015/07/21 03:16:00]

I think we need to pass "this" rather than base::Unretained(this) to ensure that
a reference gets held.

Another option here, rather than making SiteEngagementEvictionPolicy refcounted
would be to bind result_callback_ to
GetEvictionOriginTask::ReplyWithEvictionOrigin (and make that function
standalone as well). This is slightly safer because it means that if for some
reason the other thread doesn't respond (e.g. if it has been shutdown for some
reason) then we won't leak this object. Then you could remove the
GetEvictionOriginTask class altogether and just inline this PostTaskAndReply
call and all the other functions would be standalone in the anonymous namespace.

[calamity, 2015/07/21 04:57:09]

Done.

+}
+
+void SiteEngagementEvictionPolicy::GetEvictionOriginTask::
+    ReplyWithEvictionOrigin(const GURL& origin_to_evict) {
+  result_callback_.Run(origin_to_evict);
+
+  // Once this has run, the last reference will be released, deleting this
+  // task.
+}
+
+// static
+GURL SiteEngagementEvictionPolicy::GetEvictionOriginTask::
+    GetSiteEngagementEvictionOriginOnUIThread(
+        content::BrowserContext* browser_context,
+        const std::map<GURL, int64>& usage_map,
+        int64 global_quota) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+  Profile* profile = Profile::FromBrowserContext(browser_context);
+  SiteEngagementService* service =
+     g_browser_process->profile_manager()->IsValidProfile(profile) ?
+     SiteEngagementService::Get(profile) : nullptr;
+  if (!service)
+    return GURL();
+
+  return CalculateEvictionOrigin(service, usage_map, global_quota);
+}
+
+// static
+GURL SiteEngagementEvictionPolicy::GetEvictionOriginTask::
+    CalculateEvictionOrigin(SiteEngagementService* service,
+                            const std::map<GURL, int64>& usage_map,
+                            int64 global_quota) {
+  // This heuristic is intended to optimize for two criteria:
+  // - evict the site that the user cares about least
+  // - evict the least number of sites to get under the quota limit
+  //
+  // The heuristic for deciding the next eviction origin calculates a soft
+  // quota for each origin which is the amount the origin should be allowed to
+  // use based on its engagement and the global quota. The origin that most
+  // exceeds its soft quota is chosen.
+  GURL origin_to_evict;
+  int64 max_overuse = std::numeric_limits<int64>::min();
+  int total_engagement_points = service->GetTotalEngagementPoints();
+
+  for (const auto& usage : usage_map) {
+    int64 overuse =
+        usage.second - GetSoftQuotaForOrigin(usage.first,
+                                             service->GetScore(usage.first),
+                                             total_engagement_points,
+                                             global_quota);
+    if (overuse > max_overuse) {
+      max_overuse = overuse;
+      origin_to_evict = usage.first;
+    }
+  }
+  return origin_to_evict;
+}

[raymes, 2015/07/21 03:16:00]

nit: I think we can make the above 2 functions standalone in the anonymous
namespace

[calamity, 2015/07/21 04:57:09]

Done.

+
+SiteEngagementEvictionPolicy::SiteEngagementEvictionPolicy(
+    content::BrowserContext* browser_context)
+    : browser_context_(browser_context) {
+}
+
+SiteEngagementEvictionPolicy::~SiteEngagementEvictionPolicy() {
+}
+
+void SiteEngagementEvictionPolicy::GetEvictionOrigin(
+    const std::map<GURL, int64>& usage_map,
+    int64 global_quota,
+    const storage::GetEvictionOriginCallback& callback) {
+  scoped_refptr<GetEvictionOriginTask> task = new GetEvictionOriginTask(
+      browser_context_, usage_map, global_quota, callback);
+  // The task will delete itself upon completion.
+  task->Start();
+}

[raymes, 2015/07/21 03:16:00]

nit: I think we should reorder this to match the header
(SiteENgagementEvictionPolicy first)

[calamity, 2015/07/21 04:57:09]

Done.