If users clear history, the previews recency rule should be cleared

components/previews/core/previews_black_list.cc

 // 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.
 
 #include "components/previews/core/previews_black_list.h"
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram.h"
 #include "base/optional.h"
@@ skipping 135 matching lines @@
   // synchronous operations will be accurate. Otherwise, queue the task to run
   // asynchronously.
   if (loaded_) {
     ClearBlackListSync(begin_time, end_time);
   } else {
     QueuePendingTask(base::Bind(&PreviewsBlackList::ClearBlackListSync,
                                 base::Unretained(this), begin_time, end_time));
   }
 }
 
 void PreviewsBlackList::ClearBlackListSync(base::Time begin_time,

[bengr, 2017/05/15 21:15:14]

How robust is your blacklist logic to the system clock being changed?

[RyanSturm, 2017/05/15 21:58:58]

About as robust as anything that is in
ChromeBrowsingDataRemoverDelegate::RemoveEmbedderData. That being said, changing
the system clock can have all sorts of repercussions. Specifically, this will
remove things that are time-stamped between begin_time and end_time. If the user
clears all browsing history, begin will be Time() and end will be Time::Max(),
so it will clear everything in that case, which is likely the most important
(for instance NTP category ranking only clears anything when all browsing data
is cleared).

[bengr, 2017/05/16 17:06:36]

OK. I filed https://bugs.chromium.org/p/chromium/issues/detail?id=722890 for a
rainy day.

                                            base::Time end_time) {
   DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(loaded_);
   DCHECK_LE(begin_time, end_time);
+
+  // Clear last_opt_out_time_ if the period being cleared is larger than the
+  // short black list timeout and the last time the user opted out was before
+  // |end_time|.
+  if (end_time - begin_time > params::SingleOptOutDuration() &&
+      last_opt_out_time_ && last_opt_out_time_.value() < end_time) {
+    last_opt_out_time_.reset();
+  }
   black_list_item_map_.reset();
   host_indifferent_black_list_item_.reset();
   loaded_ = false;
   // Delete relevant entries and reload the blacklist into memory.
   if (opt_out_store_) {
     opt_out_store_->ClearBlackList(begin_time, end_time);
     opt_out_store_->LoadBlackList(base::Bind(
         &PreviewsBlackList::LoadBlackListDone, weak_factory_.GetWeakPtr()));
   } else {
     LoadBlackListDone(base::MakeUnique<BlackListItemMap>(),
@@ skipping 50 matching lines @@
 // static
 std::unique_ptr<PreviewsBlackListItem>
 PreviewsBlackList::CreateHostIndifferentBlackListItem() {
   return base::MakeUnique<PreviewsBlackListItem>(
       params::MaxStoredHistoryLengthForHostIndifferentBlackList(),
       params::HostIndifferentBlackListOptOutThreshold(),
       params::HostIndifferentBlackListPerHostDuration());
 }
 
 }  // namespace previews