Last_n: do not delete the currently loading offline snapshot.

The code to delete existing offline pages created previously by last_n
was also causing the deletion of the very offline snapshot currently
being loaded. This change fixes that by verifying that the snapshot that
would be deleted is not the same as the one being loaded.

It also changes slightly the way the information of the lastest last_n
snapshot is cleared: now that only happens when the snapshot is actually
being deleted.

BUG=718212

Review-Url: https://codereview.chromium.org/2854213005
Cr-Commit-Position: refs/heads/master@{#469411}

chrome/browser/android/offline_pages/recent_tab_helper.cc

@@ -212,10 +212,22 @@ void RecentTabHelper::DidFinishNavigation(
         downloads_ongoing_snapshot_info_.get(), false);
   }
 
-  // If the previous page was saved, delete it now.
-  if (last_n_latest_saved_snapshot_info_) {
+  // If currently loading an offline page get a pointer to it. It will be null
+  // otherwise.
+  const OfflinePageItem* current_offline_page =
+      OfflinePageUtils::GetOfflinePageFromWebContents(web_contents());
+
+  // If the previous page was saved, delete it now unless we are currently
+  // loading that very snapshot.
+  if (last_n_latest_saved_snapshot_info_ &&
+      (!current_offline_page ||
+       current_offline_page->offline_id !=
+           last_n_latest_saved_snapshot_info_->request_id)) {
+    DVLOG(1) << " - Deleting previous last_n snapshot with offline_id "
+             << last_n_latest_saved_snapshot_info_->request_id;
     std::vector<int64_t> id{last_n_latest_saved_snapshot_info_->request_id};
     page_model_->DeletePagesByOfflineId(id, DeletePageCallback());
+    last_n_latest_saved_snapshot_info_.reset();
   }
 
   // Cancel any and all in flight snapshot tasks from the previous page.
@@ -234,9 +246,13 @@ void RecentTabHelper::DidFinishNavigation(
   bool can_save =
       !navigation_handle->IsErrorPage() && !navigation_handle->IsPost() &&
       OfflinePageModel::CanSaveURL(web_contents()->GetLastCommittedURL()) &&
-      OfflinePageUtils::GetOfflinePageFromWebContents(web_contents()) ==
-          nullptr;
-  DVLOG_IF(1, !can_save) << " - Page can not be saved for offline usage";
+      current_offline_page == nullptr;
+  DVLOG_IF(1, !can_save)
+      << " - Page can not be saved for offline usage (reasons: "
+      << !navigation_handle->IsErrorPage() << ", "
+      << !navigation_handle->IsPost() << ", "
+      << OfflinePageModel::CanSaveURL(web_contents()->GetLastCommittedURL())
+      << ", " << (current_offline_page == nullptr) << ")";
 
   UMA_HISTOGRAM_BOOLEAN("OfflinePages.CanSaveRecentPage", can_save);
 
@@ -310,6 +326,7 @@ void RecentTabHelper::WasHidden() {
       base::Bind(&RecentTabHelper::ContinueSnapshotWithIdsToPurge,
                  weak_ptr_factory_.GetWeakPtr(),
                  last_n_ongoing_snapshot_info_.get()));
+  last_n_latest_saved_snapshot_info_.reset();
 }
 
 void RecentTabHelper::WasShown() {
@@ -507,7 +524,6 @@ void RecentTabHelper::CancelInFlightSnapshots() {
   downloads_ongoing_snapshot_info_.reset();
   downloads_latest_saved_snapshot_info_.reset();
   last_n_ongoing_snapshot_info_.reset();
-  last_n_latest_saved_snapshot_info_.reset();
 }
 
 }  // namespace offline_pages

chrome/browser/android/offline_pages/recent_tab_helper.h

@@ -133,11 +133,13 @@ class RecentTabHelper
   std::unique_ptr<SnapshotProgressInfo> downloads_latest_saved_snapshot_info_;
 
   // Snapshot progress information for a last_n triggered request. Null if
-  // last_n is not currently capturing the current page.
+  // last_n is not currently capturing the current page. It is cleared upon non
+  // ignored navigations.
   std::unique_ptr<SnapshotProgressInfo> last_n_ongoing_snapshot_info_;
 
   // Snapshot information for the last successful snapshot requested by
   // last_n. Null if no successful one has ever completed for the current page.
+  // It is cleared when the referenced snapshot is deleted.
   std::unique_ptr<SnapshotProgressInfo> last_n_latest_saved_snapshot_info_;
 
   // If empty, the tab does not have AndroidId and can not capture pages.