[ios] Enable preload with slim-navigation-manager.

Previously, preload required CopyStateFromAndPrune and
CanPruneAllButLastCommittedItem within NavigationManager, but those
are not supported with SlimNav. Here, preload is implemented by
creating a preload WebState with navigation history by restoring the
current WebState.

The performance of this implementation depends on Slim
Navigation Manager's session restoration performance which can be
measured with existing kRestoreNavigationTime metric.

Original implementation of this CL was written by eugenebut@,
simply updated in this resurrected CL.

Bug: 834116
Change-Id: I86c9ecf7e3ed213ebaeddce6264dff3bbb8a8c94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815702
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701228}

ios/chrome/browser/prerender/preload_controller.h

@@ -71,10 +71,11 @@ class WebState;
 // Returns whether |webState| is the WebState used for pre-rendering.
 - (BOOL)isWebStatePrerendered:(web::WebState*)webState;
 
-// Returns the currently prerendered WebState, or nil if none exists.  After
-// this method is called, the PrerenderController reverts to a non-prerendering
-// state.
-- (std::unique_ptr<web::WebState>)releasePrerenderContents;
+// Returns the currently prerendered WebState, or nil if it was not created
+// while |oldWebState| was active, or nil if none exists.  After this method is
+// called, the PrerenderController reverts to a non-prerendering state.
+- (std::unique_ptr<web::WebState>)releasePrerenderContentsForWebState:
+    (web::WebState*)oldWebState;
 
 @end
 

ios/chrome/browser/prerender/preload_controller.mm

@@ -32,6 +32,7 @@
 #import "ios/web/public/navigation/web_state_policy_decider_bridge.h"
 #include "ios/web/public/thread/web_thread.h"
 #import "ios/web/public/ui/java_script_dialog_presenter.h"
+#include "ios/web/public/web_client.h"
 #import "ios/web/public/web_state.h"
 #import "ios/web/public/web_state_observer_bridge.h"
 #import "ios/web/web_state/ui/crw_web_controller.h"
@@ -186,6 +187,10 @@ class PreloadJavaScriptDialogPresenter : public web::JavaScriptDialogPresenter {
 // there is no prerender scheduled.
 @property(nonatomic, readonly) const GURL& scheduledURL;
 
+// Contains the original webState that, for SlimNav, session history will be
+// created from.
+@property(nonatomic, assign) web::WebState* webStateToReplace;
+
 // Whether or not the preference is enabled.
 @property(nonatomic, getter=isPreferenceEnabled) BOOL preferenceEnabled;
 
@@ -299,6 +304,9 @@ class PreloadJavaScriptDialogPresenter : public web::JavaScriptDialogPresenter {
   }
 
   [self removeScheduledPrerenderRequests];
+
+  self.webStateToReplace = [self.delegate webStateToReplace];
+
   _scheduledRequest =
       std::make_unique<PrerenderRequest>(url, transition, referrer);
 
@@ -321,10 +329,13 @@ class PreloadJavaScriptDialogPresenter : public web::JavaScriptDialogPresenter {
   return webState && _webState.get() == webState;
 }
 
-- (std::unique_ptr<web::WebState>)releasePrerenderContents {
-  if (!_webState)
+- (std::unique_ptr<web::WebState>)releasePrerenderContentsForWebState:
+    (web::WebState*)oldWebState {
+  if (!_webState || oldWebState != self.webStateToReplace ||
+      _webState->GetNavigationManager()->IsRestoreSessionInProgress())
     return nullptr;
 
+  self.webStateToReplace = nullptr;
   self.successfulPrerendersPerSessionCount++;
   [self recordReleaseMetrics];
   [self removeScheduledPrerenderRequests];
@@ -501,7 +512,13 @@ class PreloadJavaScriptDialogPresenter : public web::JavaScriptDialogPresenter {
   }
 
   web::WebState::CreateParams createParams(self.browserState);
-  _webState = web::WebState::Create(createParams);
+  if (web::GetWebClient()->IsSlimNavigationManagerEnabled()) {
+    _webState = web::WebState::CreateWithStorageSession(
+        createParams, self.webStateToReplace->BuildSessionStorage());
+  } else {
+    _webState = web::WebState::Create(createParams);
+  }
+
   // Add the preload controller as a policyDecider before other tab helpers, so
   // that it can block the navigation if needed before other policy deciders
   // execute thier side effects (eg. AppLauncherTabHelper launching app).
@@ -563,6 +580,7 @@ class PreloadJavaScriptDialogPresenter : public web::JavaScriptDialogPresenter {
 
   self.prerenderedURL = GURL();
   self.startTime = base::TimeTicks();
+  self.webStateToReplace = nullptr;
 }
 
 #pragma mark - Notification Helpers

ios/chrome/browser/prerender/preload_controller_delegate.h

@@ -12,6 +12,10 @@
 // A protocol implemented by a delegate of PreloadController
 @protocol PreloadControllerDelegate
 
+// WebState from which preload controller should copy the session history.
+// This web state will be replaced on successful preload.
+- (web::WebState*)webStateToReplace;
+
 // Should preload controller request a desktop site.
 - (BOOL)preloadShouldUseDesktopUserAgent;
 

ios/chrome/browser/prerender/preload_controller_unittest.mm

@@ -116,7 +116,7 @@ TEST_F(PreloadControllerTest, DontPreloadNonWebURLs) {
                    referrer:kReferrer
                  transition:kTransition
                 immediately:YES];
-  EXPECT_FALSE([controller_ releasePrerenderContents]);
+  EXPECT_FALSE([controller_ releasePrerenderContentsForWebState:nil]);
 
   // Attempt to prerender the NTP and verify that no WebState was created
   // to preload.
@@ -124,7 +124,7 @@ TEST_F(PreloadControllerTest, DontPreloadNonWebURLs) {
                    referrer:kReferrer
                  transition:kTransition
                 immediately:YES];
-  EXPECT_FALSE([controller_ releasePrerenderContents]);
+  EXPECT_FALSE([controller_ releasePrerenderContentsForWebState:nil]);
 
   // Attempt to prerender the flags UI and verify that no WebState was created
   // to preload.
@@ -132,7 +132,7 @@ TEST_F(PreloadControllerTest, DontPreloadNonWebURLs) {
                    referrer:kReferrer
                  transition:kTransition
                 immediately:YES];
-  EXPECT_FALSE([controller_ releasePrerenderContents]);
+  EXPECT_FALSE([controller_ releasePrerenderContentsForWebState:nil]);
 }
 
 TEST_F(PreloadControllerTest, TestIsPrerenderingEnabled_preloadAlways) {

ios/chrome/browser/prerender/prerender_egtest.mm

@@ -65,11 +65,6 @@ std::unique_ptr<net::test_server::HttpResponse> StandardResponse(
         @"Disabled for iPad due to alternate letters educational screen.");
   }
 
-  if ([ChromeEarlGrey isSlimNavigationManagerEnabled]) {
-    // TODO(crbug.com/834116): Fix and enable this test.
-    EARL_GREY_TEST_DISABLED(@"Prerender is not supported by slim-nav yet.");
-  }
-
   [ChromeEarlGrey clearBrowsingHistory];
   // Set server up.
   int visitCounter = 0;

ios/chrome/browser/prerender/prerender_service.mm

@@ -39,13 +39,6 @@ void PrerenderService::StartPrerender(const GURL& url,
                                       const web::Referrer& referrer,
                                       ui::PageTransition transition,
                                       bool immediately) {
-  // PrerenderService is not compatible with WKBasedNavigationManager because it
-  // loads the URL in a new WKWebView, which doesn't have the current session
-  // history. TODO(crbug.com/814789): decide whether PrerenderService needs to
-  // be supported after evaluating the performance impact in Finch experiment.
-  if (web::GetWebClient()->IsSlimNavigationManagerEnabled())
-    return;
-
   [controller_ prerenderURL:url
                    referrer:referrer
                  transition:transition
@@ -62,12 +55,18 @@ bool PrerenderService::MaybeLoadPrerenderedURL(
     return false;
   }
 
+  web::WebState* web_state = web_state_list->GetActiveWebState();
   std::unique_ptr<web::WebState> new_web_state =
-      [controller_ releasePrerenderContents];
+      [controller_ releasePrerenderContentsForWebState:web_state];
+  if (!new_web_state) {
+    CancelPrerender();
+    return false;
+  }
+
   DCHECK_NE(WebStateList::kInvalidIndex, web_state_list->active_index());
 
   web::NavigationManager* active_navigation_manager =
-      web_state_list->GetActiveWebState()->GetNavigationManager();
+      web_state->GetNavigationManager();
   int lastIndex = active_navigation_manager->GetLastCommittedItemIndex();
   UMA_HISTOGRAM_COUNTS_100("Prerender.PrerenderLoadedOnIndex", lastIndex);
 
@@ -79,8 +78,13 @@ bool PrerenderService::MaybeLoadPrerenderedURL(
   web::NavigationManager* new_navigation_manager =
       new_web_state->GetNavigationManager();
 
-  if (new_navigation_manager->CanPruneAllButLastCommittedItem()) {
-    new_navigation_manager->CopyStateFromAndPrune(active_navigation_manager);
+  bool slim_navigation_manager_enabled =
+      web::GetWebClient()->IsSlimNavigationManagerEnabled();
+  if (new_navigation_manager->CanPruneAllButLastCommittedItem() ||
+      slim_navigation_manager_enabled) {
+    if (!slim_navigation_manager_enabled) {
+      new_navigation_manager->CopyStateFromAndPrune(active_navigation_manager);
+    }
     loading_prerender_ = true;
     web_state_list->ReplaceWebStateAt(web_state_list->active_index(),
                                       std::move(new_web_state));

ios/chrome/browser/ui/browser_view/browser_view_controller.mm

@@ -4758,6 +4758,10 @@ NSString* const kBrowserViewControllerSnackbarCategory =
   return [self userAgentType] == web::UserAgentType::DESKTOP;
 }
 
+- (web::WebState*)webStateToReplace {
+  return self.currentWebState;
+}
+
 #pragma mark - NetExportTabHelperDelegate
 
 - (void)netExportTabHelper:(NetExportTabHelper*)tabHelper