[ios] Use -[SceneState sceneSessionID] as session identifier

For device that do not support multi-window, the persistent identifier
returned by -[UISceneSession persistentIdentifier] may change when the
user "swipe" the app (as this gesture is interpreted as "close window"
under certain circumstances).

In preparation for using a stable identifier (i.e. a constant) for
devices that do not support multi-window, do not access the value from
UISceneSession directly in Chrome but instead through SceneState.

There is one location that is still using -persistentIdentifier: the
code responsible for recording the OS message asking Chrome to purge
old session data. It is find to use -persistentIdentifier there because
the value is correct for device that support multi-window, for other
device the value is not used as part of a path to save data thus it is
benign to try to delete data from a place that does not exists.

Bug: 1165798
Change-Id: I5169b5c0be961987ef8e1edc9c122947848268e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626440Reviewed-by: sebsg <sebsg@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844014}

ios/chrome/app/application_delegate/app_state.mm

@@ -468,6 +468,19 @@ initWithBrowserLauncher:(id<BrowserLauncher>)browserLauncher
     API_AVAILABLE(ios(13)) {
   NSMutableArray<NSString*>* sessionIDs =
       [NSMutableArray arrayWithCapacity:sceneSessions.count];
+  // This method is invoked by iOS to inform the application that the sessions
+  // for "closed windows" is garbage collected and that any data associated with
+  // them by the application needs to be deleted.
+  //
+  // Usually Chrome uses -[SceneState sceneSessionID] as identifier to properly
+  // support devices that do not support multi-window (and which use a constant
+  // identifier). For devices that do not support multi-window the session is
+  // saved at a constant path, so it is harmnless to delete files at a path
+  // derived from -persistentIdentifier (since there won't be files deleted).
+  // For devices that do support multi-window, there is data to delete once the
+  // session is garbage collected.
+  //
+  // Thus it is always correct to use -persistentIdentifier here.
   for (UISceneSession* session in sceneSessions) {
     [sessionIDs addObject:session.persistentIdentifier];
   }

ios/chrome/browser/ui/main/browser_view_wrangler.mm

@@ -105,9 +105,6 @@
   std::unique_ptr<Browser> _otrBrowser;
 }
 
-// Opaque session ID from _sceneState, nil when multi-window isn't enabled.
-@property(nonatomic, readonly) NSString* sessionID;
-
 @property(nonatomic, strong, readwrite) WrangledBrowser* mainInterface;
 @property(nonatomic, strong, readwrite) WrangledBrowser* incognitoInterface;
 
@@ -171,16 +168,6 @@
 
 #pragma mark - BrowserViewInformation property implementations
 
-- (NSString*)sessionID {
-  NSString* sessionID = nil;
-  if (IsMultiwindowSupported()) {
-    if (@available(iOS 13, *)) {
-      sessionID = _sceneState.scene.session.persistentIdentifier;
-    }
-  }
-  return sessionID;
-}
-
 - (void)setCurrentInterface:(WrangledBrowser*)interface {
   DCHECK(interface);
   // |interface| must be one of the interfaces this class already owns.
@@ -415,7 +402,7 @@
 - (void)setSessionIDForBrowser:(Browser*)browser
                 restoreSession:(BOOL)restoreSession {
   SnapshotBrowserAgent::FromBrowser(browser)->SetSessionID(
-      base::SysNSStringToUTF8(self.sessionID));
+      base::SysNSStringToUTF8(_sceneState.sceneSessionID));
 
   SessionRestorationBrowserAgent* restorationAgent =
       SessionRestorationBrowserAgent::FromBrowser(browser);
@@ -443,12 +430,13 @@
     NSString* previousSessionID =
         _sceneState.appState.previousSingleWindowSessionID;
     if (previousSessionID &&
-        ![self.sessionID isEqualToString:previousSessionID]) {
+        ![_sceneState.sceneSessionID isEqualToString:previousSessionID]) {
       restorationAgent->SetSessionID(
           base::SysNSStringToUTF8(previousSessionID));
       restorationAgent->RestoreSession();
 
-      restorationAgent->SetSessionID(base::SysNSStringToUTF8(self.sessionID));
+      restorationAgent->SetSessionID(
+          base::SysNSStringToUTF8(_sceneState.sceneSessionID));
       restorationAgent->SaveSession(true);
 
       // Fallback to the normal codepath. It will set the session identifier
@@ -458,7 +446,8 @@
     }
   }
 
-  restorationAgent->SetSessionID(base::SysNSStringToUTF8(self.sessionID));
+  restorationAgent->SetSessionID(
+      base::SysNSStringToUTF8(_sceneState.sceneSessionID));
   if (restoreSession)
     restorationAgent->RestoreSession();
 }

ios/chrome/browser/ui/main/scene_controller.mm

@@ -348,7 +348,7 @@ const char kMultiWindowOpenInNewWindowHistogram[] =
         // Add the scene to the list of connected scene, to restore in case of
         // crashes.
         [[PreviousSessionInfo sharedInstance]
-            addSceneSessionID:sceneState.scene.session.persistentIdentifier];
+            addSceneSessionID:sceneState.sceneSessionID];
       }
     }
   }
@@ -396,8 +396,7 @@ const char kMultiWindowOpenInNewWindowHistogram[] =
           // If Multiple scenes are not supported, the session shouldn't be
           // removed as it can be used for normal restoration.
           [[PreviousSessionInfo sharedInstance]
-              removeSceneSessionID:sceneState.scene.session
-                                       .persistentIdentifier];
+              removeSceneSessionID:sceneState.sceneSessionID];
         }
       }
     }

ios/chrome/browser/ui/main/scene_state.h

@@ -76,7 +76,8 @@ typedef NS_ENUM(NSUInteger, SceneActivationLevel) {
 @property(nonatomic, strong, readonly) id<BrowserInterfaceProvider>
     interfaceProvider;
 
-// The persistent identifier for the scene session.
+// The persistent identifier for the scene session. This should be used instead
+// of -[UISceneSession persistentIdentifier].
 @property(nonatomic, readonly) NSString* sceneSessionID;
 
 // True if First Run UI (terms of service & sync sign-in) is being presented

ios/chrome/browser/ui/main/scene_state.mm

@@ -130,8 +130,10 @@ ContentVisibility ContentVisibilityForIncognito(BOOL isIncognito) {
 - (NSString*)sceneSessionID {
   NSString* sessionID = nil;
   if (@available(ios 13, *)) {
+    if (IsMultiwindowSupported()) {
       sessionID = _scene.session.persistentIdentifier;
     }
+  }
   return sessionID;
 }