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The crash is triggered by this assert:

[SigninEarlGreyUtils assertSignedInWithIdentity:identity];

Somehow this calls -[BrowserViewController setActive] in a wait loop, which ends up calling NavigationManagerImpl::Restore() multiple times.

A solution is to early return if Restore() is called before the previous one finishes.

I misunderstood the problem earlier. The actual cause of the crash is setting web usage to false while there is an in-progress Restore(). The authentication flow (managed by ios/chrome/browser/ui/authentication/authentication_flow.mm) triggers this use case:

- In the SignOutIfNeeded step, -[MainController removeBrowsingDataForBrowserState:] calls WebStateImpl::SetWebUsageEnabled(false) immediately followed by WebStateImpl::SetWebUsageEnabled(true). This kicks off the session restore in navigation manager.
- In the subsequent ClearData state, -[MainController removeBrowsingDataForBrowserState:] is called again and interrupts the in-progress session restore.

I'm not sure if removing browsing data twice is necessary for authentication, but it seems reasonable that NavigationManager should support entering detached mode during session restore.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/169e31d00bea9e9d7875f2a14bb967634c130da6

commit 169e31d00bea9e9d7875f2a14bb967634c130da6
Author: Danyao Wang <danyao@chromium.org>
Date: Tue Nov 13 17:27:33 2018

[Nav Experiment] Use restore completion callback to clear browsing data.

This CL adds a new AddRestoreCompletionCallback APIs to
NavigationManager. Since session restore is asynchronous in slim nav,
this allows callers to registers a completion callback that
NavigationManager will execute when session restore completes.
LegacyNavigationManager always runs the completion handler synchronously.

[MainController -removeBrowsingDataForBrowserState:] is refactored to
wait for session restore completion.

This fixes SigninInteractionControllerTestCase/
testSignInSwitchManagedAccount.

Bug:  890381 
Change-Id: Iae660ab054514a9db8d75804a7a8d9599b234d7e
Reviewed-on: https://chromium-review.googlesource.com/c/1331568
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607633}
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/chrome/app/main_controller.mm
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/web/navigation/navigation_manager_impl.h
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/web/navigation/navigation_manager_impl.mm
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/web/navigation/navigation_manager_impl_unittest.mm
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/web/navigation/wk_based_navigation_manager_impl.h
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/web/navigation/wk_based_navigation_manager_impl.mm
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/web/public/navigation_manager.h
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/web/public/test/fakes/test_navigation_manager.h
[modify] https://crrev.com/169e31d00bea9e9d7875f2a14bb967634c130da6/ios/web/public/test/fakes/test_navigation_manager.mm