Can someone from iOS team try reproducing this bug? I'm having hard time making chrome start in my iOS setup.

The code fails in tab_switcher_panel_overlay_view.mm:462. Ed, could you check if this failure is related to your change https://chromium-review.googlesource.com/659517.

+ sdefresne@ and marq@
Changes in tab_switcher_controller.mm could be related too. Could you guys take a look?

I've reproduced this in the debugger. This happens because the CommandDispatcher has no dispatching target for the selector -showSyncSettings and thus an NSException is raised.

Code that invokes the dispatcher is:

  - (void)showSyncSettings {
    SyncSetupService::SyncServiceState syncState =
        GetSyncStateForBrowserState(_browserState);
    if (ShouldShowSyncSignin(syncState)) {
      [self.dispatcher
          showSignin:[[ShowSigninCommand alloc]
                         initWithOperation:AUTHENTICATION_OPERATION_REAUTHENTICATE
                               accessPoint:signin_metrics::AccessPoint::
                                               ACCESS_POINT_UNKNOWN]];
    } else if (ShouldShowSyncSettings(syncState)) {
      [self.dispatcher showSyncSettings];  // crash happens here
    } else if (ShouldShowSyncPassphraseSettings(syncState)) {
      [self.dispatcher showSyncPassphraseSettings];
    }
  }

Information from debugger:

2017-11-03 11:24:49.410 Chromium[63312:405694] -[CommandDispatcher showSyncSettings]: unrecognized selector sent to instance 0x6000002afa80
(lldb) po self.dispatcher
<CommandDispatcher: 0x6000004b4820>


The dispatcher passed to this TabSwitcherPanelOverlayView is the dispatcher created by TabSwitcherController. That dispatcher only accepts the commands from BrowserCommands and ApplicationCommands, but not ApplicationSettingsCommands.

  @implementation TabSwitcherController
  
  - (instancetype)initWithBrowserState:(ios::ChromeBrowserState*)browserState
                          mainTabModel:(TabModel*)mainTabModel
                           otrTabModel:(TabModel*)otrTabModel
                        activeTabModel:(TabModel*)activeTabModel
            applicationCommandEndpoint:(id<ApplicationCommands>)endpoint {
    DCHECK(mainTabModel);
    DCHECK(otrTabModel);
    DCHECK(activeTabModel == otrTabModel || activeTabModel == mainTabModel);
    self = [super initWithNibName:nil bundle:nil];
    if (self) {
      _browserState = browserState;

      _dispatcher = [[CommandDispatcher alloc] init];
      [_dispatcher startDispatchingToTarget:self
                                forProtocol:@protocol(BrowserCommands)];
      [_dispatcher startDispatchingToTarget:endpoint
                                forProtocol:@protocol(ApplicationCommands)];
      // self.dispatcher shouldn't be used in this init method, so duplicate the
      // typecast to pass dispatcher into child objects.
      id<ApplicationCommands, BrowserCommands> passableDispatcher =
          static_cast<id<ApplicationCommands, BrowserCommands>>(_dispatcher);

Looking at TabSwitcherPanelOverlayView initializer, changing the prototype of the dispatcher from id<ApplicationCommands, BrowserCommands> to id<ApplicationCommands, ApplicationSettingsCommands, BrowserCommands> I get one compilation error at the following code (out of the two non-test invocation of the initializer):

@implementation TabSwitcherPanelController

  - (void)setZeroTabStateOverlayVisible:(BOOL)show {
    if (show == [self isOverlayVisible])
      return;

    DCHECK(TabSwitcherSessionTypeIsLocalSession(_sessionType));

    if (!_overlayView) {
      _overlayView =
          [[TabSwitcherPanelOverlayView alloc] initWithFrame:[_panelView bounds]
                                                browserState:_browserState
                                                  dispatcher:self.dispatcher];

This dispatcher should probably be modified to also implements ApplicationSettingsCommands which would probably ultimately mean that TabSwitcherController will need to implement that protocol.

Looking at ios/chrome/browser/ui/commands/application_commands.h, I think this regression was introduced by https://chromium-review.googlesource.com/659517 refactoring.

The dispatcher in TabSwitcherController needed to be modified to handle
ApplicationSettingsCommands.
CL in flight:
https://chromium-review.googlesource.com/c/chromium/src/+/757886

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/79bea66adb3a154103e4605da3f2387b63dbfc3c

commit 79bea66adb3a154103e4605da3f2387b63dbfc3c
Author: edchin <edchin@chromium.org>
Date: Wed Nov 08 19:26:15 2017

[ios] Must register separately for ApplicationSettingsCommands

-startDispatchingToTarget:forProtocol: doesn't pick up protocols the
passed protocol conforms to, so ApplicationSettingsCommands must be
explicitly registered to the endpoint as well.
Without this, we get a crash: unrecognized selector.

Bug:  780280 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I03a13c665dde5c10b3615c623d1a3dd8047da8ad
Reviewed-on: https://chromium-review.googlesource.com/757886
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514904}
[modify] https://crrev.com/79bea66adb3a154103e4605da3f2387b63dbfc3c/ios/chrome/browser/ui/tab_switcher/tab_switcher_controller.mm

Verified fix for the issue - In Tab Switcher mode - Tap on Enable sync from Other Devices and it will crash the app.

Build - 64.0.3263.0 Canary 
Devices - iPad, iPad 4, iPad Mini
iOS - 10.3.3, 9.3.5, 11.2

Did fix in comment 8 make it to M63 branch? Fix landed on Nov 08, branch cut for M63 was October 12.
Looking at some new crashes in M63 (app store 1% release), this crash is in M63 release.

I just noticed this as well. It initially landed in 64.0.3263.0 and was never merged into M63. It was not marked as a blocker either so there was no expectation to have it in M63. It is currently top 6 crash with 15 unique clients affected so does not seem too severe yet. Let's wait and see.

I would've liked this to be in M63. Enabling sync from remote tabs then crashing is not a great user experience. How complex/risky is the fix ?

I understand Mardini. This crash only affects few iPad users and there is a way to go around the bug through the setting screen. We currently have only 26 unique clients affected by this crash and I think we should proceed with the rollout and monitor the impact of this crash as we go.

Re-opening this bug. Crashes are happening in iOS 11.1. and 11.2 on new stable build 63.0.3239.73
As of 2017/12/06
Total number of new occurrences so far in : 27

is it iOS 11 only?

Apparently not

Note that the fix is already in M64 (see comment 8).

Since fix is already in M64 and there's no plan to refresh M63 for this, I'm marking this as Fix and removing M63 label.

Please verify.

Verified on iPad Pro 12'5 iOS 10.3.3 , iPad mini iOS 11.2 on build 64.0.3282.22 Beta as per the steps #0
App does not crash on Enable sync in tab switcher mode.