Started flaking extensively recently. A large percentage of the tryjobs on linux-chromeos-rel are failing at least one browser_tests variant failing one of these tests.

https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux-chromeos-rel?limit=200

A few examples:

https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux-chromeos-rel/50278
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux-chromeos-rel/50522
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux-chromeos-rel/50547
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux-chromeos-rel/50538

Flakiness dashboard:

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=browser_tests&tests=ExistingUserControllerActiveDirectoryTest.PolicyChangeTriggersFileUpdate

Example shard from https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux-chromeos-rel/50522 :

https://chromium-swarm.appspot.com/task?id=3ec83174b0a6a010&refresh=10&show_raw=1

Stack:

[2346:2346:0718/122626.380441:FATAL:barrier_closure.cc(32)] Check failed: !num_callbacks_left_.IsZero(). 
#0 0x000004b175fc base::debug::StackTrace::StackTrace()
#1 0x000004a7f7cb logging::LogMessage::~LogMessage()
#2 0x000004a69722 base::(anonymous namespace)::BarrierInfo::Run()
#3 0x0000015df15e _ZN4base8internal7InvokerINS0_9BindStateIZN8chromeos12_GLOBAL__N_125KerberosFilesChangeWaiterC1EbEUlRKNS_17RepeatingCallbackIFvvEEERKNS_8FilePathEbE_JS8_EEEFvSD_bEE3RunEPNS0_13BindStateBaseESD_b
#4 0x000004a7649d base::(anonymous namespace)::FilePathWatcherImpl::OnFilePathChangedOnOriginSequence()
#5 0x000004a76661 _ZN4base8internal7InvokerINS0_9BindStateIMNS_12_GLOBAL__N_119FilePathWatcherImplEFviRKNSt3__112basic_stringIcNS5_11char_traitsIcEENS5_9allocatorIcEEEEbbbEJNS_7WeakPtrIS4_EEiSB_bbbEEEFvvEE7RunOnceEPNS0_13BindStateBaseE
#6 0x000004b38995 base::debug::TaskAnnotator::RunTask()
#7 0x000004a87dea base::MessageLoop::RunTask()
#8 0x000004a8817a base::MessageLoop::DeferOrRunPendingTask()
#9 0x000004a88477 base::MessageLoop::DoWork()
#10 0x000004b33619 base::MessagePumpLibevent::Run()
#11 0x000004a87814 base::MessageLoop::Run()
#12 0x000004aae2b9 base::RunLoop::Run()
#13 0x000004aae83a base::RunLoop::RunUntilIdle()
#14 0x000009d657b7 policy::MockConfigurationPolicyProvider::UpdateChromePolicy()
#15 0x0000015d80d6 chromeos::ExistingUserControllerActiveDirectoryTest::ApplyPolicyAndWaitFilesChanged()
#16 0x0000015d7f38 chromeos::ExistingUserControllerActiveDirectoryTest_PolicyChangeTriggersFileUpdate_Test::RunTestOnMainThread()
#17 0x0000050eec91 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#18 0x000004bf55ab ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#19 0x000004bf439a ChromeBrowserMainParts::PreMainMessageLoopRun()
#20 0x000001bdaf4a chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()
#21 0x000002da67b1 content::BrowserMainLoop::PreMainMessageLoopRun()
#22 0x000003227845 content::StartupTaskRunner::RunAllTasksNow()
#23 0x000002da50b8 content::BrowserMainLoop::CreateStartupTasks()
#24 0x000002da8e23 content::BrowserMainRunnerImpl::Initialize()
#25 0x000002da2c42 content::BrowserMain()
#26 0x0000049ddb80 content::ContentMainRunnerImpl::Run()
#27 0x000006aeacb5 service_manager::Main()
#28 0x0000049dbe84 content::ContentMain()
#29 0x0000050ee82c content::BrowserTestBase::SetUp()
#30 0x000004b9605f InProcessBrowserTest::SetUp()
#31 0x0000016c719c policy::DevicePolicyCrosBrowserTest::SetUp()


This is causing a large number of retries on the CQ. Marking P0. Must be triaged immediately; possibly a code change caused this test to begin flaking.