Some flutter_tools tests using FakeAsync are always passing

@Hixie

There are some tests that use FakeAsync.run() that are not awaiting the call, for example:

flutter/packages/flutter_tools/test/general.shard/device_test.dart

Lines 92 to 95 in 0e735e4

    
           FakeAsync().run((FakeAsync time) async { 
        
             final FakePollingDeviceDiscovery pollingDeviceDiscovery = FakePollingDeviceDiscovery(); 
        
             await pollingDeviceDiscovery.startPolling(); 
        
             time.elapse(const Duration(milliseconds: 4001));

It seems like this is resulted in the tests always passing immediately. For example adding throw 'fail'; as the first line inside the callback does nothing - the test still passes.

I think these tests need await in front of the FakeAsync().run, however doing so right now causes them to hang - which I think is because the implementation uses await which queues a microtask but the queue is never flushed (since the code stops on the await).

Based on what @Hixie said, I think flutter_test is handling this by constantly pumping the microtask queue, however the flutter_tools tests don't run with flutter_test so don't have an equivilent. I think the relevant code in flutter_test is this bit:

flutter/packages/flutter_test/lib/src/binding.dart

Lines 1128 to 1146 in 868c4d8

    
           return Future<void>.microtask(() async { 
        
             // testBodyResult is a Future that was created in the Zone of the 
        
             // fakeAsync. This means that if we await it here, it will register a 
        
             // microtask to handle the future _in the fake async zone_. We avoid this 
        
             // by calling '.then' in the current zone. While flushing the microtasks 
        
             // of the fake-zone below, the new future will be completed and can then 
        
             // be used without fakeAsync. 
        
             final Future<void> resultFuture = testBodyResult.then<void>((_) { 
        
               // Do nothing. 
        
             }); 
        
             // Resolve interplay between fake async and real async calls. 
        
             fakeAsync.flushMicrotasks(); 
        
             while (_pendingAsyncTasks != null) { 
        
               await _pendingAsyncTasks.future; 
        
               fakeAsync.flushMicrotasks(); 
        
             } 
        
             return resultFuture; 
        
           });

cc @Hixie @jonahwilliams

For my tests, I just made a small loop that flushes the queue:

https://github.com/flutter/flutter/pull/55376/files#diff-1a0ff66910e803e1ddabc1968f7d9e8bR358-R371

I don't know if this may be suitable to use elsewhere too (it may also be worth having a lint or something to catch these - perhaps the existing unawaited futures one would work?).

Bumping to P2 due to lost test coverage of currently unknown scope, and to force to surface in the next triage meeting to discuss.

FakeAsync cases are now updated where feasible and skips added for the existing failures. Since we can't regress further, I'm bumping to a P3

What's the rough number of tests that are skipped?

5 test cases

3 test cases left

removing assignment because I will be OOO

One remaining test: https://github.com/flutter/flutter/blob/cbf885b74919bf4b0796cc216b8b80e6b3ccb266/packages/flutter_tools/test/general.shard/ios/ios_device_start_nonprebuilt_test.dart

Restore skipped iOS test by looping over `FakeAsync` elapse. (#158204)

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.

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Some flutter_tools tests using FakeAsync are always passing #60675

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	FakeAsync().run((FakeAsync time) async {
	final FakePollingDeviceDiscovery pollingDeviceDiscovery = FakePollingDeviceDiscovery();
	await pollingDeviceDiscovery.startPolling();
	time.elapse(const Duration(milliseconds: 4001));

	return Future<void>.microtask(() async {
	// testBodyResult is a Future that was created in the Zone of the
	// fakeAsync. This means that if we await it here, it will register a
	// microtask to handle the future _in the fake async zone_. We avoid this
	// by calling '.then' in the current zone. While flushing the microtasks
	// of the fake-zone below, the new future will be completed and can then
	// be used without fakeAsync.
	final Future<void> resultFuture = testBodyResult.then<void>((_) {
	// Do nothing.
	});

	// Resolve interplay between fake async and real async calls.
	fakeAsync.flushMicrotasks();
	while (_pendingAsyncTasks != null) {
	await _pendingAsyncTasks.future;
	fakeAsync.flushMicrotasks();
	}
	return resultFuture;
	});

Some flutter_tools tests using FakeAsync are always passing #60675

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DanTup commented on Jul 1, 2020

zanderso commented on Aug 5, 2020

jonahwilliams commented on Aug 13, 2020

zanderso commented on Aug 13, 2020

jonahwilliams commented on Aug 13, 2020

jonahwilliams commented on Dec 2, 2020

jonahwilliams commented on Dec 4, 2020

jonahwilliams commented on May 4, 2021

github-actions commented on Nov 20, 2024

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