Non main thread Realm instance is not notified even with `autorefresh` turned on when changes happened in other thread(including main thread)

[dismory]

Run this code in an app(so we get an main RunLoop set up by free) in iOS simulator:

// realm, realm2, token, token2 are some strong references
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    realm2 = [RLMRealm defaultRealm];
    realm2.autorefresh = YES;
    token2 = [realm2 addNotificationBlock:^(NSString *notification, RLMRealm *realm) {
        // not called
    }];
    dispatch_async(dispatch_get_main_queue(), ^{
        realm = [RLMRealm defaultRealm];
        RealmTestObjecet *object = [RealmTestObjecet new]; // RealmTestObjecet is a simple subclass of RLMObject
        object.foo = @"bar";
        [realm transactionWithBlock:^{
            [realm addObject:object];
        }];         
    });
});

Do I misunderstand something?

[dismory]

I find that even main thread Realm instance is not notified and refreshed when changes happened in other thread..

Check this test case:

#import <XCTest/XCTest.h>

@class RealmTestObjecet;

RLM_ARRAY_TYPE(RealmTestObjecet);

@interface RealmTestObjecet : RLMObject

@property NSString *text;

@end

@implementation RealmTestObjecet
@end

@interface JTRealmTests : XCTestCase {
    RLMRealm *_mainThreadRealm;
    RLMNotificationToken *_token;
}

@end

@implementation JTRealmTests

- (void)setUp
{
    [super setUp];
    // Put setup code here. This method is called before the invocation of each test method in the class.

    [RLMRealm useInMemoryDefaultRealm];
    _mainThreadRealm = [RLMRealm defaultRealm];
    _mainThreadRealm.autorefresh = YES;

    RealmTestObjecet *object = [RealmTestObjecet new];
    object.text = @"foo";
    [_mainThreadRealm transactionWithBlock:^{
        [_mainThreadRealm addObject:object];
    }];
}

- (void)tearDown
{
    [_mainThreadRealm removeNotification:_token];
    _token = nil;
    _mainThreadRealm = nil;

    // Put teardown code here. This method is called after the invocation of each test method in the class.
    [super tearDown];
}

- (void)testRealm
{
    __block BOOL notifiedFlag = NO;

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    _token = [_mainThreadRealm addNotificationBlock:^(NSString *notification, RLMRealm *realm) {
        // this is not called
        notifiedFlag = YES;
    }];

    dispatch_async(queue, ^{

        RLMRealm *realm = [RLMRealm defaultRealm];
        RealmTestObjecet *object = [[RealmTestObjecet allObjectsInRealm:realm] firstObject];
        [realm transactionWithBlock:^{
            object.text = @"bar";
        }];

        dispatch_semaphore_signal(semaphore);
    });

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    RealmTestObjecet *object = [[RealmTestObjecet allObjectsInRealm:_mainThreadRealm] firstObject];
    // not refreshed
    XCTAssertEqualObjects(object.text, @"bar", @"");
    // not notified
    XCTAssertTrue(notifiedFlag, @"");
}

@end

[tgoyne]

Autorefresh occurs between iterations of the event loop (so that you always get a consistent view of data while working and to remove the need for synchronization on every read). I don't think this is explicitly documented anywhere, so that needs to be fixed.

[dismory]

@tgoyne I know that but why it does not work as expected here?

[alazier]

The problem is that you are using semaphores. When you wait for the semaphore, the main thread is blocked, and the notification never has a chance to run. Your thread needs to be able to process events for the notification to fire. We have a test which tests this exact scenario here: https://github.com/realm/realm-cocoa/blob/master/Realm/Tests/RealmTests.m#L193

Also please note that there are currently issues with in-memory realms which we are looking into. Might want to persist your realm for the time being until we can resole the issue.

[dismory]

@alazier Look at my first test case, semaphore is not used there but still not working..

[alazier]

In your first testcase you need to run a runloop in your background queue for notifications to fire.

[bmunkholm]

@dismory Is there any way you think we could document this better?

[dismory]

@bmunkholm IMHO, it is necessary to explicitly document RunLoop and Timer implementation details used in Realm as what @tgoyne what said.

[dismory]

@alazier I rewrite the test case(which use semaphores) by running a runloop manually and it works as expected. :smile:

#import <XCTest/XCTest.h>

@class RealmTestObjecet;

RLM_ARRAY_TYPE(RealmTestObjecet);

@interface RealmTestObjecet : RLMObject

@property NSString *text;

@end

@implementation RealmTestObjecet
@end

@interface JTRealmTests : XCTestCase {
    RLMRealm *_mainThreadRealm;
    RLMNotificationToken *_token;
}

@end

@implementation JTRealmTests

- (void)setUp
{
    [super setUp];
    // Put setup code here. This method is called before the invocation of each test method in the class.

    // This will cause crash 
//    [RLMRealm useInMemoryDefaultRealm];
    _mainThreadRealm = [RLMRealm defaultRealm];

    RealmTestObjecet *object = [RealmTestObjecet new];
    object.text = @"foo";
    [_mainThreadRealm transactionWithBlock:^{
        [_mainThreadRealm addObject:object];
    }];
}

- (void)tearDown
{
    [_mainThreadRealm removeNotification:_token];
    _token = nil;
    _mainThreadRealm = nil;

    // Put teardown code here. This method is called after the invocation of each test method in the class.
    [super tearDown];
}

- (void)testRealm
{
    __block BOOL notifiedFlag = NO;

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    _token = [_mainThreadRealm addNotificationBlock:^(NSString *notification, RLMRealm *realm) {
        // this is not called
        notifiedFlag = YES;
    }];

    dispatch_async(queue, ^{

        RLMRealm * realm = [RLMRealm defaultRealm];
        RealmTestObjecet *object = [[RealmTestObjecet allObjectsInRealm:realm] firstObject];
        [realm transactionWithBlock:^{
            object.text = @"bar";
        }];

        dispatch_semaphore_signal(semaphore);
    });

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    // run main runloop manually
    [[NSRunLoop mainRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.2]];

    RealmTestObjecet *object = [[RealmTestObjecet allObjectsInRealm:_mainThreadRealm] firstObject];
    // not refreshed
    XCTAssertEqualObjects(object.text, @"bar", @"");
    // not notified
    XCTAssertTrue(notifiedFlag, @"");
}

@end

Also as you said, it will crash in Realm -sendNotifications message when using in memory default Realm.

FYI:

Tests`-[RLMRealm refresh] at RLMRealm.mm:512:
0x1066d903e:  pushq  %rbp
0x1066d903f:  movq   %rsp, %rbp
0x1066d9042:  pushq  %r14
0x1066d9044:  pushq  %rbx
0x1066d9045:  movq   %rdi, %rbx
0x1066d9048:  movq   0x2f6aa1(%rip), %rsi      ; "inWriteTransaction"
0x1066d904f:  callq  *0x2c1c33(%rip)           ; (void *)0x000000010086afc0: objc_msgSend
0x1066d9055:  testb  %al, %al
0x1066d9057:  jne    0x1066d9091               ; -[RLMRealm refresh] + 83 at RLMRealm.mm:530
0x1066d9059:  movq   0x2ff6d8(%rip), %r14      ; RLMRealm._sharedGroup
0x1066d9060:  movq   (%rbx,%r14), %rdi
0x1066d9064:  callq  0x1067c2430               ; tightdb::SharedGroup::has_changed()
0x1066d9069:  testb  %al, %al
0x1066d906b:  je     0x1066d9091               ; -[RLMRealm refresh] + 83 at RLMRealm.mm:530
0x1066d906d:  movq   (%rbx,%r14), %rdi
0x1066d9071:  movq   0x2ff6c8(%rip), %rax      ; RLMRealm._writeLogs
0x1066d9078:  movq   (%rbx,%rax), %rsi
0x1066d907c:  callq  0x1067c2a40               ; tightdb::SharedGroup::advance_read(tightdb::SharedGroup::TransactLogRegistry&)
0x1066d9081:  movq   0x2f6a70(%rip), %rsi      ; "sendNotifications"
---------------------------------------------------------------------------------------------
0x1066d9088:  movq   %rbx, %rdi
0x1066d908b:  callq  *0x2c1bf7(%rip)           ; (void *)0x000000010086afc0: objc_msgSend
0x1066d9091:  popq   %rbx
0x1066d9092:  popq   %r14
0x1066d9094:  popq   %rbp
0x1066d9095:  ret    
0x1066d9096:  movq   %rdx, %rcx
0x1066d9099:  movq   %rax, %rbx
0x1066d909c:  cmpl   $0x1, %ecx
0x1066d909f:  jne    0x1066d90c2               ; -[RLMRealm refresh] + 132 at RLMRealm.mm:529
0x1066d90a1:  movq   %rbx, %rdi
0x1066d90a4:  callq  0x106913dd0               ; symbol stub for: __cxa_begin_catch
0x1066d90a9:  movq   %rax, %rdi
0x1066d90ac:  callq  0x1066d8b2b               ; (anonymous namespace)::throw_objc_exception(std::exception&) at RLMRealm.mm:63
0x1066d90b1:  popq   %rbx
0x1066d90b2:  popq   %r14
0x1066d90b4:  popq   %rbp
0x1066d90b5:  jmpq   0x106913ddc               ; symbol stub for: __cxa_end_catch
0x1066d90ba:  movq   %rax, %rbx
0x1066d90bd:  callq  0x106913ddc               ; symbol stub for: __cxa_end_catch
0x1066d90c2:  movq   %rbx, %rdi
0x1066d90c5:  callq  0x106915984               ; symbol stub for: _Unwind_Resume
0x1066d90ca:  movq   %rax, %rdi
0x1066d90cd:  callq  0x1066ce9c0               ; __clang_call_terminate

[jpsim]

@dismory closing this issue. Please reopen if you feel there's still something we can improve here.