由于超时而返回dispatch_group_wait（）时，iOS 8崩溃

debugcn 发表于 Dev

斯克维茨

我有两个获取值的操作。我只关心两个值的总和。如果花费太长时间，我根本不会在乎它的价值。

因此，我认为对于GCD使用小组来说，这将是一件容易的事。不幸的是，下面的代码只能在iOS 9上正常工作。每次没有匹配的dispatch_group_enter()/调用时，dispatch_group_leave()我都会崩溃。

文档明确指出，我必须同时匹配这两个调用。但是，当我在on上使用超时时dispatch_group_wait()，就不可能有与Enter呼叫相同数量的请假呼叫。这就是指定超时的重点。

那么这是iOS 8中的已知错误吗？难道我做错了什么？我的最初问题是否还有另一种解决方案也可以在iOS 8上使用？

编辑：实际上，我们可以将其归结为：

var sync_group: dispatch_group_t = dispatch_group_create();
dispatch_group_enter(sync_group);
let maxWait = dispatch_time(DISPATCH_TIME_NOW, Int64(60 * NSEC_PER_SEC))
let result = dispatch_group_wait(sync_group, maxWait)

sync_group = dispatch_group_create();

在iOS 9上可以正常工作，但在最后一行的iOS 8上崩溃，因为无法释放旧的dispatch_group_t实例。任何简单的解决方法？

编辑2：原来它也在iOS 9.0上损坏。它只能像在iOS 9.1+中那样工作。

原始代码：

let qualityOfServiceClass = QOS_CLASS_BACKGROUND
let backgroundQueue = dispatch_get_global_queue(qualityOfServiceClass, 0)
dispatch_async(backgroundQueue, {

    /* We want this method to block until the network code succeeded. */
    let sync_group: dispatch_group_t = dispatch_group_create();

    /* Start async operation 1. */
    dispatch_group_enter(sync_group);
    self.someAsyncOperation1({(value: Int, finalValue: Bool) in
        if (finalValue) {
            valOp1 = value
            dispatch_group_leave(sync_group);
        }
    })

    /* Start async operation 2. */
    dispatch_group_enter(sync_group);
    self.someAsyncOperation2({(value: Int, finalValue: Bool) in
        if (finalValue) {
            valOp2 = value
            dispatch_group_leave(sync_group)
        }
    })

    /* Block current thread until all leaves were called. If it takes more then 60 sec we don't care and let go. */
    let maxWait = dispatch_time(DISPATCH_TIME_NOW, Int64(60 * NSEC_PER_SEC))
    let result = dispatch_group_wait(sync_group, maxWait)
    if (result > 0) {
       /* This will result in a crash when we leave the scope: SIGTRAP in dispatch_semaphore_dispose */            
       return
    }

    dispatch_async(dispatch_get_main_queue(), {
        let newValue = valOp1 + valOp2
        self.lastKnownNotificationCombinedCounter = newValue
        success(newValue)
    })
})

实际的崩溃循环是这样的：

Exception Type:  SIGTRAP
Exception Codes: #0 at 0x3958a2a4

Thread 2 Crashed:
0   libdispatch.dylib                    0x3958a2a4 _dispatch_semaphore_dispose$VARIANT$mp + 48
1   libdispatch.dylib                    0x3958b491 _dispatch_dispose$VARIANT$mp + 30
2   libdispatch.dylib                    0x3957ea8f -[OS_dispatch_object _xref_dispose] + 44
3   myApp                                0x00176a24 block_destroy_helper67 + 354
4   myApp                                0x00176ab8 0x2e000 + 1346232
5   myApp                                0x00178334 0x2e000 + 1352500
6   libsystem_blocks.dylib               0x395d3adb _Block_release + 216
7   Foundation                           0x2c4143b9 -[NSBlockOperation dealloc] + 58
8   libobjc.A.dylib                      0x39036d57 objc_object::sidetable_release(bool) + 164
9   libobjc.A.dylib                      0x390371a9 (anonymous namespace)::AutoreleasePoolPage::pop(void*) + 402
10  libdispatch.dylib                    0x39589423 _dispatch_root_queue_drain + 1176
11  libdispatch.dylib                    0x3958a1fb _dispatch_worker_thread3 + 104
12  libsystem_pthread.dylib              0x396fae25 _pthread_wqthread + 666
13  libsystem_pthread.dylib              0x396fab78 start_wqthread + 6

斯克维茨

我想出了以下解决方法：

private let MAX_TRIES = 20;

func dispatch_group_wait_ios8Safe(group: dispatch_group_t, _ timeout: dispatch_time_t) -> Int {

    if #available(iOS 9, *) {
        /* Just forward the call. */
        return dispatch_group_wait(group, timeout)
    } else {
        /* Forward the call to original function and store result. */
        let firstResult = dispatch_group_wait(group, timeout)
        var result = firstResult, tries = 0
        while(result > 0 && tries < MAX_TRIES) {
            dispatch_group_leave(group)
            result = dispatch_group_wait(group, DISPATCH_TIME_NOW)
            tries += 1
        }

        /* Return original result. */
        return firstResult
    }
}

因此，在有人提出更好的解决方案之前，我坚持这样做。

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