ios - 当 dispatch_group_wait() 由于超时返回时在 iOS 8 上崩溃

Question

我有两个获取值的操作。我只关心两个值的总和。如果花费的时间太长，我根本不关心值(value)。

所以我认为使用组对 GCD 来说是一件容易的事。不幸的是，下面的代码只能在 iOS 9 上正常工作。每次我没有匹配的 dispatch_group_enter()/dispatch_group_leave() 调用时，我都会崩溃。

文档明确指出我必须匹配这两个调用。但是当我在 dispatch_group_wait() 上使用超时时，不可能有与 enter 调用相同数量的 leave 调用；这就是指定超时的全部要点。

那么这是 iOS 8 中的一个已知错误吗？难道我做错了什么？对于我最初遇到的问题，还有其他适用于 iOS 8 的解决方案吗？

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 上按预期工作，但在最后一行的 iOS 8 上确实崩溃，因为无法释放旧的 dispatch_group_t 实例。有任何简单的解决方法吗？

编辑 2:原来它在 iOS 9.0 上也坏了。它只能在 iOS 9.1+ 中正常工作。

---

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

Answer 1

我想到了这个解决方法:

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，我会坚持这一点。