Question

在我的dispatch_group代码中，我使用dispatch_group_wait来超时一组Web服务调用。问题，似乎我需要计算调用dispatch_group_enter的次数，然后调用相同数量的剩余dispatch_group_leave，如果某些Web服务调用永远不会返回，导致dispatch_group_enter与dispatch_group_leave的数量不相等。为什么呢？

如果在超时发生并且触发dispatch_group_wait的情况下我不这样做，我已经看到崩溃发生了。一旦确保dispatch_group_enter / dispatch_group_leave与其通话计数匹配，崩溃就会消失。

dispatch_group_t group = dispatch_group_create();

for (...) {

  dispatch_group_enter(group);

  // Make backend call and upon return call dispatch_group_leave
  [self backendCallWithCompletionHandler:^(id results) {
     dispatch_group_leave(group);
  }

}

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            dispatch_group_wait(self.group, dispatch_time(DISPATCH_TIME_NOW, 
(int64_t)(self.timeoutInterval * NSEC_PER_SEC)));
            dispatch_async(dispatch_get_main_queue(), ^{
                //execute stuff on main thread
                // call remaining dispatch_group_leave here if dispatch_group_enter and dispatch_group_leave count don't match?
            });
    });

Answer 1

非常有趣。我在OS X 10.9.4上使用Xcode 5.1.1测试了以下简单代码。

/* test.m */
#include <dispatch/dispatch.h>

int main()
{
    {
        dispatch_group_t group = dispatch_group_create();
        dispatch_group_enter(group);
        //dispatch_group_leave(group);
    }
    return 0;
}

使用ARC编译Objective-C代码。

$ clang -O0 -g -fobjc-arc a.m

执行程序。

$ ./a.out
illegal hardware instruction ./a.out

程序崩溃了。如果调用dispatch_group_leave（group）（进入和离开都是平衡的），则根本没有例外。使用lldb获取崩溃的详细信息。

$ lldb a.out
Current executable set to 'a.out' (x86_64).
(lldb) run
Process 73482 launched: 'a.out' (x86_64)
Process 73482 stopped
* thread #1: tid = 0x3808a1, 0x00007fff87125287 libdispatch.dylib`_dispatch_semaphore_dispose + 55, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    frame #0: 0x00007fff87125287 libdispatch.dylib`_dispatch_semaphore_dispose + 55
libdispatch.dylib`_dispatch_semaphore_dispose + 55:
-> 0x7fff87125287:  ud2    
   0x7fff87125289:  leaq   0x103b9(%rip), %rcx       ; "BUG IN LIBDISPATCH: flawed group/semaphore logic"
   0x7fff87125290:  movq   %rcx, -0x104f9b1f(%rip)   ; gCRAnnotations + 8
   0x7fff87125297:  ud2    
(lldb)

异常发生在_dispatch_semaphore_dispose。我们可以幸运地在Apple OpenSource网站上阅读libdispatch源代码。看看http://opensource.apple.com/source/libdispatch/libdispatch-339.92.1/src/semaphore.c

void
_dispatch_semaphore_dispose(dispatch_object_t dou)
{
    dispatch_semaphore_t dsema = dou._dsema;

    if (dsema->dsema_value < dsema->dsema_orig) {
        DISPATCH_CLIENT_CRASH(
                "Semaphore/group object deallocated while in use");
    }

根据此源代码，dispatch_group使用dispatch_semaphore。并且dispatch_semaphore强制呼叫信号/等待必须平衡。

dispatch_semaphore_create手册：

CAVEATS
     Unbalanced dispatch semaphores cannot be released.  For a given sema-
     phore, calls to dispatch_semaphore_signal() and dispatch_semaphore_wait()
     must be balanced before dispatch_release() is called on it.

因此，调用dispatch_group_enter和dispatch_group_leave也必须保持平衡。

为什么有必要调用dispatch_group_leave与dispatch_group_enter相同的次数？

1 个答案: