使用librdkafka高级消费者接口退出时rd_kafka_destroy卡住问题

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#librdkafka #kafka #rd_kafka_destroy #卡住 #阻塞

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在使用librdkafka高级消费者接口时，遇到rd_kafka_destroy函数调用时卡住的问题。经过调试发现，问题出在一个循环条件判断上，由于消息对象的引用计数不满足释放条件，导致循环无法结束。原因是消息解析异常分支中，没有正确调用rd_kafka_message_destroy释放消息对象，从而引发内存泄露，影响了程序退出。修复此问题后，程序运行正常。

近期在使用librdkafka消费者接口时遇到一个问题：当消息消费完成或用户主动退出时经常卡住，gdb attach上去看了一下是调用rd_kafka_destroy时一直阻塞：
- 剩余3个线程，其中两个在pthread_join，另一个当前堆栈顶层函数是poll
- 查阅代码，并打印了poll的参数，该调用是有超时时间的。也就是说不会一直阻塞，问题应该出在其他地方。

(gdb) set pagination off
(gdb) info thr
  Id   Target Id         Frame 
  3    Thread 0x7f1f506ba700 (LWP 24296) "rdk:main" 0x0000003074208705 in pthread_join (threadid=139772448966400, thread_return=0x7f1f506b5d58) at pthread_join.c:92
  2    Thread 0x7f1f4f2b8700 (LWP 24299) "rdk:broker0" 0x0000003073ee1bfd in poll () at ../sysdeps/unix/syscall-template.S:81
* 1    Thread 0x7f1f514ef780 (LWP 23787) "mpp_test" 0x0000003074208705 in pthread_join (threadid=139772469946112, thread_return=0x7fff0d9410a8) at pthread_join.c:92

(gdb) thr apply all bt

Thread 3 (Thread 0x7f1f506ba700 (LWP 24296)):
Python Exception <type 'exceptions.AttributeError'> 'module' object has no attribute 'Command': 
#0  0x0000003074208705 in pthread_join (threadid=139772448966400, thread_return=0x7f1f506b5d58) at pthread_join.c:92
#1  0x0000000000475f28 in thrd_join (thr=139772448966400, res=0x0) at tinycthread.c:749
#2  0x000000000040c199 in rd_kafka_destroy_internal (rk=0x1b6c280) at rdkafka.c:850
#3  0x000000000040e7ac in rd_kafka_thread_main (arg=0x1b6c280) at rdkafka.c:1284
#4  0x0000000000475db1 in _thrd_wrapper_function (aArg=0x1b6d350) at tinycthread.c:624
#5  0x0000003074207213 in start_thread (arg=0x7f1f506ba700) at pthread_create.c:309
#6  0x0000003073eeb65d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Thread 2 (Thread 0x7f1f4f2b8700 (LWP 24299)):
Python Exception <type 'exceptions.AttributeError'> 'module' object has no attribute 'Command': 
#0  0x0000003073ee1bfd in poll () at ../sysdeps/unix/syscall-template.S:81
#1  0x000000000043d969 in rd_kafka_transport_poll (rktrans=0x7f1f40000ab0, tmout=1000) at rdkafka_transport.c:1538
#2  0x000000000043d24c in rd_kafka_transport_io_serve (rktrans=0x7f1f40000ab0, timeout_ms=1000) at rdkafka_transport.c:1397
#3  0x000000000042081d in rd_kafka_broker_serve (rkb=0x1b6de40, abs_timeout=435806751453) at rdkafka_broker.c:2293
#4  0x0000000000425e63 in rd_kafka_broker_consumer_serve (rkb=0x1b6de40) at rdkafka_broker.c:3199
#5  0x0000000000426334 in rd_kafka_broker_thread_main (arg=0x1b6de40) at rdkafka_broker.c:3311
#6  0x0000000000475db1 in _thrd_wrapper_function (aArg=0x1b4c370) at tinycthread.c:624
#7  0x0000003074207213 in start_thread (arg=0x7f1f4f2b8700) at pthread_create.c:309
#8  0x0000003073eeb65d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Thread 1 (Thread 0x7f1f514ef780 (LWP 23787)):
Python Exception <type 'exceptions.AttributeError'> 'module' object has no attribute 'Command': 
#0  0x0000003074208705 in pthread_join (threadid=139772469946112, thread_return=0x7fff0d9410a8) at pthread_join.c:92
#1  0x0000000000475f28 in thrd_join (thr=139772469946112, res=0x0) at tinycthread.c:749
#2  0x000000000040bc97 in rd_kafka_destroy_app (rk=0x1b6c280, blocking=1) at rdkafka.c:736
#3  0x000000000040bd18 in rd_kafka_destroy (rk=0x1b6c280) at rdkafka.c:749
#4  0x0000000000407718 in closeKafkaConsumerConnection (kafka_cxt=0x1b4c010) at test.c:618
#5  0x0000000000408e02 in main (argc=12, argv=0x7fff0d9412e8) at test.c:1259

根据线程2的调用堆栈查阅代码,看到rd_kafka_broker_consumer_serve 函数中有一个循环。于是在循环条件处设断点，然后continue继续执行，果然进入断点：

static void rd_kafka_broker_consumer_serve (rd_kafka_broker_t *rkb) {
    rd_kafka_assert(rkb->rkb_rk, thrd_is_current(rkb->rkb_thread));
    rd_kafka_broker_lock(rkb);

    while (!rd_kafka_broker_terminating(rkb) &&
           rkb->rkb_state == RD_KAFKA_BROKER_STATE_UP) {
        rd_ts_t now;
                rd_ts_t min_backoff;

        rd_kafka_broker_unlock(rkb);
        ……

        rd_kafka_broker_serve(rkb,
                now + (rkb->rkb_blocking_max_ms * 1000));

        rd_kafka_broker_lock(rkb);
    }
    ……
}

打印循环控制变量: 其中rd_kafka_broker_terminating(rkb)是一个宏，其展开来定义如下：(rd_refcnt_get(&(rkb)->rkb_refcnt) <= 1，也就是判断(rkb)的引用计数是否<=1。个人理解<=1说明只有当前这个指针还在引用它，没有其他指针指向这个内存，可以安全释放。

Breakpoint 4, rd_kafka_broker_consumer_serve (rkb=0x1b6de40) at rdkafka_broker.c:3159
3159               rkb->rkb_state == RD_KAFKA_BROKER_STATE_UP) {
(gdb) p rkb->rkb_state
$3 = RD_KAFKA_BROKER_STATE_UP
(gdb) p rd_refcnt_get(&(rkb)->rkb_refcnt)
$4 = 16

很显然这里是因为(rkb)->rkb_refcnt这个引用计数不满足<=1的条件。常识判断应该是有什么对象没有释放或者销毁，以下是退出关闭环节的主要代码：仔细检查了几遍，似乎并没有什么问题。

    err = rd_kafka_consumer_close(rk);
    if (err)
        log(WARNING, "Failed to close consumer: %s\n",  rd_kafka_err2str(err));
    else
        log(WARNING, "Consumer closed\n");

    rd_kafka_destroy(rk);
    rd_kafka_topic_partition_list_destroy(tpl);

    /* Let background threads clean up and terminate cleanly. */
    int run = 5;
    while (run-- > 0 && rd_kafka_wait_destroyed(1000) == -1)
        log(WARNING, "Waiting for librdkafka to decommission\n");
    if (run <= 0)
        rd_kafka_dump(stdout, rk);

作为对比，用librdkafka自带的demo测试了几次，发现没有这个问题。二者主要区别在于demo收到消息后仅增加统计计数然后就销毁。而自己的程序根据业务需要对消息做了解析，仔细检查了发现有一个解析出错的异常分支，当初测试时为了省事直接continue了，没有认真处理，也没有调用 rd_kafka_message_destroy 销毁消息对象。加上这一句后即一切正常。
总结一下 解析kafka消息时，有个别消息不满足业务规则进入异常分支，没有调用rd_kafka_message_destroy，造成内存泄露并导致rd_kafka_broker_t对象的引用计数无法释放,在退出时因不满足条件而一直循环。