[I] 父进程启动bthread后，fork出子进程后子进程踩内存 (brpc)

[via GitHub]

Triol opened a new issue, #2731:
URL: https://github.com/apache/brpc/issues/2731

   **Describe the bug (描述bug)**
   1、父进程启动bthread
   2、fork出子进程
   3、子进程的Sampler踩了内存
   
   **To Reproduce (复现方法)**
   编译执行 使用valgrind可见
   ```C++
   #include <sys/stat.h>
   #include <sys/unistd.h>
   #include <brpc/server.h>
   #include <bthread/bthread.h>
   #include <iostream>
   #include <thread>
   #include <gflags/gflags.h>
   using namespace std;
   
   void child_proc();
   void parent_proc();
   void *empty_th(void *) { std::cout << "do nothing but start bthread 
worker\n"; }
   int main(int argc, char *argv[]) {
     gflags::ParseCommandLineFlags(&argc, &argv, true);
   
     // preload: start bthread
     bthread_t tid;
     bthread_start_background(&tid, nullptr, empty_th, nullptr);
     bthread_join(tid, nullptr);
     std::cout << "start to fork\n";
     int ret = fork();
     if (ret == 0) {
       child_proc();
     } else if (ret > 0) {
        parent_proc();
     } else {
       assert(false && "cannot fork");
     }
     return 0;
   }
   
   void child_proc() {
     std::cout << "start child, pid: " << getpid() << "\n";
     for (int i = 0; i < 40; i++) {
       std::thread th([&] {
           char mem[40960];
           memset(mem, 0x7f, sizeof(mem));
           while(true) sleep(3);
       });
       th.detach();
     }
   }
   
   
   void parent_proc() {
     std::cout << "start parent, pid: " << getpid() << "\n";
     while(true) sleep(3);
   }
   
   /*
    * g++ -std=c++11 ./test.cc -lpthread -lbrpc -lgflags
    * valgrind ./a.out
    */
   ```
   ```
   10:36:53 docker /home valgrind ./a.out 
   ==13029== Memcheck, a memory error detector
   ==13029== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
   ==13029== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
   ==13029== Command: ./a.out
   ==13029== 
   do nothing but start bthread workerstart parent, pid: 13029
   do nothing but start bthread workerstart child, pid: 13041
   ==13041== Thread 2 bvar_sampler:
   ==13041== Jump to the invalid address stated on the next line
   ==13041==    at 0x7F7F7F7F7F7F7F7F: ???
   ==13041==    by 0x5269B10: bvar::detail::SamplerCollector::run() 
(sampler.cpp:173)
   ==13041==    by 0x526A7EF: 
bvar::detail::SamplerCollector::sampling_thread(void*) (sampler.cpp:113)
   ==13041==    by 0x4E44863: start_thread (pthread_create.c:477)
   ==13041==    by 0x7102C1E: clone (clone.S:95)
   ==13041==  Address 0x7f7f7f7f7f7f7f7f is not stack'd, malloc'd or (recently) 
free'd
   ==13041== 
   ```
   
   **Expected behavior (期望行为)**
   子进程能正常运作
   
   **Versions (各种版本)**
   OS:CentOS Linux release 7.8.2003 (Core)
   Compiler: gcc (GCC) 7.5.0
   brpc: master
   protobuf: protobuf-2.2.1
   
   **Additional context/screenshots (更多上下文/截图)**
   已经可以确认是timer thread线程中那几个开在栈上的bvar
   在fork之后 因线程的栈空间被强制回收，没有走bvar的析构流程，导致其sampler指向了无效地址
   我构造的demo里面开辟大量的栈空间可以稳定将其内存踩坏
   


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