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在低内存条件下测试应用程序

是否必须检查什么回报的问题malloc是有争议的，并且总是引起激烈的讨论。



有人认为我们应该尝试处理各种运行时错误，包括。和OOM情况。其他人则认为，OOM几乎无能为力，最好让应用程序崩溃。在第二个人的一边，还有一个事实，即附加的OOM处理逻辑很难测试。而且，如果代码未经测试，则几乎肯定无法正常工作。



我完全同意，您不应实施不打算测试的错误处理逻辑。它几乎肯定不会改善任何东西，甚至更糟-破坏一切。



是否尝试在库/应用程序中处理OOM的情况存在争议，我们在这里不再赘述。作为本出版物的一部分，我只想分享我的经验，即如何测试用C / C ++编写的应用程序中处理OOM情况的已实现逻辑。讨论将围绕Linux和macOS操作系统进行。由于多种原因，将绕过Windows。



介绍



我们都希望OOM永远不会发生，但是在现实生活中，由于以下原因，这并非总是可能的：



  • RAM总是有限的。
  • 不总是启用SWAP。
  • 应用程序的行为并不总是令人满意，有时会尝试分配不切实际的大量内存，从而干扰自己和其他人。
  • 32位应用程序仍然存在。
  • 过量提交并非始终启用。
  • ulimit例如，可以限制内存消耗。
  • LD_PRELOAD .


, , , OOM . , , :



  • , - .
  • OOM . .
  • . , .


, , SQLite. , . . SQLite .



, , , . OOM Killer, , . , C++, .



1.



, OOM . my_malloc my_free malloc free.



my_free . my_realloc.



my_malloc malloc . my_malloc , NULL .



, :



  • 3rd party .
  • malloc . - strdup.
  • malloc’ , , .
  • C++ malloc free.


- .



2.



Linux LD_PRELOAD. . malloc. , malloc/realloc/free (weak). , macOS LD_PRELOAD, DYLD_INSERT_LIBRARIES.



, , LD_PRELOAD DYLD_INSERT_LIBRARIES malloc/realloc NULL .



, "" . , .



, "" , , . :



  • main. , .
  • Runtime macOS " ". , , , .
  • printf macOS SIGSEGV/SIGBUS.
  • , std::bad_alloc, . , , , OOM. std::terminate. .
  • std::thread std::terminate macOS.


UPDATE: Travis CI , macOS / Xcode , std::bad_alloc , std::thread std::terminate.



Overthrower. - malloc NULL. Overthrower - , .



3.



main



, main , main runtime . main, , .. - main.



, main . Overthrower, OOM . Overthrower , .



:



  • activateOverthrower
  • deactivateOverthrower


:



#ifdef __cplusplus
extern "C" {
#endif
void activateOverthrower() __attribute__((weak));
unsigned int deactivateOverthrower() __attribute__((weak));
#ifdef __cplusplus
}
#endif


.



Overthrower LD_PRELOAD, NULL, , .



, , :



int main(int argc, char** argv)
{
    activateOverthrower();
    // Some code we want to test ...
    deactivateOverthrower();
}


activateOverthrower/deactivateOverthrower , :



TEST(Foo, Bar)
{
    activateOverthrower();
    // Some code we want to test ...
    deactivateOverthrower();
}


, -, , Overthrower , :



#ifdef __cplusplus
extern "C" {
#endif
void pauseOverthrower(unsigned int duration) __attribute__((weak));
void resumeOverthrower() __attribute__((weak));
#ifdef __cplusplus
}
#endif


:



TEST(Foo, Bar)
{
    activateOverthrower();
    // Some code we want to test ...
    pauseOverthrower(0);
    // Some fragile code we can not fix ...
    resumeOverthrower();
    // Some code we want to test ...
    deactivateOverthrower();
}


Overthrower .





__cxa_allocate_exception, , , malloc, NULL. , Linux, malloc, __cxa_allocate_exception (emergency buffer), , . .



macOS , , , , std::bad_alloc, std::terminate.



UPDATE: , macOS / Xcode .



, , , __cxa_allocate_exception malloc. - Overthrower’ malloc. Overthrower malloc __cxa_allocate_exception.



, , , macOS __cxa_atexit, Linux dlerror. .



Overthrower , malloc free. Overthrower’ , activateOverthrower deactivateOverthrower , :



overthrower got deactivation signal.
overthrower will not fail allocations anymore.
overthrower has detected not freed memory blocks with following addresses:
0x0000000000dd1e70  -       2  -         128
0x0000000000dd1de0  -       1  -         128
0x0000000000dd1030  -       0  -         128
^^^^^^^^^^^^^^^^^^  |  ^^^^^^  |  ^^^^^^^^^^
      pointer       |  malloc  |  block size
                    |invocation|
                    |  number  |


Overthrower , , .



Overthrower’, , valgrind. , OOM. , , Overthrower . Overthrower , , deactivateOverthrower , stderr .





Overthrower 3 :



  • Random — rand() % duty_cycle == 0. duty_cycle, .
  • Step — (malloc_seq_num >= delay), delay .


<--- delay --->
--------------+
              |
              | All further allocations fail
              |
              +------------------------------


  • Pulse — (malloc_seq_num > delay && malloc_seq_num <= delay + duration), delay duration .


<--- delay --->
--------------+                +------------------------------
              |                |
              |                | All further allocations pass
              |                |
              +----------------+
              <--- duration --->


:



  • OVERTHROWER_STRATEGY
  • OVERTHROWER_SEED
  • OVERTHROWER_DUTY_CYCLE
  • OVERTHROWER_DELAY
  • OVERTHROWER_DURATION


activateOverthrower. , Overthrower , /dev/urandom.



README.md.





  • Overthrower malloc /.
  • .
  • Overthrower .
  • Overthrower .
  • Overthrower , .
  • Overthrower’ .
  • Overthrower Overthrower-aware . , .
  • Overthrower本身已在Ubuntu（自14.04起）和macOS（自Sierra（10.12）和Xcode 8.3起）上进行了测试。在测试过程中，Overthrower尝试掉线。
  • 如果系统中出现真正的OOM，则Overthrower会竭尽所能不摔倒。


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