3r3-31. On October 1? 201? our team released a new version of the React Native application. We are pleased and proud of it.
 
 
But what a horror: after a few hours, the number of crashes under Android suddenly increases.
 
 
 

1?000 crashes under Android
3r33450.
 
 
Our crash monitoring tool Sentry going crazy.
 
Native Navigation where for each screen native fragments under Android are used.
 
3r33395. We have updated react-native-svg . There were a few exceptions related to SVG components, but this is hardly the case.
 
 
At the moment, we can not reproduce the error, so the best strategy:
 
 
3r3186.  
3r33395. Roll back one of the two libraries. Roll it out for 10% of users, which is trivially done in the Play Store. Check with several users if the crash persists. Thus, we will confirm or disprove the hypothesis.
 
3r3191.
 
 
But how to choose a library for rollback? Of course, you can throw a coin, but is this the best option?
 
 
3r33476.  

Getting to the point of

  3r33333. public T acquire () {
if (mPoolSize> 0) {
final int lastPooledIndex = mPoolSize - 1;
T instance = (T) mPool[lastPooledIndex];
mPool[lastPooledIndex]= null;
mPoolSize--;
return instance;
}
return null;
}    3r33333.
 
Therefore, the only way to get a negative value is    mPoolSize    - This is to reduce it at    mPoolSize = 0    . But how is this possible with the condition    mPoolSize> 0    ?
 
 
Put a breakpoint in Android Studio and take a look at what happens when you start the application. I mean, the condition here is    if    This code should work fine!
 
 
  Finally, a revelation!  

 
3r33476. 3r3302. 3r3499.
 
See in 3r3481. DynamicFromMap    static link to    SimplePool    .
 
 

  3r33333. private static final Pools.SimplePool    sPool = new Pools.SimplePool  <>(10);  3r33333.
 
After several dozen clicks of the Play button with carefully placed breakpoints, we see that the streams are  [i]  mqt_native_modules    refer to functions    SimplePool.acquire    and 3r3481. SimplePool.release    using React Native to control the style properties of the React component (below property    width    component)
 
 
3r33333.
 
 
But they are also accessed by the mainstream    main    !
 
 
   
 
Above, we see that they are used to update the property    fill    in the main thread, as a rule, for the component    react-native-svg    ! And indeed, the library    react-native-svg    started using    DynamicFromMap    3r33357. only from the seventh version of    to improve the performance of native svg-animations.
 
 
And-and-and  function can be accessed from two threads, but    DynamicFromMap    does not use 3r3481. SimplePool    thread safe way. “Thread safe,” you say?
 
 
  Thread safety, a bit of theory  

 
In single-threaded jаvascript, developers usually do not need to deal with thread safety.
 
 
On the other hand, Java supports the concept of parallel or multi-threaded programs. Multiple threads can run within the same program and can potentially access the overall data structure, which sometimes leads to unexpected results.
 
 
To take a simple example is shown below the image that streams A and B in parallel:
 
 

•  
  3r33395. read integer;  
  3r33395. increase its value;  
  3r33395. return it.  
  

Correction