[GitHub] [lucene-solr] probakowski opened a new pull request #931: LUCENE-9001: Fix race condition in SetOnce

[GitBox <gi...@apache.org>]

probakowski opened a new pull request #931: LUCENE-9001: Fix race condition in SetOnce
URL: https://github.com/apache/lucene-solr/pull/931
 
 
   # Description
   
   This change fixes race condition in SetOnce class that can cause code like below throw `NullPointerException`
   ```java
   SetOnce<String> setOnce = new SetOnce<>();
   new Thread(() -> setOnce.set("thread")).start();
   try{
       setOnce.set("main");
   } catch (SetOnce.AlreadySetException e){
       setOnce.get().hashCode(); //possible NPE!
   }
   ```
   It also adds `boolean trySet(T obj)` method that doesn't throw exception when something was set before, it returns false in that case.
   
   # Solution
   
   `AtomicBoolean` and `volatile T obj` was replaced with single `AtomicReference` that serves both purposes: checking if something was set and actually storing object. That way update is atomic - no race condition can occur.
   `Wrapper` is used to allow null object to be stored, the same way it was possible before.
   
   # Tests
   
   All tests from `TestSetOnce` pass, I've also added test for `trySet` method
   
   # Checklist
   
   Please review the following and check all that apply:
   
   - [x] I have reviewed the guidelines for [How to Contribute](https://wiki.apache.org/solr/HowToContribute) and my code conforms to the standards described there to the best of my ability.
   - [x] I have created a Jira issue and added the issue ID to my pull request title.
   - [x] I am authorized to contribute this code to the ASF and have removed any code I do not have a license to distribute.
   - [x] I have given Solr maintainers [access](https://help.github.com/en/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork) to contribute to my PR branch. (optional but recommended)
   - [x] I have developed this patch against the `master` branch.
   - [x] I have run `ant precommit` and the appropriate test suite.
   - [x] I have added tests for my changes.
   - [ ] I have added documentation for the [Ref Guide](https://github.com/apache/lucene-solr/tree/master/solr/solr-ref-guide) (for Solr changes only).
   

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