[jira] [Commented] (LUCENE-9001) Race condition in SetOnce

["Adrien Grand (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LUCENE-9001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16947428#comment-16947428 ] 

Adrien Grand commented on LUCENE-9001:
--------------------------------------

When SetOnce was initially designed, I suspect it wasn't envisioned that one would try to catch the exception since the goal was to signal a programming error. That said, your proposal helps make it less surprising so it's probably a good think to do anyway. I'll give others a couple days to comment in case they think we should rather keep the current way that SetOnce works.

Do you need the trySet semantics? I wonder because if I needed them, I'd probably go to AtomicReference directly instead of SetOnce, like we do in IndexWriter. So I'd rather keep SetOnce simple and not introduce trySet.

> Race condition in SetOnce
> -------------------------
>
>                 Key: LUCENE-9001
>                 URL: https://issues.apache.org/jira/browse/LUCENE-9001
>             Project: Lucene - Core
>          Issue Type: Bug
>            Reporter: Przemko Robakowski
>            Priority: Minor
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> There is race condition in SetOnce that can cause code below fail with NullPointerException:
> {code: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!
> }
> {code}
> This is caused by 2 separate write operations - 1 for set marker field and 1 for actual object. So it's possible that marker is already set to true (causing AlreadySetException on second write attempt) but object is still not set (causing NullPointerException).
> This can be avoided by using single AtomicReference instead to serve both purposes.



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