[jira] [Assigned] (JENA-522) DatasetGraphWithLock claims to support multiple readers but appears not to

["Andy Seaborne (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/JENA-522?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Andy Seaborne reassigned JENA-522:
----------------------------------

    Assignee: Andy Seaborne
    
> DatasetGraphWithLock claims to support multiple readers but appears not to
> --------------------------------------------------------------------------
>
>                 Key: JENA-522
>                 URL: https://issues.apache.org/jira/browse/JENA-522
>             Project: Apache Jena
>          Issue Type: Bug
>          Components: ARQ
>    Affects Versions: Jena 2.10.2
>            Reporter: Rob Vesse
>            Assignee: Andy Seaborne
>              Labels: concurrency, lock, multi-threading
>         Attachments: JENA-522.patch
>
>
> This bug originates out of a StackOverflow question pertaining to jena-text
> http://stackoverflow.com/q/18385738/107591
> However upon investigation the culprit appears to be {{DatasetGraphWithLock}}, reproducing my analysis from my SO answer here:
> {quote}
> The issue is that the dataset with inference implicitly uses ARQ's standard in-memory Dataset implementation and this does not support transactions.
> However text datasets which correspond to {{DatasetGraphText}} internally (and in your stack trace) requires the wrapped dataset to support transactions and where they do not wraps them with {{DatasetGraphWithLock}}. It is this that appears to be encountering the problem with the lock, the documentation states that this should support multiple readers but having followed the logic of the code I'm not sure that it actually allows this.
> {quote}
> I put together the following test case which illustrates the issue:
> {noformat}
>     @Test
>     public synchronized void dsg_with_lock_concurrency_02() throws InterruptedException, ExecutionException, TimeoutException {
>         ExecutorService executor = Executors.newCachedThreadPool();
>         try {
>             final DatasetGraphWithLock dsg = new DatasetGraphWithLock(DatasetGraphFactory.createMem());
>             Callable<Boolean> callable = new Callable<Boolean>() {
>                 @Override
>                 public Boolean call() throws Exception {
>                     // Get a read lock
>                     dsg.begin(ReadWrite.READ);
>                     // Hold the lock for a short time
>                     try {
>                         Thread.sleep(500);
>                     } catch (InterruptedException e) {
>                         // Ignore error
>                     }
>                     // Release the lock
>                     dsg.commit();
>                     return true;
>                 }
>             };
>             // Run the callable a bunch of times
>             List<Future<Boolean>> futures = new ArrayList<Future<Boolean>>();
>             for (int i = 0; i < 100; i++) {
>                 futures.add(executor.submit(callable));
>             }
>             // Check all the futures come back OK
>             for (Future<Boolean> f : futures) {
>                 Assert.assertTrue(f.get(3, TimeUnit.SECONDS));
>             }
>         } finally {
>             executor.shutdownNow();
>         }
>     }
> {noformat}
> So the problem appears to be that DatasetGraphWithLock claims multi-reader support but in reality does not allow this.

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