[JENA-522] DatasetGraphWithLock claims to support multiple readers but appears not to

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Jena 2.11.0
• Fix Version/s: Jena 2.11.0
• Component/s: ARQ
• Labels: concurrency lock multi-threading
• Assignee: Andy Seaborne
• Reporter: Rob Vesse
• Created: 22/Aug/13 23:31
• Updated: 06/Sep/13 08:51
• Resolved: 23/Aug/13 18:31

Description

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:

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.

I put together the following test case which illustrates the issue:

    @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();
        }
    }

So the problem appears to be that DatasetGraphWithLock claims multi-reader support but in reality does not allow this.

Issue Links

• relates to: JENA-523 DatasetGraphWithLock throws exception on abort but this can occur in Fuseki