Chronicle-Queue message length changes issue when 'padToCacheAlign' == true

Question

The following code fails on the second assert statement at the 20th iteration - note I'm just recreating my code that caused the issue; the count is not relevant, rather the number of bytes written is.

    SingleChronicleQueue writer = SingleChronicleQueueBuilder.binary("/tmp/broken").build();

    ExcerptAppender excerptAppender = writer.acquireAppender();

    try(DocumentContext dc = excerptAppender.writingDocument())
    {
        dc.wire().bytes().writeSkip(36);
    }

    for(int i = 0; i < 20; i++)
    {
        try (DocumentContext dc = excerptAppender.writingDocument())
        {
            dc.wire().bytes().writeSkip(14);
        }
    }

    SingleChronicleQueue reader = SingleChronicleQueueBuilder.binary("/tmp/broken").build();

    ExcerptTailer tailer = reader.createTailer();

    try(DocumentContext dc = tailer.readingDocument())
    {
        assert dc.isPresent() && dc.wire().bytes().readRemaining() == 36;
    }

    for(int i = 0; i < 20; i++)
    {
        try(DocumentContext dc = tailer.readingDocument())
        {
            //Fails on the 20th read .. with 16 bytes being returned
            assert dc.isPresent() && dc.wire().bytes().readRemaining() == 14;
        }
    }

The issue appears to be in the SingleChronicleQueueExcerpts class where padding is added to the message to cache align it to 64 bytes. I wasn't anticipating having to add my own message lengths to my writes, but it doesn't seem avoidable if chronicle-queue is not padding it's own header to the cache line boundary.

thanks in advance

Answer 1

The problem this is trying to work around is that CAS operations are not actually atomic across cache lines !! On ARM it just gets a SIGBUS but on x64 it just works 99.999% of the time.

This was discovered by us after the format was being used by customers so we ended up with this work around. The next major version should fix this. I suggest adding a stop bit encoded length to the start which will only be a byte or two.