[ASTERIXDB-1220] Race condition in AObjectPrinter singleton instance

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Component/s: *DB - AsterixDB, EXT - External data
• Assignee: Till Westmann
• Reporter: Wail Y. Alkowaileet
• Created: 09/Dec/15 14:06
• Updated: 26/May/16 20:03
• Resolved: 26/May/16 20:03

Description

Hi,

The bug appears clearly when two queries race to print their output using AObjectPrinter singleton in the same DataFormat.

            public void print(byte[] b, int start, int l, PrintStream ps) throws AlgebricksException {
                try {
                    listAccessor.set(b, start, l);
                    arg.first = ps;
                    listAccessor.accept(printVisitor, arg);
                } catch (Exception ioe) {
                    throw new AlgebricksException(ioe);
                }
            }

Since there's only one instance of AObjectPrinter, this will be a critical section and things will lose track (such as ps and listAccessor).

I think new printer should be instantiated for each query.

To produce the problem:
Running the query in two clients:

//As in runtimets/queries/open-closed/query-issue55.3.query.aql
for $x in [[1,3],[4,5,2],[-1,-3,0],["a"]]
return $x

interchange the execution sequence in the two threads such as:
Thread 1 run: listAccessor.set(b, start, l);
Thread 2 run: listAccessor.set(b, start, l);
Thread 1 resume running.
Thread 2 resume running.