Concurrency fixes

[David Aspinall <da+pgtrac@…>]

Thread safety in the code needs to be carefully reviewed. Non-deterministic buggy behaviour happens quite often. I'm using this ticket to list places that need work.

[David Aspinall <da+pgtrac@…>]

Uses of wait() in the code need to be reviewed. Search for "wait(" or look for notes added in FIXMEs. I've fixed some but not all. Notes:

• wait() may spuriously wake-up (or wake-up on other conditions), so a specific condition should be checked in a loop
• notify() may have been sent before wait() was reached, so (unbounded) wait() should not be unconditional

Solution pattern:

synchronized(lock) {
    while (!condition) {
         lock.wait();
    }

where some other thread sets the condition to true before notify or notifyAll. Only use notify if you can be sure there is no more than one thread waiting on the lock.

For more, see the Java Concurrency Tutorial, including this page: ​http://java.sun.com/docs/books/tutorial/essential/concurrency/guardmeth.html

[David Aspinall <da+pgtrac@…>]

FindBugs is recommended for helping locate concurrency problems, including these patterns:

• IS inconsistent synchronisation policy
• MWN mis-matched wait
• Wa wait not in loop
• UW unconditional wait

See #65 for more on FindBugs.

In project properties select "multi-threaded correctness". At the moment we have 25 warnings in this category.

[David Aspinall]

Milestone PG-Eclipse-1.1.0 deleted