Issue521701

I expected, as in CPython, that built-in lists be
thread-safe, in that each operation (.append, .remove)
be atomic. This is not the case -- the attached script
is a test case. I've also seen java.lang.NullException
or some such error in my real code.

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I don't think this is a bug in Jython.
The python docs specify nothing about thread safety of 
standard types and the threading module is explicitly 
recommended for inter thread communication.

CPython's central lock happens to enforce the thread
safety of it's implementation of standard types.

In Java the thread safety of standard types can be obtained
by using synchronized methods. In many JVM's synchronized
methods take more time to execute than non synchronized
ones, so this would slow down Jython.
Btw. the (internal) dictionaries used for namespaces are
thread safe in Jython.

Jim, check it out, a 7 year old list thread safety test. It's very basic 
but we should incorporate it =]

Sure, it's a good approach in terms of ensuring the structural integrity
of the list, and straightforward enough to add other methods with
similar guarantees.

Added modified version of suggested thread safety test case in r6452.