Thread safety for PDU classes

[brutzman]

Found an interesting problem in DisThreadedNetworkInterface.selfTest() method where ESPDU changes to marking values were getting clobbered during fast sends/updates. This was shown by print statements showing the same marking value in rapid progression (rather than marking 1, marking 2, etc.).

Avoided this problem (for now) by creating a new PDU each time. I think this anomaly indicates that the library isn't fully thread safe, e.g. an ESPDU instance might get modified while still being sent (or reported upon). Some more thought will be needed for making all PDU methods thread safe.

Log of test results found in DisThreadedNetworkInterfaceSelfTestLog.txt

[brutzman]

Problem definition:

• User invokes sending a PDU, which is then temporarily blocked while synchronized send buffers are processed
• User modifies that object before the send occurs
• PDU actually being sent does not match the PDU originally sent

Any performance reduction from synchronization is miniscule in comparison to possible loss of object integrity. Indeed synchronization blocking only occurs if it is needed. If we inadvertently create a deadlock, that will certainly be discovered as part of our extensive version-controlled unit tests and example applications - further fine-tuning can then occur.

Next steps:

1. Done. For DisThreadedNetworkInterface.sendPDU() method, make a deep copy of the PDU prior to adding it to the buffered list for threaded sending. Java garbage collection should dispose of these extra objects reasonably well.

• This is currently failing on FixedAndVariableDatumRoundTripTest

3. Done. For JavaGenerator creation of each of the concrete PDU classes, add keyword synchronized to
   • all set accessor methods
   • all clear, copy and equal* accessor methods
   • all marshall and unmarshall accessor methods
   • all toString accessor methods

[terry-norbraten]

FixedAndVariableDatumRoundTripTest is now fixed (Issue #26)

[brutzman]

@terry-norbraten has also fixed some unnecessary duplicate creation of DISThreadedNetworkInterface instance in PduRecorder, further helping with threading.

We are looking pretty good, and so closing.