NOXSolver: clear pending_exception

[peterrum]

FYI @vovannikov

[peterrum]

Needed to be able to reuse the solver.

[bangerth]

This doesn't quite seem right. If there is still an exception pending, we should assert. Can you create a testcase that shows how this can happen?

[peterrum]

@bangerth I don't think the call_and_possibly_capture_exception() logic is correct in NOXSolver. Let's assume that the linear solver fails and NOX does not stop but decreases the step size!?

[peterrum]

@bangerth Any feedback. The NOXSolver on master is not working in our application after the update.

[bangerth]

@peterrum Apologies for beging slow to get to this -- I spent the weekend from Friday afternoon to this morning in the mountains.

The logic is that call_and_possibly_capture_exception() saves any exceptions for later use because, at least as far as I understand, NOX does not have the concept of "recoverable exceptions". At least in other libraries, if there are no recoverable exceptions, as soon as a user callback returns an error code, the library in question stops any operations and returns to user code (in our case, to deal.II code) where we would note that an exception has been saved, and re-throw it. That happens here:
https://github.com/dealii/dealii/blob/master/include/deal.II/trilinos/nox.templates.h#L1132-L1216

The logic here implicitly assumes that if NOX returns success (NOX::StatusTest::Converged), then there should not be a pending exception. That could also usefully be asserted, in line 1146 just before the return statement.

So, if NOX does have the ability to recover from exceptions, then that would be useful to know -- ideally with a testcase that demonstrates that, and by adding the assertions I mention above.

[bangerth]

@peterrum Ping?

[bangerth]

@peterrum Ping again?

[peterrum]

Ping?

Ping again?

@vovannikov Is working on a simplified test that triggers the problem.

[vovannikov]

The nullification of the pending_exception won't resolve the issue we observed in our code. The problem comes from a bit different part of NOX, see #16329.