Issues with new model registration in NEST

[pnbabu]

Following #2990, the model registration and the semantics of ResetKernel() have changed. After incorporating these changes to NESTML models, the tests fail when there are multiple calls to the ResetKernel(). For example,

nest.ResetKernel()
nest.Install("nestmlmodule")

nrn = nest.Create(“neuron1”)
...
nest.Simulate(100.)
...
nest.ResetKernel()  # removes the neuron1 and neuron2 from the list of registered models
# nest.Install("nestmlmodule”)   # Error: Module 'nestmlmodule' is loaded already.
nrn = nest.Create(“neuron2”)  # Neuron cannot be created - Unknown neuron model

In the above example nestmlmodule is an external module consisting of models neuron1 and neuron2. According to the new semantics of ResetKernel(), a call to the function deregisters the models loaded from an external module. Due to this, the second call to ResetKernel() de-registers neuron1 and neuron2 which were previously installed from the nest.Install() call. Hence, the neuron2 cannot be created and the script fails.

A second call to nest.Install() after the second ResetKernel() also fails as the same module cannot be loaded again. This is a known error as the external modules are still wrapped as SLI modules and can't be unloaded/reloaded.

As a result of these issues, the NESTML master breaks with the latest NEST master. Therefore, for the new model registration to work with NESTML, the module unloading/reloading needs to be in place.

[terhorstd]

@heplesser, it seems the module is unloaded but the name stays registered. The second Install() should not fail.

[heplesser]

Yes, this is a problem I had overlooked. I am not sure if this is already solved in the "new pynest" branch so we could take the solution from there. The key point would be to remember which new-style modules we have loaded and call their init() method. One challenge is that right now, we only unload neuron and synapse models on ResetKernel() but not backends or connection rules, while the module init() registers them all, see e.g., https://github.com/nest/nest-extension-module/blob/d3812c192cbfd86e1d777bf65d2c39c77686784c/src/mymodule.cpp#L106-L128