coreneuron_modtests::datareturn_py test fails on GPU

[olupton]

Describe the issue
The coreneuron_modtests::datareturn_py CTest test in NEURON fails when a GPU-enabled CoreNEURON build is used. Note that, because of this issue, the test is disabled by default and to see the problem you will have to re-enable it (remove CONFLICTS gpu in NEURON's test/CMakeLists.txt)

To Reproduce
Build NEURON against CoreNEURON with GPU support enabled, run

ctest -R coreneuron_modtests::datareturn_py

from the build directory.

Expected behavior
Success.

Logs
The main error is

FATAL ERROR: variable in data clause is partially present on the device: name=(unknown)

full log: datareturn_py.log.

System (please complete the following information)

• OS: BB5
• Compiler: NVHPC 21.2
• Version: master
• Backend: GPU

Additional context
Identified in the context of neuronsimulator/nrn#1122.

This test calls CoreNEURON multiple times in a single process, and the failure occurs on the second call. The failure is thought to be related to CoreNEURON not correctly freeing all its device-side data.

cc: @pramodk @iomaganaris

[olupton]

Closed by neuronsimulator/nrn#1132.