test (#535)

* test

* Trigger MPI CI

* Use runscript with openmpi.

* Install pytest-xdist.

* Use coverage directly in mpi tests.

* Revert trigger comment.

* Remove placeholder diff.md [skip ci]

---------

Co-authored-by: Shuli Shu <08cnbj@gmail.com>

.coveragerc

@@ -5,6 +5,9 @@ omit =
     tests/*
     mpitests/*
 
+[coverage:run]
+parallel = true
+
 [report]
 # Regexes for lines to exclude from consideration
 exclude_lines =

.github/workflows/tests_linux_x86_mpi_gpu.yml

@@ -236,11 +236,15 @@ jobs:
           CMAKE_ARGS="-DCMAKE_C_COMPILER=mpicc -DCMAKE_CXX_COMPILER=mpicxx -DENABLE_MPI=ON -DCMAKE_CUDA_COMPILER=/usr/local/cuda/bin/nvcc -DCMAKE_CUDA_ARCHITECTURES=${{ env.CI_CUDA_ARCH }} -DPython_EXECUTABLE=${{ steps.python_path.outputs.python }}" \
           PL_BACKEND=lightning_gpu python -m pip install -e . --verbose
 
+      # There are issues running py-cov with MPI. A solution is to use coverage as reported
+      # [here](https://github.com/pytest-dev/pytest-cov/issues/237#issuecomment-544824228)
       - name: Run unit tests for MPI-enabled lightning.gpu device
         run: |
           source /etc/profile.d/modules.sh && module use /opt/modules/ && module load ${{ matrix.mpilib }}
-          PL_DEVICE=lightning.gpu /opt/mpi/${{ matrix.mpilib }}/bin/mpirun -np 2 python -m pytest ./mpitests $COVERAGE_FLAGS
-          mv coverage.xml coverage-${{ github.job }}-lightning_gpu_${{ matrix.mpilib }}-main.xml
+          PL_DEVICE=lightning.gpu /opt/mpi/${{ matrix.mpilib }}/bin/mpirun -np 2 \
+          coverage run --rcfile=.coveragerc --source=pennylane_lightning -p -m mpi4py -m pytest ./mpitests --tb=native
+          coverage combine
+          coverage xml -o coverage-${{ github.job }}-lightning_gpu_${{ matrix.mpilib }}-main.xml
           # PL_DEVICE=lightning.gpu /opt/mpi/${{ matrix.mpilib }}/bin/mpirun --oversubscribe -n 4 pytest -s -x mpitests/test_device.py -k test_create_device $COVERAGE_FLAGS
 
       - name: Upload code coverage results