Transpiler is crashed when transpiling 2q gates on non-connected qubit pairs

[nkanazawa1989]

Information

• Qiskit Terra version: master
• Python version: 3.7
• Operating system: 18.04.3 LTS

What is the current behavior?

Transpiler cannot handle coupling map correctly.

Steps to reproduce the problem

from qiskit import transpile, QuantumCircuit
from qiskit.test.mock import FakeMontreal
from qiskit import quantum_info as qi

backend = FakeMontreal()

qc = QuantumCircuit(4)
qc.append(qi.random_unitary(4).to_instruction(), [0,1])
transpile(qc, backend) # this is ok

qc = QuantumCircuit(4)
qc.append(qi.random_unitary(4).to_instruction(), [0,3])
transpile(qc, backend) # see error code

...
~/qiskit/qiskit-terra/qiskit/transpiler/passes/routing/stochastic_swap.py in _layer_permutation(self, layer_partition, layout, qubit_subset, coupling, trials)
    195                                                                      int_gates, cdist2,
    196                                                                      cdist, edges, scale,
--> 197                                                                      self.rng)
    198 
    199             logger.debug("layer_permutation: final distance for this trial = %s", dist)

~/qiskit/qiskit-terra/qiskit/transpiler/passes/routing/cython/stochastic_swap/swap_trial.pyx in qiskit.transpiler.passes.routing.cython.stochastic_swap.swap_trial.swap_trial()

~/qiskit/qiskit-terra/qiskit/transpiler/passes/routing/cython/stochastic_swap/swap_trial.cpython-37m-x86_64-linux-gnu.so in View.MemoryView.memoryview_cwrapper()

~/qiskit/qiskit-terra/qiskit/transpiler/passes/routing/cython/stochastic_swap/swap_trial.cpython-37m-x86_64-linux-gnu.so in View.MemoryView.memoryview.__cinit__()

ValueError: buffer source array is read-only

What is the expected behavior?

Suggested solutions

[nkanazawa1989]

perhaps we need to update unittest too

[nkanazawa1989]

sorry I solved this by myself. I just pulled the master but actually needed to compile cython again.

[mtreinish]

Yeah, #5183 updated the cython code (it was necessary because the distance matrix is now returned directly from retworkx but is read only because it lives in the rust heap and not python's so the matrix parameter in cython is const now), if you have a local build from source whenever the cython source changes you'll have to rebuild it. Fortunately the cython code doesn't generally change to frequently so this shouldn't come up too often.