DAGCircuit.substitute_node_with_dag mutates input_dag if node has a condition

[kdk]

Information

• Qiskit Terra version: 63449c4

What is the current behavior?

From https://travis-ci.com/github/Qiskit/qiskit-terra/jobs/361028025#L6256 , DAGCircuit.substitute_node_with_dag(node, input_dag) will mutate input_dag to add the condition from node. This will lead to issues of input_dag is later re-used in a non-conditional context.

Steps to reproduce the problem

>>> import qiskit as qk
>>> qc = qk.QuantumCircuit(2,1)
>>> qc.cx(0,1).c_if(qc.cregs[0], 1)
>>> qc.cx(1,0)
>>> print(qc)
            ┌───┐
q_0: ───■───┤ X ├
      ┌─┴─┐ └─┬─┘
q_1: ─┤ X ├───■──
      └─┬─┘      
     ┌──┴──┐     
c: 1/╡ = 1 ╞═════
     └─────┘     
>>> qk.transpile(qc, basis_gates=['u3', 'cz'])

---------------------------------------------------------------------------
DAGCircuitError                           Traceback (most recent call last)
<ipython-input-2-ee46705662ef> in <module>
      4 
      5 
----> 6 qk.transpile(qc, basis_gates=['u3', 'cz']).draw()

~/q/qiskit-terra/qiskit/compiler/transpile.py in transpile(circuits, backend, basis_gates, coupling_map, backend_properties, initial_layout, layout_method, routing_method, translation_method, seed_transpiler, optimization_level, pass_manager, callback, output_name)
    217     # Transpile circuits in parallel
    218     circuits = parallel_map(_transpile_circuit, list(zip(circuits, transpile_args)),
--> 219                             task_args=(faulty_qubits_map, backend))
    220 
    221     if len(circuits) == 1:

~/q/qiskit-terra/qiskit/tools/parallel.py in parallel_map(task, values, task_args, task_kwargs, num_processes)
    106         return []
    107     if len(values) == 1:
--> 108         return [task(values[0], *task_args, **task_kwargs)]
    109 
    110     Publisher().publish("terra.parallel.start", len(values))

~/q/qiskit-terra/qiskit/compiler/transpile.py in _transpile_circuit(circuit_config_tuple, faulty_qubits_map, backend)
    320 
    321     result = pass_manager.run(circuit, callback=transpile_config['callback'],
--> 322                               output_name=transpile_config['output_name'])
    323 
    324     if faulty_qubits_map:

~/q/qiskit-terra/qiskit/transpiler/passmanager.py in run(self, circuits, output_name, callback)
    212         """
    213         if isinstance(circuits, QuantumCircuit):
--> 214             return self._run_single_circuit(circuits, output_name, callback)
    215         elif len(circuits) == 1:
    216             return self._run_single_circuit(circuits[0], output_name, callback)

~/q/qiskit-terra/qiskit/transpiler/passmanager.py in _run_single_circuit(self, circuit, output_name, callback)
    275         if callback is None and self.callback:  # TODO to remove with __init__(callback)
    276             callback = self.callback
--> 277         result = running_passmanager.run(circuit, output_name=output_name, callback=callback)
    278         self.property_set = running_passmanager.property_set
    279         return result

~/q/qiskit-terra/qiskit/transpiler/runningpassmanager.py in run(***failed resolving arguments***)
    113         for passset in self.working_list:
    114             for pass_ in passset:
--> 115                 dag = self._do_pass(pass_, dag, passset.options)
    116 
    117         circuit = dag_to_circuit(dag)

~/q/qiskit-terra/qiskit/transpiler/runningpassmanager.py in _do_pass(self, pass_, dag, options)
    143         # Run the pass itself, if not already run
    144         if pass_ not in self.valid_passes:
--> 145             dag = self._run_this_pass(pass_, dag)
    146 
    147             # update the valid_passes property

~/q/qiskit-terra/qiskit/transpiler/runningpassmanager.py in _run_this_pass(self, pass_, dag)
    155             # Measure time if we have a callback or logging set
    156             start_time = time()
--> 157             new_dag = pass_.run(dag)
    158             end_time = time()
    159             run_time = end_time - start_time

~/q/qiskit-terra/qiskit/transpiler/passes/basis/basis_translator.py in run(self, dag)
    153                     dag.substitute_node(node, bound_target_dag.op_nodes()[0].op, inplace=True)
    154                 else:
--> 155                     dag.substitute_node_with_dag(node, bound_target_dag)
    156             else:
    157                 raise TranspilerError('BasisTranslator did not map {}.'.format(node.name))

~/q/qiskit-terra/qiskit/dagcircuit/dagcircuit.py in substitute_node_with_dag(self, node, input_dag, wires)
    810             wires = input_dag.wires
    811 
--> 812         self._check_wires_list(wires, node)
    813 
    814         # Create a proxy wire_map to identify fragments and duplicates

~/q/qiskit-terra/qiskit/dagcircuit/dagcircuit.py in _check_wires_list(self, wires, node)
    686         if len(wires) != wire_tot:
    687             raise DAGCircuitError("expected %d wires, got %d"
--> 688                                   % (wire_tot, len(wires)))
    689 
    690     def _make_pred_succ_maps(self, node):

DAGCircuitError: 'expected 2 wires, got 3'

What is the expected behavior?

Suggested solutions

Around https://github.com/Qiskit/qiskit-terra/blob/700beea/qiskit/dagcircuit/dagcircuit.py#L796 , we should make a copy of input_dag if it needs to be modified prior to insertion.