Fix split barriers leaking during disjoint layout processing

qiskit/transpiler/passes/layout/disjoint_utils.py

@@ -107,7 +107,10 @@ def split_barriers(dag: DAGCircuit):
         num_qubits = len(node.qargs)
         if num_qubits == 1:
             continue
-        barrier_uuid = uuid.uuid4()
+        if node.op.label:
+            barrier_uuid = f"{node.op.label}_uuid={uuid.uuid4()}"
+        else:
+            barrier_uuid = f"_none_uuid={uuid.uuid4()}"
         split_dag = DAGCircuit()
         split_dag.add_qubits([Qubit() for _ in range(num_qubits)])
         for i in range(num_qubits):
@@ -124,10 +127,13 @@ def combine_barriers(dag: DAGCircuit, retain_uuid: bool = True):
     qubit_indices = {bit: index for index, bit in enumerate(dag.qubits)}
     uuid_map: dict[uuid.UUID, DAGOpNode] = {}
     for node in dag.op_nodes(Barrier):
-        if isinstance(node.op.label, uuid.UUID):
-            barrier_uuid = node.op.label
+        if node.op.label:
+            if "_uuid=" in node.op.label:
+                barrier_uuid = node.op.label
+            else:
+                continue
             if barrier_uuid in uuid_map:
-                other_node = uuid_map[node.op.label]
+                other_node = uuid_map[barrier_uuid]
                 num_qubits = len(other_node.qargs) + len(node.qargs)
                 new_op = Barrier(num_qubits, label=barrier_uuid)
                 new_node = dag.replace_block_with_op([node, other_node], new_op, qubit_indices)
@@ -136,8 +142,11 @@ def combine_barriers(dag: DAGCircuit, retain_uuid: bool = True):
                 uuid_map[barrier_uuid] = node
     if not retain_uuid:
         for node in dag.op_nodes(Barrier):
-            if isinstance(node.op.label, uuid.UUID):
+            if isinstance(node.op.label, str) and node.op.label.startswith("_none_uuid="):
                 node.op.label = None
+            elif isinstance(node.op.label, str) and "_uuid=" in node.op.label:
+                original_label = "_uuid=".join(node.op.label.split("_uuid=")[:-1])
+                node.op.label = original_label
 
 
 def require_layout_isolated_to_component(
@@ -202,4 +211,7 @@ def separate_dag(dag: DAGCircuit) -> List[DAGCircuit]:
         new_dag.remove_clbits(*idle_clbits)
         combine_barriers(new_dag)
         decomposed_dags.append(new_dag)
+    # Reverse split barriers on input dag to avoid leaking out internal transformations as
+    # part of splitting
+    combine_barriers(dag, retain_uuid=False)
     return decomposed_dags

releasenotes/notes/fix-leaking-split-barrier-7c143d6b13b96ced.yaml

@@ -0,0 +1,12 @@
+---
+fixes:
+  - |
+    Fixed an issue when using :func:`.transpile` or running a preset pass
+    manager (such as generated by :func:`.generate_preset_pass_manager`) when
+    targetting a backend that has disjoint connectivity adding extra barriers
+    to the output :class:`.QuantumCircuit`. In some cases several
+    single qubit :class:`.Barrier` directives would be included in the output
+    circuit right before any final measurements in the circuit. This was
+    internal state generated by the internal processing for disjoint
+    connectivity that was incorrectly being added into the output circuit.
+    Fixed `#11649 <https://github.com/Qiskit/qiskit/issues/11649>`__

test/python/compiler/test_transpiler.py

@@ -3260,6 +3260,19 @@ def test_transpile_target_with_qubits_without_ops_circuit_too_large_disconnected
         with self.assertRaises(TranspilerError):
             transpile(qc, target=target, optimization_level=opt_level)
 
+    @data(0, 1, 2, 3)
+    def test_barrier_no_leak_disjoint_connectivity(self, opt_level):
+        """Test that we don't leak an internal labelled barrier from disjoint layout processing."""
+        cmap = CouplingMap([(0, 1), (1, 2), (3, 4)])
+        qc = QuantumCircuit(cmap.size(), cmap.size())
+        qc.cx(0, 1)
+        qc.cx(1, 2)
+        qc.cx(2, 0)
+        qc.cx(3, 4)
+        qc.measure(qc.qubits, qc.clbits)
+        out = transpile(qc, coupling_map=cmap, optimization_level=opt_level)
+        self.assertNotIn("barrier", out.count_ops())
+
     @data(0, 1, 2, 3)
     def test_transpile_does_not_affect_backend_coupling(self, opt_level):
         """Test that transpiliation of a circuit does not mutate the `CouplingMap` stored by a V2