Fix pickle handling for SingletonGate class (#10871)

* Fix pickle handling for SingletonGate class

This commit fixes an oversight in #10314 where the handling of pickle
wasn't done correctly. Because the SingletonGate class defines __new__
and based on the parameters to the gate.__class__() call determines
whether we get a new mutable copy or a shared singleton immutable
instance we need special handling in pickle. By default pickle will call
__new__() without any arguments and then rely on __setstate__ to update
the state in the new object. This works fine if the original instance
was a singleton but in the case of mutable copies this will create a
singleton object instead of a mutable copy. To fix this a __reduce__
method is added to ensure arguments get passed to __new__ forcing a
mutable object to be created in deserialization. Then a __setstate__
method is defined to correctly update the mutable object post creation.

* Use __getnewargs_ex__ insetad of __reduce__ & __setstate__

This commit pivots the pickle interface methods used to implement
__getnewargs_ex__ instead of the combination of __reduce__ and
__setstate__. Realistically, all we need to do here is pass that
we have mutable arguments to new to trigger it to create a separate
object, the rest of pickle was working correctly. This makes the
interface being used for pickle a lot clearer.

* Improve assertion in immutable pickle test

qiskit/circuit/singleton_gate.py

@@ -80,6 +80,11 @@ def __init__(self, *args, _condition=None, **kwargs):
         super().__init__(*args, **kwargs)
         self._condition = _condition
 
+    def __getnewargs_ex__(self):
+        if not self.mutable:
+            return ((), {})
+        return ((self.label, self._condition, self.duration, self.unit), {})
+
     def c_if(self, classical, val):
         if not isinstance(classical, (ClassicalRegister, Clbit)):
             raise CircuitError("c_if must be used with a classical register or classical bit")

test/python/circuit/test_singleton_gate.py

@@ -18,6 +18,8 @@
 """
 
 import copy
+import io
+import pickle
 
 from qiskit.circuit.library import HGate, SXGate
 from qiskit.circuit import Clbit, QuantumCircuit, QuantumRegister, ClassicalRegister
@@ -251,3 +253,27 @@ def test_positional_label(self):
         label_gate = SXGate("I am a little label")
         self.assertIsNot(gate, label_gate)
         self.assertEqual(label_gate.label, "I am a little label")
+
+    def test_immutable_pickle(self):
+        gate = SXGate()
+        self.assertFalse(gate.mutable)
+        with io.BytesIO() as fd:
+            pickle.dump(gate, fd)
+            fd.seek(0)
+            copied = pickle.load(fd)
+        self.assertFalse(copied.mutable)
+        self.assertIs(copied, gate)
+
+    def test_mutable_pickle(self):
+        gate = SXGate()
+        clbit = Clbit()
+        condition_gate = gate.c_if(clbit, 0)
+        self.assertIsNot(gate, condition_gate)
+        self.assertEqual(condition_gate.condition, (clbit, 0))
+        self.assertTrue(condition_gate.mutable)
+        with io.BytesIO() as fd:
+            pickle.dump(condition_gate, fd)
+            fd.seek(0)
+            copied = pickle.load(fd)
+        self.assertEqual(copied, condition_gate)
+        self.assertTrue(copied.mutable)

test/python/compiler/test_transpiler.py

@@ -2173,6 +2173,20 @@ def run(self, dag):
             added_cal = qc_test.calibrations["sx"][((0,), ())]
             self.assertEqual(added_cal, ref_cal)
 
+    @data(0, 1, 2, 3)
+    def test_parallel_singleton_conditional_gate(self, opt_level):
+        """Test that singleton mutable instance doesn't lose state in parallel."""
+        backend = FakeNairobiV2()
+        circ = QuantumCircuit(2, 1)
+        circ.h(0)
+        circ.measure(0, circ.clbits[0])
+        circ.z(1).c_if(circ.clbits[0], 1)
+        res = transpile(
+            [circ, circ], backend, optimization_level=opt_level, seed_transpiler=123456769
+        )
+        self.assertTrue(res[0].data[-1].operation.mutable)
+        self.assertEqual(res[0].data[-1].operation.condition, (res[0].clbits[0], 1))
+
     @data(0, 1, 2, 3)
     def test_backendv2_and_basis_gates(self, opt_level):
         """Test transpile() with BackendV2 and basis_gates set."""