Make PrimitiveJob serializable

[kt474]

Summary

In qiskit-ibm-runtime we want to add job methods to our QiskitRuntimeLocalService (Issue Qiskit/qiskit-ibm-runtime#1607). In order to save PrimitiveJob objects locally we need a way to serialize them. It's currently not serializable due to the use of ThreadPoolExecutor:

qiskit/qiskit/primitives/primitive_job.py

Lines 47 to 48 in 35d0954

	executor = ThreadPoolExecutor(max_workers=1) # pylint: disable=consider-using-with
	self._future = executor.submit(self._function, *self._args, **self._kwargs)

Credit to @t-imamichi

Details and comments

Also fixes #12787

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core
• @ajavadia
• @levbishop
• @t-imamichi

[coveralls]

Pull Request Test Coverage Report for Build 14136417982

Details

• 22 of 25 (88.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.03%) to 88.093%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/primitives/primitive_job.py	22	25	88.0%

Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/unitary_synthesis.rs	1	94.79%
crates/qasm2/src/expr.rs	1	94.23%
crates/qasm2/src/lex.rs	3	92.73%

Totals
Change from base Build 14129997569:	0.03%
Covered Lines:	72775
Relevant Lines:	82612

---

💛 - Coveralls

[t-imamichi]

It looks good overall. Could you add a reno to describe that both DataBin and PrimitiveJob become serializable?

[t-imamichi]

LGTM. Could you ask someone else to review it too just in case?

[raynelfss]

The removal of the unimplemented __setattr__ method makes sense to me, but I have a question about the _prepare_dump method.

[raynelfss]

When is this function called during serialization? I can't see anywhere where the method is called in the PR itself?

[kt474]

It would be called by the user if they want to serialize PrimitiveJob - example here

[raynelfss]

Would it be better to make this happen in one of the serialization methods that python has built-in? I believe this could be executed on one of the methods, avoiding that function call. You could include the lines here and override one of the reserved methods for serialization: __reduce__, __getstate__, __getnewargs__, depending on your needs.

Note: This might require you to also implement a __setstate__ method.

[t-imamichi]

Thank you for your suggestion. I think the following should work (to remove non-pickable object _future) by replacing _prepare_dump with the following.

    def __getstate__(self):
        _ = self.result()
        _ = self.status()
        state = self.__dict__.copy()
        state["_future"] = None
        return state

    def __setstate__(self, state):
        self.__dict__.update(state)
        self._future = None

[t-imamichi]

I made a patch to this PR kt474#1.

[raynelfss]

This looks much better! Thank you for these additions @kt474, @t-imamichi 🚀