Don't block for the first job in a session (Qiskit#1170)

* don't block if backend not selected

* add reno

* add test

qiskit_ibm_runtime/session.py

@@ -23,6 +23,7 @@
 from .runtime_job import RuntimeJob
 from .utils.result_decoder import ResultDecoder
 from .ibm_backend import IBMBackend
+from .exceptions import RuntimeJobTimeoutError
 from .utils.default_session import set_cm_session
 from .utils.deprecation import deprecate_arguments
 
@@ -180,7 +181,10 @@ def run(
                 self._setup_lock.release()
 
         if self._backend is None:
-            self._backend = job.backend().name
+            try:
+                self._backend = job.backend(0).name
+            except RuntimeJobTimeoutError:
+                self._backend = None
 
         return job
 

releasenotes/notes/backend-blocking-job-70ebcf44855cbdfd.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed an issue where the first job in a cloud channel session without a backend passed 
+    in would block other jobs from starting. This would happen because the first job would
+    attempt to retrieve the backend from the job which required the job to be in a 
+    completed state. 
+

test/integration/test_session.py

@@ -101,6 +101,19 @@ def test_session_from_id(self, service):
         job = sampler.run(ReferenceCircuits.bell(), shots=400)
         self.assertEqual(session_id, job.session_id)
 
+    @run_integration_test
+    def test_session_no_backend(self, service):
+        """Test session without passing in backend."""
+        if self.dependencies.channel == "ibm_quantum":
+            self.skipTest("Not supported on ibm_quantum")
+        with Session(service) as session:
+            sampler = Sampler(session=session)
+            job1 = sampler.run(ReferenceCircuits.bell(), shots=400)
+            job2 = sampler.run(ReferenceCircuits.bell(), shots=400)
+        self.assertTrue(job1.backend())
+        self.assertTrue(job2.backend())
+        self.assertEqual(job1.backend().name, job2.backend().name)
+
 
 class TestBackendRunInSession(IBMIntegrationTestCase):
     """Integration tests for Backend.run in Session."""