Remove quasi-monitoring checkpoint file dfk.pkl

This file is saved alongside checkpoints, but stores an extremely
light summary of DFK state rather than of checkpoints: the DFK's
task count and run directory path.

That is information that falls into the space of monitoring, not
of checkpointing - so this PR removes that file, in favour of the
much richer monitoring database.

Author: benclifford

parsl/dataflow/dflow.py

@@ -1263,7 +1263,7 @@ def checkpoint(self, tasks: Optional[Sequence[TaskRecord]] = None) -> str:
         Returns:
             Checkpoint dir if checkpoints were written successfully.
             By default the checkpoints are written to the RUNDIR of the current
-            run under RUNDIR/checkpoints/{tasks.pkl, dfk.pkl}
+            run under RUNDIR/checkpoints/tasks.pkl
         """
         with self.checkpoint_lock:
             if tasks:
@@ -1273,18 +1273,11 @@ def checkpoint(self, tasks: Optional[Sequence[TaskRecord]] = None) -> str:
                 self.checkpointable_tasks = []
 
             checkpoint_dir = '{0}/checkpoint'.format(self.run_dir)
-            checkpoint_dfk = checkpoint_dir + '/dfk.pkl'
             checkpoint_tasks = checkpoint_dir + '/tasks.pkl'
 
             if not os.path.exists(checkpoint_dir):
                 os.makedirs(checkpoint_dir, exist_ok=True)
 
-            with open(checkpoint_dfk, 'wb') as f:
-                state = {'rundir': self.run_dir,
-                         'task_count': self.task_count
-                         }
-                pickle.dump(state, f)
-
             count = 0
 
             with open(checkpoint_tasks, 'ab') as f:

parsl/tests/test_checkpointing/test_python_checkpoint_1.py

@@ -27,8 +27,5 @@ def test_initial_checkpoint_write():
 
     cpt_dir = parsl.dfk().checkpoint()
 
-    cptpath = cpt_dir + '/dfk.pkl'
-    assert os.path.exists(cptpath), f"DFK checkpoint missing: {cptpath}"
-
     cptpath = cpt_dir + '/tasks.pkl'
     assert os.path.exists(cptpath), f"Tasks checkpoint missing: {cptpath}"