Extract MPI code from execute_task()

The `execute_task()` function is used by multiple executors, but the MPI
code is specific to HTEX.

parsl/executors/high_throughput/process_worker_pool.py

@@ -590,28 +590,28 @@ def update_resource_spec_env_vars(mpi_launcher: str, resource_spec: Dict, node_i
         os.environ[key] = prefix_table[key]
 
 
-def execute_task(bufs, mpi_launcher: Optional[str] = None):
-    """Deserialize the buffer and execute the task.
+def _init_mpi_env(mpi_launcher: str, resource_spec: Dict):
+    node_list = resource_spec.get("MPI_NODELIST")
+    if node_list is None:
+        return
+    nodes_for_task = node_list.split(',')
+    logger.info(f"Launching task on provisioned nodes: {nodes_for_task}")
+    update_resource_spec_env_vars(mpi_launcher=mpi_launcher, resource_spec=resource_spec, node_info=nodes_for_task)
+
 
+def execute_task(bufs: bytes):
+    """Deserialize the buffer and execute the task.
     Returns the result or throws exception.
     """
     user_ns = locals()
     user_ns.update({'__builtins__': __builtins__})
 
-    f, args, kwargs, resource_spec = unpack_res_spec_apply_message(bufs, user_ns, copy=False)
+    f, args, kwargs, resource_spec = unpack_res_spec_apply_message(bufs, copy=False)
 
     for varname in resource_spec:
         envname = "PARSL_" + str(varname).upper()
         os.environ[envname] = str(resource_spec[varname])
 
-    if resource_spec.get("MPI_NODELIST"):
-        worker_id = os.environ['PARSL_WORKER_RANK']
-        nodes_for_task = resource_spec["MPI_NODELIST"].split(',')
-        logger.info(f"Launching task on provisioned nodes: {nodes_for_task}")
-        assert mpi_launcher
-        update_resource_spec_env_vars(mpi_launcher,
-                                      resource_spec=resource_spec,
-                                      node_info=nodes_for_task)
     # We might need to look into callability of the function from itself
     # since we change it's name in the new namespace
     prefix = "parsl_"
@@ -786,8 +786,10 @@ def manager_is_alive():
             ready_worker_count.value -= 1
         worker_enqueued = False
 
+        _init_mpi_env(mpi_launcher=mpi_launcher, resource_spec=req["resource_spec"])
+
         try:
-            result = execute_task(req['buffer'], mpi_launcher=mpi_launcher)
+            result = execute_task(req['buffer'])
             serialized_result = serialize(result, buffer_threshold=1000000)
         except Exception as e:
             logger.info('Caught an exception: {}'.format(e))