Fix writing index variables for parallel batches (#123)

* Fix writing index variables for parallel batches

Issue when running batches in parallel using a multi-process scheduler:
model state is not returned by execution of dask delayed functions, so
the state was lost.

Fix by calling store.write_index_vars for each model run. It is not a
big deal to write those datasets many times (should be same values
across all simulations).

* clean up

xsimlab/drivers.py

@@ -328,8 +328,7 @@ def run_model(self):
         self.store.write_input_xr_dataset()
 
         if self.batch_dim is None:
-            model = self.model
-            self._run_one_model(self.dataset, model, parallel=self.parallel)
+            self._run_one_model(self.dataset, self.model, parallel=self.parallel)
 
         else:
             ds_gby_batch = self.dataset.groupby(self.batch_dim)
@@ -348,8 +347,6 @@ def run_model(self):
             if self.parallel:
                 dask.compute(futures, scheduler=self.scheduler)
 
-        self.store.write_index_vars(model=model)
-
     def _run_one_model(self, dataset, model, batch=-1, parallel=False):
         """Run one simulation.
 
@@ -406,3 +403,5 @@ def _run_one_model(self, dataset, model, batch=-1, parallel=False):
         self.store.write_output_vars(batch, -1, model=model)
 
         model.execute("finalize", rt_context, **execute_kwargs)
+
+        self.store.write_index_vars(model=model)

xsimlab/tests/test_xr_accessor.py

@@ -477,6 +477,10 @@ def test_run_batch_dim(self, dims, data, clock, parallel, scheduler):
         class P:
             in_var = xs.variable(dims=[(), "x"])
             out_var = xs.variable(dims=[(), "x"], intent="out")
+            idx_var = xs.index(dims="x")
+
+            def initialize(self):
+                self.idx_var = [0, 1]
 
             def run_step(self):
                 self.out_var = self.in_var * 2