Update TNG tests and add another dont_cache check for loading

tests/tests.yaml

@@ -1,5 +1,5 @@
 answer_tests:
-  local_arepo_002:
+  local_arepo_003:
     - yt/frontends/arepo/tests/test_outputs.py
 
   local_artio_001:

yt/frontends/arepo/tests/test_outputs.py

@@ -1,3 +1,5 @@
+import os
+import tempfile
 from collections import OrderedDict
 from yt.testing import requires_file, ParticleSelectionComparison
 from yt.utilities.answer_testing.framework import \
@@ -8,6 +10,7 @@
 
 bullet_h5 = "ArepoBullet/snapshot_150.hdf5"
 tng59_h5 = "TNGHalo/halo_59.hdf5"
+_tng59_bbox = [[45135.0, 51343.0], [51844.0, 56184.0], [60555.0, 63451.0]]
 
 
 @requires_file(bullet_h5)
@@ -61,15 +64,25 @@ def test_tng_hdf5():
     ]
 )
 
-
 @requires_ds(tng59_h5)
 def test_arepo_tng59():
-    ds = data_dir_load(tng59_h5)
+    ds = data_dir_load(tng59_h5, kwargs = {'bounding_box': _tng59_bbox})
     for test in sph_answer(ds, 'halo_59', 10107142,
                            tng59_fields):
         test_arepo_tng59.__name__ = test.description
         yield test
 
+@requires_ds(tng59_h5)
+def test_index_override():
+    # This tests that we can supply an index_filename, and that when we do, it
+    # doesn't get written if our bounding_box is overwritten.
+    tmpfd, tmpname = tempfile.mkstemp(suffix=".ewah")
+    os.close(tmpfd)
+    ds = data_dir_load(tng59_h5, kwargs = {'index_filename': tmpname,
+                                           'bounding_box': _tng59_bbox})
+    ds.index
+    assert len(open(tmpname, "r").read()) == 0
+
 @requires_file(tng59_h5)
 def test_arepo_tng59_selection():
     ds = data_dir_load(tng59_h5)

yt/geometry/particle_geometry_handler.py

@@ -3,6 +3,7 @@
 import numpy as np
 import os
 import weakref
+import struct
 
 from yt.funcs import \
     get_pbar, \
@@ -128,13 +129,16 @@ def _initialize_index(self):
         else:
             fname = ds.index_filename
 
+        dont_load = dont_cache and not hasattr(ds, 'index_filename')
         try:
+            if dont_load:
+                raise OSError
             rflag = self.regions.load_bitmasks(fname)
             rflag = self.regions.check_bitmasks()
             self._initialize_frontend_specific()
             if rflag == 0:
                 raise OSError
-        except OSError:
+        except (OSError, struct.error):
             self.regions.reset_bitmasks()
             self._initialize_coarse_index()
             self._initialize_refined_index()