Fix map_blocks HLG layering

This fixes an issue with the HighLevelGraph noted in
pydata#3584, and exposed by a recent
change in Dask to do more HLG fusion.

xarray/core/parallel.py

@@ -7,6 +7,7 @@
 except ImportError:
     pass
 
+import collections
 import itertools
 import operator
 from typing import (
@@ -222,6 +223,7 @@ def _wrapper(func, obj, to_array, args, kwargs):
     indexes.update({k: template.indexes[k] for k in new_indexes})
 
     graph: Dict[Any, Any] = {}
+    new_layers = collections.defaultdict(dict)
     gname = "{}-{}".format(
         dask.utils.funcname(func), dask.base.tokenize(dataset, args, kwargs)
     )
@@ -310,10 +312,14 @@ def _wrapper(func, obj, to_array, args, kwargs):
                     # unchunked dimensions in the input have one chunk in the result
                     key += (0,)
 
-            graph[key] = (operator.getitem, from_wrapper, name)
+            new_layers[gname_l][key] = (operator.getitem, from_wrapper, name)
 
     graph = HighLevelGraph.from_collections(gname, graph, dependencies=[dataset])
 
+    for gname_l, layer in new_layers.items():
+        graph.dependencies[gname_l] = {gname}
+        graph.layers[gname_l] = layer
+
     result = Dataset(coords=indexes, attrs=template.attrs)
     for name, gname_l in var_key_map.items():
         dims = template[name].dims

xarray/tests/test_dask.py

@@ -1189,6 +1189,13 @@ def func(obj):
     assert_identical(expected.compute(), actual.compute())
 
 
+def test_map_blocks_hlg_layers():
+    ds = xr.Dataset({'x': (('y',), dask.array.ones(10, chunks=(5,)))})
+    mapped = ds.map_blocks(lambda x: x)
+
+    xr.testing.assert_equal(mapped, ds)  # does not work
+
+
 def test_make_meta(map_ds):
     from ..core.parallel import make_meta