html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/pull/3584#issuecomment-573099009,https://api.github.com/repos/pydata/xarray/issues/3584,573099009,MDEyOklzc3VlQ29tbWVudDU3MzA5OTAwOQ==,2448579,2020-01-10T16:11:01Z,2020-01-10T16:11:01Z,MEMBER,Thanks @crusaderky ,"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,530657789
https://github.com/pydata/xarray/pull/3584#issuecomment-572197513,https://api.github.com/repos/pydata/xarray/issues/3584,572197513,MDEyOklzc3VlQ29tbWVudDU3MjE5NzUxMw==,2448579,2020-01-08T18:31:08Z,2020-01-08T18:31:08Z,MEMBER,gentle ping @crusaderky ,"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,530657789
https://github.com/pydata/xarray/pull/3584#issuecomment-561232232,https://api.github.com/repos/pydata/xarray/issues/3584,561232232,MDEyOklzc3VlQ29tbWVudDU2MTIzMjIzMg==,2448579,2019-12-03T15:57:16Z,2019-12-03T15:57:16Z,MEMBER,"The tests fail on `dask == 2.8.1` with this interesting bug. Here's a reproducible example.

``` python
import dask
import xarray as xr

ds = xr.Dataset({'x': (('y',), dask.array.ones(10, chunks=(3,)))})
mapped = ds.map_blocks(lambda x: x)
mapped.compute()  # works

xr.testing.assert_equal(mapped, ds)  # does not work
xr.testing.assert_equal(mapped, ds.compute()) # works
xr.testing.assert_equal(mapped.compute(), ds)  # works
xr.testing.assert_equal(mapped.compute(), ds.compute())  # works
```

The traceback is
```
~/miniconda3/envs/dcpy/lib/python3.7/site-packages/dask/array/optimization.py in optimize(dsk, keys, fuse_keys, fast_functions, inline_functions_fast_functions, rename_fused_keys, **kwargs)
     41     if isinstance(dsk, HighLevelGraph):
     42         dsk = optimize_blockwise(dsk, keys=keys)
---> 43         dsk = fuse_roots(dsk, keys=keys)
     44 
     45     # Low level task optimizations

~/miniconda3/envs/dcpy/lib/python3.7/site-packages/dask/blockwise.py in fuse_roots(graph, keys)
    819             isinstance(layer, Blockwise)
    820             and len(deps) > 1
--> 821             and not any(dependencies[dep] for dep in deps)  # no need to fuse if 0 or 1
    822             and all(len(dependents[dep]) == 1 for dep in deps)
    823         ):

~/miniconda3/envs/dcpy/lib/python3.7/site-packages/dask/blockwise.py in <genexpr>(.0)
    819             isinstance(layer, Blockwise)
    820             and len(deps) > 1
--> 821             and not any(dependencies[dep] for dep in deps)  # no need to fuse if 0 or 1
    822             and all(len(dependents[dep]) == 1 for dep in deps)
    823         ):

KeyError: 'lambda-6720ab0e3639d5c63fc06dfc66a3ce47-x'
```

This key is not in `dependencies`. From https://github.com/dask/dask/blob/67fb5363009c583c175cb577776a4f2f4e811410/dask/blockwise.py#L816-L826 

``` python
    for name, layer in graph.layers.items():
        deps = graph.dependencies[name]
        if (
            isinstance(layer, Blockwise)
            and len(deps) > 1
            and not any(dependencies[dep] for dep in deps)  # no need to fuse if 0 or 1
            and all(len(dependents[dep]) == 1 for dep in deps)
        ):
            new = toolz.merge(layer, *[layers[dep] for dep in deps])
            new, _ = fuse(new, keys, ave_width=len(deps))
```

I'm not sure whether this is a bug in `fuse_roots`, `HighLevelGraph.from_collections` or in how `map_blocks` calls `HighLevelGraph.from_collections` here:
https://github.com/pydata/xarray/blob/69c85b85a1ef5d13b6ca51b6b655a1f719cc5abf/xarray/core/parallel.py#L315

cc @mrocklin 
","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,530657789