Error in read_record_data when processing 6-hourly UM data

I'm getting an error when trying to collapse UM files to go from sub-daily to daily frequency. The data are 6-hourly maximum air temperature (max across all timesteps).

I'm new to cf-python so unsure about where/how to start with debugging this, and read_record_data doesn't tell me much about the source of the error:

umfile: error condition detected in routine read_record_data_core_dbl
umfile: error condition detected in routine read_record_data
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/decorators.py", line 71, in precede_with_kwarg_deprecation_check
    operation_method_result = operation_method(self, *args, **kwargs)
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cfdm/decorators.py", line 171, in verbose_override_wrapper
    return method_with_verbose_kwarg(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/field.py", line 6931, in collapse
    f = f._collapse_grouped(
        ^^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cfdm/decorators.py", line 171, in verbose_override_wrapper
    return method_with_verbose_kwarg(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/field.py", line 8628, in _collapse_grouped
    f = self.concatenate(fl, axis=iaxis, cull_graph=True)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/field.py", line 3189, in concatenate
    new_data = Data.concatenate(
               ^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/data/data.py", line 4068, in concatenate
    d.cull_graph()
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/data/data.py", line 11470, in cull_graph
    dsk, _ = cull(dx.dask, dx.__dask_keys__())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/dask/optimization.py", line 61, in cull
    dependencies_k = get_dependencies(dsk, k, as_list=True)  # fuse needs lists
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/dask/core.py", line 306, in get_dependencies
    return keys_in_tasks(dsk, [arg], as_list=as_list)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/dask/core.py", line 194, in keys_in_tasks
    if w in keys:
       ^^^^^^^^^
  File "<frozen _collections_abc>", line 780, in __contains__
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/dask/highlevelgraph.py", line 517, in __getitem__
    return self.layers[key[0]][key]  # type: ignore
                       ~~~^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/data/array/umarray.py", line 190, in __getitem__
    array = rec.get_data().reshape(self.shape)
            ^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/umread_lib/umfile.py", line 435, in get_data
    return c.read_record_data(
           ^^^^^^^^^^^^^^^^^^^
  File "/apps/jasmin/jaspy/miniforge_envs/jaspy3.11/mf3-23.11.0-0/envs/jaspy3.11-mf3-23.11.0-0-v20240815/lib/python3.11/site-packages/cf/umread_lib/cInterface.py", line 591, in read_record_data
    raise umfile.UMFileException("Error reading record data")
cf.umread_lib.umfile.UMFileException: Error reading record data

Any help would be appreciated!

Start by reproducing the 6-hourly UM collapse to daily frequency described in the issue. Read cf/field.py around collapse and concatenate, then follow the traceback through cf/data/array/umarray.py, cf/umread_lib/umfile.py, and cf/umread_lib/cInterface.py. Done means identifying the cause of the record-reading failure and confirming that the collapse completes successfully.