METSRW tries to call property for a non-existent object

[peterVG]

METSRW tries to call a property for a non-existent object (inside a function that creates a new AMDSec by parsing the root element).

See https://github.com/artefactual-labs/mets-reader-writer/blob/master/metsrw/metadata.py#L94-L103

The error that is raised:

[2020-07-15 15:57:58,502: ERROR/ForkPoolWorker-4] Task AIPscan.Aggregator.tasks.get_mets[a24ce0ce-6e25-40d5-b117-ce5435944741] raised unexpected: AttributeError("'NoneType' object has no attribute 'tag'")
Traceback (most recent call last):
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/celery/app/trace.py", line 412, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/Users/peter/Development/AIPscan/flask_celery.py", line 18, in __call__
    return TaskBase.__call__(self, *args, **kwargs)
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/celery/app/trace.py", line 704, in __protected_call__
    return self.run(*args, **kwargs)
  File "/Users/peter/Development/AIPscan/AIPscan/Aggregator/tasks.py", line 271, in get_mets
    mets = metsrw.METSDocument.fromfile(downloadFile)
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/metsrw/mets.py", line 593, in fromfile
    return cls.fromtree(etree.parse(path, parser=parser))
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/metsrw/mets.py", line 617, in fromtree
    mets._parse_tree(tree)
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/metsrw/mets.py", line 543, in _parse_tree
    tree, structMap, normative_parent_elem=normative_struct_map
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/metsrw/mets.py", line 367, in _parse_tree_structmap
    tree, elem, normative_parent_elem=normative_elem
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/metsrw/mets.py", line 371, in _parse_tree_structmap
    self._add_amdsecs_to_fs_entry(elem.get("ADMID"), fs_entry, tree)
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/metsrw/mets.py", line 515, in _add_amdsecs_to_fs_entry
    amdsec = metadata.AMDSec.parse(amdsec_elem)
  File "/Users/peter/Development/AIPscan/venv/lib/python3.7/site-packages/metsrw/metadata.py", line 94, in parse
    if root.tag != utils.lxmlns("mets") + "amdSec":
AttributeError: 'NoneType' object has no attribute 'tag'

The work around for now is to patch METSRW/metadata.py with the following code:

    @classmethod
    def parse(cls, root):
        """
        Create a new AMDSec by parsing root.

        :param root: Element or ElementTree to be parsed into an object.
        """
        if root:
            if root.tag != utils.lxmlns("mets") + "amdSec":
                raise exceptions.ParseError(
                    "AMDSec can only parse amdSec elements with METS namespace."
                )
            section_id = root.get("ID")
            subsections = []
            for child in root:
                subsection = SubSection.parse(child)
                subsections.append(subsection)
            return cls(section_id, subsections)
        else:
            return

[ross-spencer]

Related to archivematica/Issues#1129

[ross-spencer]

Given the related issue and this ticket here: #47 I feel we're doing enough to handle this in AIPscan now, and we can close this in favor of reporting the AIP errors summary. We will need to track metsrw and AIP changes upstream but this one might just resolve itself once Archivematica outputs the METS correctly.