Malformed WFS Capabilities causes cryptic stack trace

[JakobMiksch]

I discovered that opening this WFS https://www.geoproxy.geoportal-th.de/geoproxy/services/STRNETZ_wfs with owslib leads to a crash when using version 1.0.0 or 2.0.0. With version 1.1.0 it works. Examples:

version 1.0.0

from owslib.wfs import WebFeatureService
wfs = WebFeatureService(url="https://www.geoproxy.geoportal-th.de/geoproxy/services/STRNETZ_wfs", version="1.0.0")

Causes this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/dev/.local/lib/python3.11/site-packages/owslib/wfs.py", line 48, in WebFeatureService
    return wfs100.WebFeatureService_1_0_0(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dev/.local/lib/python3.11/site-packages/owslib/feature/wfs100.py", line 112, in __init__
    self._buildMetadata(parse_remote_metadata)
  File "/home/dev/.local/lib/python3.11/site-packages/owslib/feature/wfs100.py", line 121, in _buildMetadata
    self.identification = ServiceIdentification(serviceelem, self.version)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dev/.local/lib/python3.11/site-packages/owslib/feature/wfs100.py", line 310, in __init__
    self.type = testXMLValue(self._root.find(nspath("Name")))
                             ^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'find'

version 2.0.0

from owslib.wfs import WebFeatureService
wfs = WebFeatureService(url="https://www.geoproxy.geoportal-th.de/geoproxy/services/STRNETZ_wfs", version="2.0.0")

Causes this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/dev/.local/lib/python3.11/site-packages/owslib/wfs.py", line 56, in WebFeatureService
    return wfs200.WebFeatureService_2_0_0(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/dev/.local/lib/python3.11/site-packages/owslib/feature/wfs200.py", line 87, in __init__
    self._buildMetadata(parse_remote_metadata)
  File "/home/dev/.local/lib/python3.11/site-packages/owslib/feature/wfs200.py", line 122, in _buildMetadata
    for elem in self._capabilities.find(nspath("OperationsMetadata"))[:]:
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
TypeError: 'NoneType' object is not subscriptable

version 1.1.0 works

---

Apparently the WFS Capabilites were kind of malformed, but I was surprised that a internal stack trace of owslib was returned. I would have expected that some "official" exception like MalformedXmlExcpetion or so would have been returned. At least, that would be a bit more user friendly and informative.

maybe related: #999

[geographika]

Hi @JakobMiksch !

Are you able to create a pull request similar to #1000? If you can add the XML output (stripping out layers if large, but ensuring the exception is still thrown), and then add a new test to parse the file.

The fix is likely to be a quick check for None - I can help with that if needed.
Also are these properties required by the WFS spec or optional? The first should at least log an error or return an exception, whereas if a property is optional it should parse successfully.

[JakobMiksch]

@geographika I had a deeper look at it. The WFS service: https://www.geoproxy.geoportal-th.de/geoproxy/services/STRNETZ_wfs?SERVICE=WFS&REQUEST=GetCapabilities&version=2.0.0 seems to always return version 1.1.0, even though other versions are requested via query param or argument in WebFeatureService(url, version="2.0.0"). This confuses OWSlib and causes crashes.

My idea would be to check if the actual version of the WFS is the one that is requested. On mismatch, raise exception.

def _buildMetadata(self, parse_remote_metadata=False):
    """set up capabilities metadata objects: """

    actual_wfs_version = self._capabilities.attrib.get("version")

    if actual_wfs_version != self.version:
        raise Exception(f'WFS Version mismatch. Actual: {actual_wfs_version}, Requested: {self.version}')

what do you think about that?

[geographika]

@JakobMiksch - sounds a good approach to me. Would be great to add this in with a test.

Other classes define a custom CapabilitiesError(Exception): that could be added and raised rather than a bare Exception (although I don't see any examples of it in use).

So maybe add in the following to wfs.py and import it into wfs200.py:

class CapabilitiesError(Exception):
    pass

[JakobMiksch]

@geographika here the PR #1039
Adding CapabilitiesError to wfs.py did not work, because it would be a circular dependency. I added it to owslib/feature/common.py instead.