ToolProvider: Fix for issue 74

[rghostin]

[c.f issue #74]

---

In some cases, the ToolProvider.consumer_secret is not set. This prevents the signing of outcome requests' XML.

Code snippets below help reproduce the events.

• In a launch view:

def lti_launch(request):
    if request.method == "POST":
        tool_provider = DjangoToolProvider.from_django_request(request=request)
        # tool_provider.consumer_key was set from the oauth_consumer_key post parameter; consumer_secret is still None
        oauth_validator = SigOnlyRequestValidator()
        is_valid_oauth = tool_provider.is_valid_request(oauth_validator)    
        [...]

• In the method ToolProvider.is_valid_request:

validator = ProxyValidator(validator)
valid, request = endpoint.validate_request([...])
# At this point the proxy contains the secret in validator.secret, though the tool_provider.consumer_secret is still None
if valid and not self.consumer_key and not self.consumer_secret:     # ! Potentially faulty line ! Condition is False, so tool_provider.consumer_secret stays to None
    self.consumer_key = self.launch_params['oauth_consumer_key']
    self.consumer_secret = validator.secret
return valid

• Problems arise later on during the usage of the ToolProvider instance:

tool_provider.post_replace_result(score=1)    # unable to sign the request since consumer_secret is None (OutcomeRequest.has_required_attributes returns False)

---

Solution:
In the method ToolProvider.is_valid_request, the condition should be formulated as:

if valid:
    # Gather the key and secret
    if not self.consumer_key: 
        self.consumer_key = self.launch_params['oauth_consumer_key']
    if not self.consumer_secret:
        self.consumer_secret = validator.secret

[codecov]

Codecov Report

Merging #75 into master will decrease coverage by 1.15%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
- Coverage   86.05%   84.90%   -1.16%     
==========================================
  Files          16       16              
  Lines         667      669       +2     
  Branches      115      117       +2     
==========================================
- Hits          574      568       -6     
- Misses         74       81       +7     
- Partials       19       20       +1     

Impacted Files	Coverage Δ
src/lti/tool_provider.py	85.54% <80.00%> (-4.59%)	⬇️
src/lti/utils.py	70.00% <0.00%> (-10.00%)	⬇️
src/lti/launch_params.py	88.00% <0.00%> (-2.67%)	⬇️

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