Check for errors during client.fetch_schema()

[luketaverne]

Hi everyone!

We (@wang2bo2 and I) came across a bug when our JWT tokens expired.

Current behavior / steps to reproduce

• Use an expired JWT token
• The server returns an error for the expired token, but the error is not checked client side (see PR changes).
• The result is the error being added added to the client as the data/schema.
• This causes another error to be raised, which isn't immediately obvious that it's due to an expired JWT. See stacktrace below.

... [project trace omitted] ...
File "/root/.cache/pypoetry/virtualenvs/project-xS3fZVNL-py3.8/lib/python3.8/site-packages/gql/client.py", line 617, in __enter__
         self.session.fetch_schema()
File "/root/.cache/pypoetry/virtualenvs/project-xS3fZVNL-py3.8/lib/python3.8/site-packages/gql/client.py", line 806, in fetch_schema
         self.client.schema = build_client_schema(self.client.introspection)
File "/root/.cache/pypoetry/virtualenvs/project-xS3fZVNL-py3.8/lib/python3.8/site-packages/gql/utilities/build_client_schema.py", line 79, in build_client_schema
         raise TypeError(
TypeError: Invalid or incomplete introspection result. Ensure that you are passing the 'data' attribute of an introspection response and no 'errors' were returned alongside: None.

Expected behavior

• The error from the server should be raised inside the client sdk.

Fix

The error handling used elsewhere in the library was copied over to the sync and async fetch_schema functions.

Thank you all for maintaining this useful SDK 🙂

[codecov-commenter]

Codecov Report

Merging #328 (64ac830) into master (321c606) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #328   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         2156      2159    +3     
=========================================
+ Hits          2156      2159    +3     

Impacted Files	Coverage Δ
gql/client.py	100.00% <100.00%> (ø)

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[leszekhanusz]

Thanks for your PR!
I modified the message so that we could see directly that the error comes from fetching the schema and added some tests.

[leszekhanusz]

Fixed in v3.3.0