Raise an exception if a "plugins" block exists in metadata.json

[asg017]

refs #2183 #2093

From this comment in #2183: If a "plugins" block appears in metadata.json, it means that a user hasn't migrated over their plugin configuration from metadata.json to datasette.yaml, which is a breaking change in Datasette 1.0.

This PR will ensure that an error is raised whenever that happens.

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📚 Documentation preview 📚: https://datasette--2190.org.readthedocs.build/en/2190/

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03% ⚠️

Comparison is base (6ed7908) 92.69% compared to head (fc7dbe0) 92.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2190      +/-   ##
==========================================
- Coverage   92.69%   92.67%   -0.03%     
==========================================
  Files          40       40              
  Lines        6039     6044       +5     
==========================================
+ Hits         5598     5601       +3     
- Misses        441      443       +2     

Files Changed	Coverage Δ
datasette/app.py	94.19% <100.00%> (ø)
datasette/cli.py	82.53% <100.00%> (ø)
datasette/utils/__init__.py	94.83% <100.00%> (+0.03%)	⬆️

... and 1 file with indirect coverage changes

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[asg017]

TODO: add similar checks for permissions/allow/canned queries

[simonw]

This looks good and works well. The error from this currently looks like:

datasette -m metadata.json -p 8844
Traceback (most recent call last):
  File "/Users/simon/.local/share/virtualenvs/datasette-AWNrQs95/bin/datasette", line 33, in <module>
    sys.exit(load_entry_point('datasette', 'console_scripts', 'datasette')())
  File "/Users/simon/.local/share/virtualenvs/datasette-AWNrQs95/lib/python3.10/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/Users/simon/.local/share/virtualenvs/datasette-AWNrQs95/lib/python3.10/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/Users/simon/.local/share/virtualenvs/datasette-AWNrQs95/lib/python3.10/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/simon/.local/share/virtualenvs/datasette-AWNrQs95/lib/python3.10/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/simon/.local/share/virtualenvs/datasette-AWNrQs95/lib/python3.10/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/Users/simon/Dropbox/Development/datasette/datasette/cli.py", line 98, in wrapped
    return fn(*args, **kwargs)
  File "/Users/simon/Dropbox/Development/datasette/datasette/cli.py", line 546, in serve
    metadata_data = fail_if_plugins_in_metadata(parse_metadata(metadata.read()))
  File "/Users/simon/Dropbox/Development/datasette/datasette/utils/__init__.py", line 1282, in fail_if_plugins_in_metadata
    raise Exception(
Exception: Datasette no longer accepts plugin configuration in --metadata. Move your "plugins" configuration blocks to a separate file - we suggest calling that datasette..json - and start Datasette with datasette -c datasette..json. See https://docs.datasette.io/en/latest/configuration.html for more details.

With wrapping:

Exception: Datasette no longer accepts plugin configuration in --metadata. Move your "plugins" configuration blocks to a separate file - we suggest calling that datasette..json - and start Datasette with datasette -c datasette..json. See https://docs.datasette.io/en/latest/configuration.html for more details.

I think we should link directly to documentation that tells people how to perform this upgrade.

[simonw]

It would be nice if we could catch that and turn that into a less intimidating Click exception too.

[simonw]

I'm going to land this and open a new issue for the upgrade instructions.