Ability to enable/disable specific features via --config

[simonw]

--config settings from #274 can currently only be integers.

I'd like them to be available as boooeans too. Then we can use them to have that are turned on by default but can be turned off.

First features to get this treatment:

• allow_sql - whether or not the ?sql= parameter is allowed and form is displayed
• allow_facet - is ?_facet= allowed or do we only run facets defined in metadata.json
• allow_download - do we let users download the full SQLite database file?
• suggest_facets - do we attempt to calculate suggested facets?

Refs #275

[simonw]

I'm not crazy about the enable_ prefix on these.

[simonw]

This will also give us a mechanism for turning on and off the cross-database joins feature from #283

[simonw]

Maybe allow_sql, allow_facet and allow_download

[simonw]

allow_sql should only affect the ?sql= parameter and whether or not the form is displayed. You should still be able to use and execute canned queries even if this option is turned off.

[simonw]

Demo:

datasette publish now --branch=master fixtures.db \
    --source="#284 Demo" \
    --source_url="https://github.com/simonw/datasette/issues/284" \
    --extra-options "--config allow_sql:off --config allow_facet:off --config allow_download:off" \
    --name=datasette-demo-284

now alias https://datasette-demo-284-jogjwngegj.now.sh datasette-demo-284.now.sh

https://datasette-demo-284.now.sh/

Note the following:

• https://datasette-demo-284.now.sh/fixtures-fda0fea has no SQL input textarea
• https://datasette-demo-284.now.sh/fixtures-fda0fea has no database download link
• https://datasette-demo-284.now.sh/fixtures-fda0fea.db returns 403 forbidden
• https://datasette-demo-284.now.sh/fixtures-fda0fea?sql=select%20*%20from%20sqlite_master throws error 400
• https://datasette-demo-284.now.sh/fixtures-fda0fea/facetable shows no suggested facets
• https://datasette-demo-284.now.sh/fixtures-fda0fea/facetable?_facet=city_id throws error 400