Make API discoverable from the root

[fpagnoux]

Connected to #615

• On the Web API, expose a welcome message (with a 300 code) on / instead of a 404 error.

For instance, curl -i localhost:5000 gives:

HTTP/1.1 300 MULTIPLE CHOICES
(...)

{
  "welcome": "This is the root of an OpenFisca Web API. To learn how to use it, check the general documentation (https://openfisca.org/doc/api) and the OpenAPI specification of this instance (http://localhost:5000/spec)."
}

• This message can be customized:

If the Web API is started with openfisca serve -p 3000 --welcome-message "Welcome to the OpenFisca-France Web API. To learn how to use it, check our interactive swagger documentation: https://fr.openfisca.org/legislation/swagger."

Then curl -i localhost:5000 gives:

HTTP/1.1 300 MULTIPLE CHOICES
(...)

{
  "welcome": "Welcome to the OpenFisca-France Web API. To learn how to use it, check our interactive swagger documenation: https://fr.openfisca.org/legislation/swagger"
}

(Like other configuration variables, this custom message can also be defined in a configuration file. Check the openfisca serve documentation)

[Anna-Livia]

Great work :)
I have one question and then it is good to go ^^

[Anna-Livia]

This condition is checked in def resolve_host. Isn't it redundant to have it here also ?

[fpagnoux]

The spec route can be called before the / one. In that case, data['host'] is still None when we get here, so we need to resolve the host.

[fpagnoux]

However, I was just told that re-using the host from one request to another was risky (one local request, and everyone is seeing a localhost:3000 as a host) and not very stateless, so we need a different approach 😞

[fpagnoux]

Ok, adapted the code :)