Add support for ASGI root_path for openapi docs

[iksteen]

ASGI specifies the root_path connection scope variable. This is the path the application is mounted under (the equivalent of WSGI's SCRIPT_NAME).

This PR ensures that this root is used as a prefix for the openapi.json URL, as a prefix for paths contained in openapi.json and by the oauth2 redirect endpoint to redirect back to the swagger docs.

To override the path the ASGI server provides you can still explicitly set the openapi_prefix variable.

Do note that this is a breaking change for applications that overriden openapi() themselves.

This should fix/related to #829

[codecov]

Codecov Report

Merging #1199 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master     #1199    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          224       230     +6     
  Lines         6728      6862   +134     
==========================================
+ Hits          6728      6862   +134     

Impacted Files	Coverage Δ
fastapi/applications.py	100.00% <100.00%> (ø)
tests/test_deprecated_openapi_prefix.py	100.00% <100.00%> (ø)
...t_tutorial/test_behind_a_proxy/test_tutorial001.py	100.00% <100.00%> (ø)
...t_tutorial/test_behind_a_proxy/test_tutorial002.py	100.00% <100.00%> (ø)
fastapi/utils.py	100.00% <0.00%> (ø)
fastapi/routing.py	100.00% <0.00%> (ø)
fastapi/encoders.py	100.00% <0.00%> (ø)
fastapi/openapi/utils.py	100.00% <0.00%> (ø)
tests/test_skip_defaults.py	100.00% <0.00%> (ø)
... and 13 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6e1cd45...ac517f0. Read the comment docs.

[nunomen]

I guess this PR solves the same issue that I was having (I didn't even know about issue #829). I solved my problem by just adding the prefix to the redirect url. You can see what I did on my proposed PR #1159. But it seems to me that your solution involves more steps that I may have been missing. Could you explain?

I will probably delete #1159 since it is redundant, now that you've done this 😄

[tiangolo]

Great! 🚀

Thanks for the patience, it took me a bit of time to review this as I wanted to first re-read the WSGI and ASGI specs to make sure I wasn't missing anything.

And then to test it all with an actual reverse proxy with path prefix stripping (what the root_path would be used for).

I refactored it a bit and took the opportunity to deprecate openapi_prefix in favor of the new root_path 🔥 🚀

And I added a bunch of docs to explain how the root_path works and to document how to test it all with a real proxy using Traefik.

Thanks for your contribution! 🚀 🎉 🍰

This is available in FastAPI 0.56.0. 🍰