103 Early hints

[padziu]

Hi!

I would like to ask if there are any plans to introduce support for Early Hints (HTTP 103) in free nginx in the future. Specifically, I am interested in two areas:

Configuring specific assets:
Will there be a way to configure Early Hints directly in the nginx configuration, such as defining which assets (CSS, JS, fonts) should be included in the 103 responses?

Pass-through support (i think that is most important):
Will nginx support forwarding HTTP 103 responses from upstream servers without modification? This would allow Early Hints to be handled by backend while nginx simply passes them along to the client.

Early Hints is becoming an increasingly valuable mechanism for improving performance, and it would be great to know if this is something that might be supported natively in nginx.

Thank you in advance for your response.

[mdounin]

Looking into support for 103 Early Hints (and improved handling of 1xx responses in general) is in my TODO list, but there are no defined aspects yet.

The most basic improvement I currently consider is simply skipping 1xx responses while proxying: currently, proxying instead removes Expect request header to make sure 100 Continue responses won't appear on the wire, but there is no general mechanism to ignore such responses, and any 1xx response will break things. Introducing skipping 1xx responses will make sure that 103 responses can be safely used by backends.

Further improvements will likely include sending such responses to the client, at least if configured to. This will need to be done with care though, since various header processing functions might not expect informational responses to be sent before the actual response.

[mdounin]

For the record, as of fd953ff interim 1xx responses are properly skipped. This should make freenginx compatible with upstream servers which return 103 (Early Hints), as well as other 1xx response codes, such as unexpected 100 (Continue) or upcoming 104 (Upload Resumption Supported).