net/http: support status code 102 (Processing) in ResponseWriter

[majewsky]

HTTP status code 102 (Processing) is defined by RFC 2518, section 10.1:

This status code SHOULD only be sent when the server has a reasonable expectation that the request will take significant time to complete. [...] The server MUST send a final response after the request has been completed.

As of now (Go 1.13.6), there is no way to actually send a 102 response because ResponseWriter.WriteHeader() can only be called once. Any successive calls are rejected (here).

Since the ResponseWriter interface cannot be extended without breaking backwards compatibility, a new interface should be added (and implemented by the existing ResponseWriter implementations) that exposes a function for sending an interim response with status 102.

[toothrot]

WebDAV specific features might be better suited for x/net/webdav. I'm not positive we could make this change in net/http while maintaining API compatibility.

/cc @bradfitz @nigeltao

[bradfitz]

I don't think we'd need API changes. We could just permit WriteHeader(102) followed by a non-1xx status later