net/http: add ResponseController.EnableFullDuplex

[neild]

This proposal aims to address #15527.

The net/http HTTP/1 server does not permit reading from an inbound request body after starting to write the response. (See the ResponseWriter.Write documentation).

This limitation is because the server drains any unread portion of the request body before writing the response headers, to avoid deadlocking clients that attempt to write a complete request before reading the response. (See #15527 (comment) for more context.)

I propose that we offer an opt-in mechanism to disable this behavior, permitting a server handler to write some or all of the response interleaved with reads from the request.

// SetBidi indicates whether the request handler will interleave reads from Request.Body with
// writes to the ResponseWriter.
//
// For HTTP/1 requests, the Go HTTP server by default consumes any unread portion of the request
// body before beginning to write the response, preventing handlers from concurrently reading from
// the request and writing the response. Calling SetBidi(true) disables this behavior and permits
// handlers to continue to read from the request while concurrently writing the response.
//
// For HTTP/2 requests, the Go HTTP server always permits concurrent reads and responses.
func (c *ResponseController) SetBidi(bidi bool) error {}

[rsc]

"bidi" often means bidirectional text like Unicode LTR/RTL.
It seems like a very short name for a rarely used feature.
Is there a standard name for this behavior in the HTTP specs?
It's unclear from the docs whether c.SetBidi(false) has an effect on HTTP/2 (or returns an error?).

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[neild]

I don't believe there is a standard name for this behavior. I'm not committed to the name here; EnableInterleavedReadsAndWrites would be unambiguous if long. EnableResponseInterleaving? EnableInterleave? EnableConcurrentResponse?

SetBidi(false) will return an error for HTTP/2 requests. We already support interleaving for HTTP/2, and I don't see any value in disabling it. The reason for draining the inbound request before responding is to work better with naive clients, and a naive HTTP/2 client is somewhat of an oxymoron.

Ideally we wouldn't have a knob here, but I don't see how to avoid it; there are valid reasons to want both possible behaviors here, and changing our current default will break some users.

[seankhliao]

informational rfc: bidirectional http
internet draft: full duplex

[neild]

I like EnableFullDuplex.

And perhaps only permit enabling full duplex, to avoid any questions about what it means to disable it with HTTP/2 or HTTP/3.

func (c *ResponseController) EnableFullDuplex() error {}

[rsc]

With the renaming to EnableFullDuplex, have all the concerns about this proposal been addressed?

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group