net/http: add an interface for server push

This interface will be implemented by golang.org/x/net/http2 in
https://go-review.googlesource.com/c/29439/.

Updates golang/go#13443

Change-Id: Ib6bdd403b0878cfe36fa9875c07c2c7239232556
Reviewed-on: https://go-review.googlesource.com/32012
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/http.go

@@ -100,3 +100,42 @@ var (
 	_ io.WriterTo   = NoBody
 	_ io.ReadCloser = NoBody
 )
+
+// PushOptions describes options for Pusher.Push.
+type PushOptions struct {
+	// Method specifies the HTTP method for the promised request.
+	// If set, it must be "GET" or "HEAD". Empty means "GET".
+	Method string
+
+	// Header specifies additional promised request headers. This cannot
+	// include HTTP/2 pseudo header fields like ":path" and ":scheme",
+	// which will be added automatically.
+	Header Header
+}
+
+// Pusher is the interface implemented by ResponseWriters that support
+// HTTP/2 server push. For more background, see
+// https://tools.ietf.org/html/rfc7540#section-8.2.
+type Pusher interface {
+	// Push initiates an HTTP/2 server push. This constructs a synthetic
+	// request using the given target and options, serializes that request
+	// into a PUSH_PROMISE frame, then dispatches that request using the
+	// server's request handler. If opts is nil, default options are used.
+	//
+	// The target must either be an absolute path (like "/path") or an absolute
+	// URL that contains a valid host and the same scheme as the parent request.
+	// If the target is a path, it will inherit the scheme and host of the
+	// parent request.
+	//
+	// The HTTP/2 spec disallows recursive pushes and cross-authority pushes.
+	// Push may or may not detect these invalid pushes; however, invalid
+	// pushes will be detected and canceled by conforming clients.
+	//
+	// Handlers that wish to push URL X should call Push before sending any
+	// data that may trigger a request for URL X. This avoids a race where the
+	// client issues requests for X before receiving the PUSH_PROMISE for X.
+	//
+	// Push returns ErrNotSupported if the client has disabled push or if push
+	// is not supported on the underlying connection.
+	Push(target string, opts *PushOptions) error
+}