control/controlhttp: add AcceptHTTP hook to add coalesced Server->Client write

New plan for #5972. Instead of sending the public key in the clear
(from earlier unreleased 246274b8e9) where the client might have to
worry about it being dropped or tampered with and retrying, we'll
instead send it post-Noise handshake but before the HTTP/2 connection
begins.

This replaces the earlier extraHeaders hook with a different sort of
hook that allows us to combine two writes on the wire in one packet.

Updates #5972

Change-Id: I42cdf7c1859b53ca4dfa5610bd1b840c6986e09c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlhttp/server.go

@@ -9,7 +9,10 @@ import (
 	"encoding/base64"
 	"errors"
 	"fmt"
+	"io"
+	"net"
 	"net/http"
+	"time"
 
 	"nhooyr.io/websocket"
 	"tailscale.com/control/controlbase"
@@ -18,16 +21,20 @@ import (
 	"tailscale.com/types/key"
 )
 
-// AcceptHTTP upgrades the HTTP request given by w and r into a
-// Tailscale control protocol base transport connection.
+// AcceptHTTP upgrades the HTTP request given by w and r into a Tailscale
+// control protocol base transport connection.
 //
-// AcceptHTTP always writes an HTTP response to w. The caller must not
-// attempt their own response after calling AcceptHTTP.
+// AcceptHTTP always writes an HTTP response to w. The caller must not attempt
+// their own response after calling AcceptHTTP.
 //
-// extraHeader optionally specifies extra header(s) to send in the
-// 101 Switching Protocols Upgrade response. It must not include the "Upgrade"
-// or "Connection" headers; they will be replaced.
-func AcceptHTTP(ctx context.Context, w http.ResponseWriter, r *http.Request, private key.MachinePrivate, extraHeader http.Header) (*controlbase.Conn, error) {
+// earlyWrite optionally specifies a func to write to the noise connection
+// (encrypted). It receives the negotiated version and a writer to write to, if
+// desired.
+func AcceptHTTP(ctx context.Context, w http.ResponseWriter, r *http.Request, private key.MachinePrivate, earlyWrite func(protocolVersion int, w io.Writer) error) (*controlbase.Conn, error) {
+	return acceptHTTP(ctx, w, r, private, earlyWrite)
+}
+
+func acceptHTTP(ctx context.Context, w http.ResponseWriter, r *http.Request, private key.MachinePrivate, earlyWrite func(protocolVersion int, w io.Writer) error) (_ *controlbase.Conn, retErr error) {
 	next := r.Header.Get("Upgrade")
 	if next == "" {
 		http.Error(w, "missing next protocol", http.StatusBadRequest)
@@ -58,9 +65,6 @@ func AcceptHTTP(ctx context.Context, w http.ResponseWriter, r *http.Request, pri
 		return nil, errors.New("can't hijack client connection")
 	}
 
-	for k, vv := range extraHeader {
-		w.Header()[k] = vv
-	}
 	w.Header().Set("Upgrade", upgradeHeaderValue)
 	w.Header().Set("Connection", "upgrade")
 	w.WriteHeader(http.StatusSwitchingProtocols)
@@ -69,18 +73,41 @@ func AcceptHTTP(ctx context.Context, w http.ResponseWriter, r *http.Request, pri
 	if err != nil {
 		return nil, fmt.Errorf("hijacking client connection: %w", err)
 	}
+
+	defer func() {
+		if retErr != nil {
+			conn.Close()
+		}
+	}()
+
 	if err := brw.Flush(); err != nil {
-		conn.Close()
 		return nil, fmt.Errorf("flushing hijacked HTTP buffer: %w", err)
 	}
 	conn = netutil.NewDrainBufConn(conn, brw.Reader)
 
-	nc, err := controlbase.Server(ctx, conn, private, init)
+	cwc := newWriteCorkingConn(conn)
+
+	nc, err := controlbase.Server(ctx, cwc, private, init)
 	if err != nil {
-		conn.Close()
 		return nil, fmt.Errorf("noise handshake failed: %w", err)
 	}
 
+	if earlyWrite != nil {
+		if deadline, ok := ctx.Deadline(); ok {
+			if err := conn.SetDeadline(deadline); err != nil {
+				return nil, fmt.Errorf("setting conn deadline: %w", err)
+			}
+			defer conn.SetDeadline(time.Time{})
+		}
+		if err := earlyWrite(nc.ProtocolVersion(), nc); err != nil {
+			return nil, err
+		}
+	}
+
+	if err := cwc.uncork(); err != nil {
+		return nil, err
+	}
+
 	return nc, nil
 }
 
@@ -128,3 +155,61 @@ func acceptWebsocket(ctx context.Context, w http.ResponseWriter, r *http.Request
 
 	return nc, nil
 }
+
+// corkConn is a net.Conn wrapper that initially buffers all writes until uncork
+// is called. If the conn is corked and a Read occurs, the Read will flush any
+// buffered (corked) write.
+//
+// Until uncorked, Read/Write/uncork may be not called concurrently.
+//
+// Deadlines still work, but a corked write ignores deadlines until a Read or
+// uncork goes to do that Write.
+//
+// Use newWriteCorkingConn to create one.
+type corkConn struct {
+	net.Conn
+	corked bool
+	buf    []byte // corked data
+}
+
+func newWriteCorkingConn(c net.Conn) *corkConn {
+	return &corkConn{Conn: c, corked: true}
+}
+
+func (c *corkConn) Write(b []byte) (int, error) {
+	if c.corked {
+		c.buf = append(c.buf, b...)
+		return len(b), nil
+	}
+	return c.Conn.Write(b)
+}
+
+func (c *corkConn) Read(b []byte) (int, error) {
+	if c.corked {
+		if err := c.flush(); err != nil {
+			return 0, err
+		}
+	}
+	return c.Conn.Read(b)
+}
+
+// uncork flushes any buffered data and uncorks the connection so future Writes
+// don't buffer. It may not be called concurrently with reads or writes and
+// may only be called once.
+func (c *corkConn) uncork() error {
+	if !c.corked {
+		panic("usage error; uncork called twice") // worth panicking to catch misuse
+	}
+	err := c.flush()
+	c.corked = false
+	return err
+}
+
+func (c *corkConn) flush() error {
+	if len(c.buf) == 0 {
+		return nil
+	}
+	_, err := c.Conn.Write(c.buf)
+	c.buf = nil
+	return err
+}