net/netutil: move some net utils from control/controlhttp to netutil

In prep for reuse elsewhere.

Change-Id: I1b804edf76ac66b9108e6f434e77eab7a7472d69
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlhttp/client.go

@@ -35,6 +35,7 @@ import (
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/netns"
+	"tailscale.com/net/netutil"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tshttpproxy"
 	"tailscale.com/types/key"
@@ -232,22 +233,5 @@ func (a *dialParams) tryURL(u *url.URL, init []byte) (net.Conn, error) {
 		return nil, errors.New("http Transport did not provide a writable body")
 	}
 
-	return &wrappedConn{switchedConn, rwc}, nil
-}
-
-type wrappedConn struct {
-	net.Conn
-	rwc io.ReadWriteCloser
-}
-
-func (w *wrappedConn) Read(bs []byte) (int, error) {
-	return w.rwc.Read(bs)
-}
-
-func (w *wrappedConn) Write(bs []byte) (int, error) {
-	return w.rwc.Write(bs)
-}
-
-func (w *wrappedConn) Close() error {
-	return w.rwc.Close()
+	return netutil.NewAltReadWriteCloserConn(rwc, switchedConn), nil
 }

control/controlhttp/server.go

@@ -5,15 +5,14 @@
 package controlhttp
 
 import (
-	"bufio"
 	"context"
 	"encoding/base64"
 	"errors"
 	"fmt"
-	"net"
 	"net/http"
 
 	"tailscale.com/control/controlbase"
+	"tailscale.com/net/netutil"
 	"tailscale.com/types/key"
 )
 
@@ -62,9 +61,7 @@ func AcceptHTTP(ctx context.Context, w http.ResponseWriter, r *http.Request, pri
 		conn.Close()
 		return nil, fmt.Errorf("flushing hijacked HTTP buffer: %w", err)
 	}
-	if brw.Reader.Buffered() > 0 {
-		conn = &drainBufConn{conn, brw.Reader}
-	}
+	conn = netutil.NewDrainBufConn(conn, brw.Reader)
 
 	nc, err := controlbase.Server(ctx, conn, private, init)
 	if err != nil {
@@ -74,22 +71,3 @@ func AcceptHTTP(ctx context.Context, w http.ResponseWriter, r *http.Request, pri
 
 	return nc, nil
 }
-
-// drainBufConn is a net.Conn with an initial bunch of bytes in a
-// bufio.Reader. Read drains the bufio.Reader until empty, then passes
-// through subsequent reads to the Conn directly.
-type drainBufConn struct {
-	net.Conn
-	r *bufio.Reader
-}
-
-func (b *drainBufConn) Read(bs []byte) (int, error) {
-	if b.r == nil {
-		return b.Conn.Read(bs)
-	}
-	n, err := b.r.Read(bs)
-	if b.r.Buffered() == 0 {
-		b.r = nil
-	}
-	return n, err
-}

net/netutil/netutil.go

@@ -6,6 +6,7 @@
 package netutil
 
 import (
+	"bufio"
 	"io"
 	"net"
 	"sync"
@@ -63,3 +64,55 @@ type dummyAddr string
 
 func (a dummyAddr) Network() string { return string(a) }
 func (a dummyAddr) String() string  { return string(a) }
+
+// NewDrainBufConn returns a net.Conn conditionally wrapping c,
+// prefixing any bytes that are in initialReadBuf, which may be nil.
+func NewDrainBufConn(c net.Conn, initialReadBuf *bufio.Reader) net.Conn {
+	r := initialReadBuf
+	if r != nil && r.Buffered() == 0 {
+		r = nil
+	}
+	return &drainBufConn{c, r}
+}
+
+// drainBufConn is a net.Conn with an initial bunch of bytes in a
+// bufio.Reader. Read drains the bufio.Reader until empty, then passes
+// through subsequent reads to the Conn directly.
+type drainBufConn struct {
+	net.Conn
+	r *bufio.Reader
+}
+
+func (b *drainBufConn) Read(bs []byte) (int, error) {
+	if b.r == nil {
+		return b.Conn.Read(bs)
+	}
+	n, err := b.r.Read(bs)
+	if b.r.Buffered() == 0 {
+		b.r = nil
+	}
+	return n, err
+}
+
+// NewAltReadWriteCloserConn returns a net.Conn that wraps rwc (for
+// Read, Write, and Close) and c (for all other methods).
+func NewAltReadWriteCloserConn(rwc io.ReadWriteCloser, c net.Conn) net.Conn {
+	return wrappedConn{c, rwc}
+}
+
+type wrappedConn struct {
+	net.Conn
+	rwc io.ReadWriteCloser
+}
+
+func (w wrappedConn) Read(bs []byte) (int, error) {
+	return w.rwc.Read(bs)
+}
+
+func (w wrappedConn) Write(bs []byte) (int, error) {
+	return w.rwc.Write(bs)
+}
+
+func (w wrappedConn) Close() error {
+	return w.rwc.Close()
+}