refactor: move third_party code to separate dir

internal/third_party/tailscale/httpproxy/proxy.go

@@ -0,0 +1,79 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// HTTP proxy code
+
+package httpproxy
+
+import (
+	"context"
+	"io"
+	"net"
+	"net/http"
+	"net/http/httputil"
+	"strings"
+)
+
+// Handler returns an HTTP proxy http.Handler using the
+// provided backend dialer.
+func Handler(dialer func(ctx context.Context, netw, addr string) (net.Conn, error)) http.Handler {
+	rp := &httputil.ReverseProxy{
+		Director: func(r *http.Request) {}, // no change
+		Transport: &http.Transport{
+			DialContext: dialer,
+		},
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != "CONNECT" {
+			backURL := r.RequestURI
+			if strings.HasPrefix(backURL, "/") || backURL == "*" {
+				http.Error(w, "bogus RequestURI; must be absolute URL or CONNECT", 400)
+				return
+			}
+			rp.ServeHTTP(w, r)
+			return
+		}
+
+		// CONNECT support:
+
+		dst := r.RequestURI
+		c, err := dialer(r.Context(), "tcp", dst)
+		if err != nil {
+			w.Header().Set("Connect-Error", err.Error())
+			http.Error(w, err.Error(), 500)
+			return
+		}
+		defer c.Close()
+
+		cc, ccbuf, err := w.(http.Hijacker).Hijack()
+		if err != nil {
+			http.Error(w, err.Error(), 500)
+			return
+		}
+		defer cc.Close()
+
+		io.WriteString(cc, "HTTP/1.1 200 OK\r\n\r\n")
+
+		var clientSrc io.Reader = ccbuf
+		if ccbuf.Reader.Buffered() == 0 {
+			// In the common case (with no
+			// buffered data), read directly from
+			// the underlying client connection to
+			// save some memory, letting the
+			// bufio.Reader/Writer get GC'ed.
+			clientSrc = cc
+		}
+
+		errc := make(chan error, 1)
+		go func() {
+			_, err := io.Copy(cc, c)
+			errc <- err
+		}()
+		go func() {
+			_, err := io.Copy(c, clientSrc)
+			errc <- err
+		}()
+		<-errc
+	})
+}