net/tshttpproxy: new package, support WPAD/PAC proxies on Windows

Updates tailscale/corp#553

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

net/tshttpproxy/tshttpproxy.go

@@ -0,0 +1,33 @@
+// Copyright (c) 2020 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.
+
+// Package tshttpproxy contains Tailscale additions to httpproxy not available
+// in golang.org/x/net/http/httpproxy. Notably, it aims to support Windows better.
+package tshttpproxy
+
+import (
+	"net/http"
+	"net/url"
+)
+
+// sysProxyFromEnv, if non-nil, specifies a platform-specific ProxyFromEnvironment
+// func to use if http.ProxyFromEnvironment doesn't return a proxy.
+// For example, WPAD PAC files on Windows.
+var sysProxyFromEnv func(*http.Request) (*url.URL, error)
+
+func ProxyFromEnvironment(req *http.Request) (*url.URL, error) {
+	u, err := http.ProxyFromEnvironment(req)
+	if u != nil && err == nil {
+		return u, nil
+	}
+
+	if sysProxyFromEnv != nil {
+		u, err := sysProxyFromEnv(req)
+		if u != nil && err == nil {
+			return u, nil
+		}
+	}
+
+	return nil, err
+}