net/http: document setting of Proxy-Authorization header

Add a test for setting a proxy username/password in
the HTTP_PROXY environment variable as well.

Fixes #61505

Change-Id: I31c3fa94c7bc463133321e9af9289fd47da75b46
Reviewed-on: https://go-review.googlesource.com/c/go/+/512555
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/net/http/transport.go

@@ -117,6 +117,10 @@ type Transport struct {
 	// "https", and "socks5" are supported. If the scheme is empty,
 	// "http" is assumed.
 	//
+	// If the proxy URL contains a userinfo subcomponent,
+	// the proxy request will pass the username and password
+	// in a Proxy-Authorization header.
+	//
 	// If Proxy is nil or returns a nil *URL, no proxy is used.
 	Proxy func(*Request) (*url.URL, error)
 

src/net/http/transport_test.go

@@ -6810,3 +6810,36 @@ func testRequestSanitization(t *testing.T, mode testMode) {
 		resp.Body.Close()
 	}
 }
+
+func TestProxyAuthHeader(t *testing.T) {
+	// Not parallel: Sets an environment variable.
+	run(t, testProxyAuthHeader, []testMode{http1Mode}, testNotParallel)
+}
+func testProxyAuthHeader(t *testing.T, mode testMode) {
+	const username = "u"
+	const password = "@/?!"
+	cst := newClientServerTest(t, mode, HandlerFunc(func(rw ResponseWriter, req *Request) {
+		// Copy the Proxy-Authorization header to a new Request,
+		// since Request.BasicAuth only parses the Authorization header.
+		var r2 Request
+		r2.Header = Header{
+			"Authorization": req.Header["Proxy-Authorization"],
+		}
+		gotuser, gotpass, ok := r2.BasicAuth()
+		if !ok || gotuser != username || gotpass != password {
+			t.Errorf("req.BasicAuth() = %q, %q, %v; want %q, %q, true", gotuser, gotpass, ok, username, password)
+		}
+	}))
+	u, err := url.Parse(cst.ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	u.User = url.UserPassword(username, password)
+	t.Setenv("HTTP_PROXY", u.String())
+	cst.tr.Proxy = ProxyURL(u)
+	resp, err := cst.c.Get("http://_/")
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+}