net/http: don't write redirect body if content-type is set

Fixes #25166

Change-Id: Id1fe18899579365519ac08ebedf74cd23c0fbd9f
Reviewed-on: https://go-review.googlesource.com/110296
Run-TryBot: Sam Whited <sam@samwhited.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -2589,20 +2589,29 @@ func TestRedirect(t *testing.T) {
 
 // Test that Content-Type header is set for GET and HEAD requests.
 func TestRedirectContentTypeAndBody(t *testing.T) {
+	var unsetCT = []string{"sentinalValNoCT"}
+
 	var tests = []struct {
+		initCT   []string
 		method   string
 		wantCT   string
 		wantBody string
 	}{
-		{MethodGet, "text/html; charset=utf-8", "<a href=\"/foo\">Found</a>.\n\n"},
-		{MethodHead, "text/html; charset=utf-8", ""},
-		{MethodPost, "", ""},
-		{MethodDelete, "", ""},
-		{"foo", "", ""},
+		{unsetCT, MethodGet, "text/html; charset=utf-8", "<a href=\"/foo\">Found</a>.\n\n"},
+		{unsetCT, MethodHead, "text/html; charset=utf-8", ""},
+		{unsetCT, MethodPost, "", ""},
+		{unsetCT, MethodDelete, "", ""},
+		{unsetCT, "foo", "", ""},
+		{[]string{"application/test"}, MethodGet, "application/test", ""},
+		{[]string{}, MethodGet, "", ""},
+		{nil, MethodGet, "", ""},
 	}
 	for _, tt := range tests {
 		req := httptest.NewRequest(tt.method, "http://example.com/qux/", nil)
 		rec := httptest.NewRecorder()
+		if len(tt.initCT) != 1 || &tt.initCT[0] != &unsetCT[0] {
+			rec.Header()["Content-Type"] = tt.initCT
+		}
 		Redirect(rec, req, "/foo", 302)
 		if got, want := rec.Header().Get("Content-Type"), tt.wantCT; got != want {
 			t.Errorf("Redirect(%q) generated Content-Type header %q; want %q", tt.method, got, want)

src/net/http/server.go

@@ -2003,6 +2003,9 @@ func StripPrefix(prefix string, h Handler) Handler {
 //
 // The provided code should be in the 3xx range and is usually
 // StatusMovedPermanently, StatusFound or StatusSeeOther.
+// If Content-Type has not been set Redirect sets the header to
+// "text/html; charset=utf-8" and writes a small HTML body.
+// Setting the Content-Type header to nil also prevents writing the body.
 func Redirect(w ResponseWriter, r *Request, url string, code int) {
 	// parseURL is just url.Parse (url is shadowed for godoc).
 	if u, err := parseURL(url); err == nil {
@@ -2039,9 +2042,14 @@ func Redirect(w ResponseWriter, r *Request, url string, code int) {
 		}
 	}
 
-	w.Header().Set("Location", hexEscapeNonASCII(url))
+	h := w.Header()
+	h.Set("Location", hexEscapeNonASCII(url))
+
+	if _, ok := h["Content-Type"]; ok {
+		return
+	}
 	if r.Method == "GET" || r.Method == "HEAD" {
-		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		h.Set("Content-Type", "text/html; charset=utf-8")
 	}
 	w.WriteHeader(code)