net/http/cgi: serve 500, not 200, on invalid responses from child processes

Per RFC 3875 section 6 rules.

Fixes #7198

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/68960049

src/pkg/net/http/cgi/host.go

@@ -223,6 +223,8 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	linebody := bufio.NewReaderSize(stdoutRead, 1024)
 	headers := make(http.Header)
 	statusCode := 0
+	headerLines := 0
+	sawBlankLine := false
 	for {
 		line, isPrefix, err := linebody.ReadLine()
 		if isPrefix {
@@ -239,8 +241,10 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 			return
 		}
 		if len(line) == 0 {
+			sawBlankLine = true
 			break
 		}
+		headerLines++
 		parts := strings.SplitN(string(line), ":", 2)
 		if len(parts) < 2 {
 			h.printf("cgi: bogus header line: %s", string(line))
@@ -266,6 +270,11 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 			headers.Add(header, val)
 		}
 	}
+	if headerLines == 0 || !sawBlankLine {
+		rw.WriteHeader(http.StatusInternalServerError)
+		h.printf("cgi: no headers")
+		return
+	}
 
 	if loc := headers.Get("Location"); loc != "" {
 		if strings.HasPrefix(loc, "/") && h.PathLocationHandler != nil {
@@ -277,6 +286,12 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		}
 	}
 
+	if statusCode == 0 && headers.Get("Content-Type") == "" {
+		rw.WriteHeader(http.StatusInternalServerError)
+		h.printf("cgi: missing required Content-Type in headers")
+		return
+	}
+
 	if statusCode == 0 {
 		statusCode = http.StatusOK
 	}

src/pkg/net/http/cgi/matryoshka_test.go

@@ -128,6 +128,7 @@ func TestKillChildAfterCopyError(t *testing.T) {
 }
 
 // Test that a child handler writing only headers works.
+// golang.org/issue/7196
 func TestChildOnlyHeaders(t *testing.T) {
 	h := &Handler{
 		Path: os.Args[0],
@@ -143,6 +144,26 @@ func TestChildOnlyHeaders(t *testing.T) {
 	}
 }
 
+// golang.org/issue/7198
+func Test500WithNoHeaders(t *testing.T)     { want500Test(t, "/immediate-disconnect") }
+func Test500WithNoContentType(t *testing.T) { want500Test(t, "/no-content-type") }
+func Test500WithEmptyHeaders(t *testing.T)  { want500Test(t, "/empty-headers") }
+
+func want500Test(t *testing.T, path string) {
+	h := &Handler{
+		Path: os.Args[0],
+		Root: "/test.go",
+		Args: []string{"-test.run=TestBeChildCGIProcess"},
+	}
+	expectedMap := map[string]string{
+		"_body": "",
+	}
+	replay := runCgiTest(t, h, "GET "+path+" HTTP/1.0\nHost: example.com\n\n", expectedMap)
+	if replay.Code != 500 {
+		t.Errorf("Got code %d; want 500", replay.Code)
+	}
+}
+
 type neverEnding byte
 
 func (b neverEnding) Read(p []byte) (n int, err error) {
@@ -158,6 +179,16 @@ func TestBeChildCGIProcess(t *testing.T) {
 		// Not in a CGI environment; skipping test.
 		return
 	}
+	switch os.Getenv("REQUEST_URI") {
+	case "/immediate-disconnect":
+		os.Exit(0)
+	case "/no-content-type":
+		fmt.Printf("Content-Length: 6\n\nHello\n")
+		os.Exit(0)
+	case "/empty-headers":
+		fmt.Printf("\nHello")
+		os.Exit(0)
+	}
 	Serve(http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
 		rw.Header().Set("X-Test-Header", "X-Test-Value")
 		req.ParseForm()