An answer from the app without a Content-Type reaches the client without one (closes #127)

Go's server adds a Content-Type it guesses from the body when the
response's headers have no Content-Type key, which happened to some
of the app's answers and not others, as the first part of the body or
the headers went out first. When the app sends none, smallwebwaf now
sets the key with no value on the client's response, which sends none.

Model: opus-5-5

internal/proxy/passthrough_test.go

@@ -192,6 +192,36 @@ func wantRequestFields(t *testing.T, line logLine, host string, sent, received i
 	}
 }
 
+func TestAddsNoContentTypeTheAppDidNotSend(t *testing.T) {
+	t.Parallel()
+
+	const typed = "text/plain"
+
+	app := startApp(t, func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/typed" {
+			w.Header().Set("Content-Type", typed)
+		} else {
+			// The key with no value keeps Go's server from adding one.
+			w.Header()["Content-Type"] = nil
+		}
+
+		_, _ = io.WriteString(w, "<!doctype html><title>page</title>")
+	})
+	addr, _ := startProxy(t, app.URL, nil)
+
+	got := get(t, addr, "/")
+	if got.header.Values("Content-Type") != nil {
+		t.Errorf("client got Content-Type %q, want none",
+			got.header.Get("Content-Type"))
+	}
+
+	got = get(t, addr, "/typed")
+	if got.header.Get("Content-Type") != typed {
+		t.Errorf("client got Content-Type %q, want %q",
+			got.header.Get("Content-Type"), typed)
+	}
+}
+
 func TestStreamsTheRequestBody(t *testing.T) {
 	t.Parallel()
 

internal/proxy/request.go

@@ -431,6 +431,13 @@ func (rq *request) modifyResponse(res *http.Response) error {
 		return errResponseTooLarge
 	}
 
+	// Go's server adds a Content-Type it guesses from the body to a
+	// response whose headers have no Content-Type key. The key with no
+	// value sends none, as the app sent none.
+	if res.Header.Get("Content-Type") == "" {
+		rq.out.Header()["Content-Type"] = nil
+	}
+
 	res.Body = &responseBody{body: limitBody(res.Body, maxBytes), rq: rq}
 	rq.startClientResponseTimeout()
 

internal/proxy/request_internal_test.go

@@ -0,0 +1,55 @@
+package proxy
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"sneak.berlin/go/smallwebwaf/internal/config"
+)
+
+func TestAnAnswerWithoutAContentTypeGetsNone(t *testing.T) {
+	t.Parallel()
+
+	srv := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			rq := &request{
+				h:   &handler{config: &config.Config{}},
+				out: &responseWriter{ResponseWriter: w},
+			}
+			app := &http.Response{
+				StatusCode: http.StatusOK,
+				Header:     http.Header{},
+				Body:       http.NoBody,
+			}
+
+			err := rq.modifyResponse(app)
+			if err != nil {
+				t.Errorf("modifyResponse: %v", err)
+			}
+
+			// The headers go out with this, the first part of the body,
+			// which Go's server would guess a Content-Type from.
+			_, _ = io.WriteString(w, "<!doctype html><title>page</title>")
+		}))
+	t.Cleanup(srv.Close)
+
+	req, err := http.NewRequestWithContext(t.Context(), http.MethodGet, srv.URL,
+		http.NoBody)
+	if err != nil {
+		t.Fatalf("new request: %v", err)
+	}
+
+	res, err := srv.Client().Do(req)
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+
+	_ = res.Body.Close()
+
+	if res.Header.Values("Content-Type") != nil {
+		t.Errorf("client got Content-Type %q, want none",
+			res.Header.Get("Content-Type"))
+	}
+}