net/http/httptest: record trailing headers in ResponseRecorder

Trailers() returns the headers that were set by the handler after the
headers were written "to the wire" (in this case HeaderMap) and that
were also specified in a proper header called "Trailer".

Neither HeaderMap or trailerMap (used for Trailers()) are manipulated by
the handler code, instead a third stagingMap is given to the
handler. This avoid a reference kept by handler to affect the recorded
results.

If a handler just modify the header but doesn't call any Write or Flush
method from ResponseWriter (or Flusher) interface, HeaderMap will not be
updated. In this case, calling Flush in the recorder is enough to get
the HeaderMap filled.

Fixes #14531.
Fixes #8857.

Change-Id: I42842341ec3e95c7b87d7e6f178c65cd03d63cc3
Reviewed-on: https://go-review.googlesource.com/20047Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httptest/recorder.go

@@ -18,6 +18,9 @@ type ResponseRecorder struct {
 	Body      *bytes.Buffer // if non-nil, the bytes.Buffer to append written data to
 	Flushed   bool
 
+	stagingMap http.Header // map that handlers manipulate to set headers
+	trailerMap http.Header // lazily filled when Trailers() is called
+
 	wroteHeader bool
 }
 
@@ -36,10 +39,10 @@ const DefaultRemoteAddr = "1.2.3.4"
 
 // Header returns the response headers.
 func (rw *ResponseRecorder) Header() http.Header {
-	m := rw.HeaderMap
+	m := rw.stagingMap
 	if m == nil {
 		m = make(http.Header)
-		rw.HeaderMap = m
+		rw.stagingMap = m
 	}
 	return m
 }
@@ -59,16 +62,15 @@ func (rw *ResponseRecorder) writeHeader(b []byte, str string) {
 		str = str[:512]
 	}
 
-	_, hasType := rw.HeaderMap["Content-Type"]
-	hasTE := rw.HeaderMap.Get("Transfer-Encoding") != ""
+	m := rw.Header()
+
+	_, hasType := m["Content-Type"]
+	hasTE := m.Get("Transfer-Encoding") != ""
 	if !hasType && !hasTE {
 		if b == nil {
 			b = []byte(str)
 		}
-		if rw.HeaderMap == nil {
-			rw.HeaderMap = make(http.Header)
-		}
-		rw.HeaderMap.Set("Content-Type", http.DetectContentType(b))
+		m.Set("Content-Type", http.DetectContentType(b))
 	}
 
 	rw.WriteHeader(200)
@@ -92,11 +94,21 @@ func (rw *ResponseRecorder) WriteString(str string) (int, error) {
 	return len(str), nil
 }
 
-// WriteHeader sets rw.Code.
+// WriteHeader sets rw.Code. After it is called, changing rw.Header
+// will not affect rw.HeaderMap.
 func (rw *ResponseRecorder) WriteHeader(code int) {
-	if !rw.wroteHeader {
-		rw.Code = code
-		rw.wroteHeader = true
+	if rw.wroteHeader {
+		return
+	}
+	rw.Code = code
+	rw.wroteHeader = true
+	if rw.HeaderMap == nil {
+		rw.HeaderMap = make(http.Header)
+	}
+	for k, vv := range rw.stagingMap {
+		vv2 := make([]string, len(vv))
+		copy(vv2, vv)
+		rw.HeaderMap[k] = vv2
 	}
 }
 
@@ -107,3 +119,33 @@ func (rw *ResponseRecorder) Flush() {
 	}
 	rw.Flushed = true
 }
+
+// Trailers returns any trailers set by the handler. It must be called
+// after the handler finished running.
+func (rw *ResponseRecorder) Trailers() http.Header {
+	if rw.trailerMap != nil {
+		return rw.trailerMap
+	}
+	trailers, ok := rw.HeaderMap["Trailer"]
+	if !ok {
+		rw.trailerMap = make(http.Header)
+		return rw.trailerMap
+	}
+	rw.trailerMap = make(http.Header, len(trailers))
+	for _, k := range trailers {
+		switch k {
+		case "Transfer-Encoding", "Content-Length", "Trailer":
+			// Ignore since forbidden by RFC 2616 14.40.
+			continue
+		}
+		k = http.CanonicalHeaderKey(k)
+		vv, ok := rw.stagingMap[k]
+		if !ok {
+			continue
+		}
+		vv2 := make([]string, len(vv))
+		copy(vv2, vv)
+		rw.trailerMap[k] = vv2
+	}
+	return rw.trailerMap
+}

src/net/http/httptest/recorder_test.go

@@ -47,6 +47,37 @@ func TestRecorder(t *testing.T) {
 			return nil
 		}
 	}
+	hasNotHeaders := func(keys ...string) checkFunc {
+		return func(rec *ResponseRecorder) error {
+			for _, k := range keys {
+				_, ok := rec.HeaderMap[http.CanonicalHeaderKey(k)]
+				if ok {
+					return fmt.Errorf("unexpected header %s", k)
+				}
+			}
+			return nil
+		}
+	}
+	hasTrailer := func(key, want string) checkFunc {
+		return func(rec *ResponseRecorder) error {
+			if got := rec.Trailers().Get(key); got != want {
+				return fmt.Errorf("trailer %s = %q; want %q", key, got, want)
+			}
+			return nil
+		}
+	}
+	hasNotTrailers := func(keys ...string) checkFunc {
+		return func(rec *ResponseRecorder) error {
+			trailers := rec.Trailers()
+			for _, k := range keys {
+				_, ok := trailers[http.CanonicalHeaderKey(k)]
+				if ok {
+					return fmt.Errorf("unexpected trailer %s", k)
+				}
+			}
+			return nil
+		}
+	}
 
 	tests := []struct {
 		name   string
@@ -130,6 +161,39 @@ func TestRecorder(t *testing.T) {
 			},
 			check(hasHeader("Content-Type", "text/html; charset=utf-8")),
 		},
+		{
+			"Header is not changed after write",
+			func(w http.ResponseWriter, r *http.Request) {
+				hdr := w.Header()
+				hdr.Set("Key", "correct")
+				w.WriteHeader(200)
+				hdr.Set("Key", "incorrect")
+			},
+			check(hasHeader("Key", "correct")),
+		},
+		{
+			"Trailer headers are correctly recorded",
+			func(w http.ResponseWriter, r *http.Request) {
+				w.Header().Set("Non-Trailer", "correct")
+				w.Header().Set("Trailer", "Trailer-A")
+				w.Header().Add("Trailer", "Trailer-B")
+				w.Header().Add("Trailer", "Trailer-C")
+				io.WriteString(w, "<html>")
+				w.Header().Set("Non-Trailer", "incorrect")
+				w.Header().Set("Trailer-A", "valuea")
+				w.Header().Set("Trailer-C", "valuec")
+				w.Header().Set("Trailer-NotDeclared", "should be omitted")
+			},
+			check(
+				hasStatus(200),
+				hasHeader("Content-Type", "text/html; charset=utf-8"),
+				hasHeader("Non-Trailer", "correct"),
+				hasNotHeaders("Trailer-A", "Trailer-B", "Trailer-C", "Trailer-NotDeclared"),
+				hasTrailer("Trailer-A", "valuea"),
+				hasTrailer("Trailer-C", "valuec"),
+				hasNotTrailers("Non-Trailer", "Trailer-B", "Trailer-NotDeclared"),
+			),
+		},
 	}
 	r, _ := http.NewRequest("GET", "http://foo.com/", nil)
 	for _, tt := range tests {