net/http: add MaxBytesHandler

Fixes #39567

Change-Id: I226089b678a6a13d7ce69f360a23fc5bd297d550
GitHub-Last-Rev: 6435fd5881
GitHub-Pull-Request: golang/go#48104
Reviewed-on: https://go-review.googlesource.com/c/go/+/346569
Trust: Damien Neil <dneil@google.com>
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

src/net/http/serve_test.go

@@ -6682,3 +6682,63 @@ func testQuerySemicolon(t *testing.T, query string, wantX string, allowSemicolon
 		}
 	}
 }
+
+func TestMaxBytesHandler(t *testing.T) {
+	setParallel(t)
+	defer afterTest(t)
+
+	for _, maxSize := range []int64{100, 1_000, 1_000_000} {
+		for _, requestSize := range []int64{100, 1_000, 1_000_000} {
+			t.Run(fmt.Sprintf("max size %d request size %d", maxSize, requestSize),
+				func(t *testing.T) {
+					testMaxBytesHandler(t, maxSize, requestSize)
+				})
+		}
+	}
+}
+
+func testMaxBytesHandler(t *testing.T, maxSize, requestSize int64) {
+	var (
+		handlerN   int64
+		handlerErr error
+	)
+	echo := HandlerFunc(func(w ResponseWriter, r *Request) {
+		var buf bytes.Buffer
+		handlerN, handlerErr = io.Copy(&buf, r.Body)
+		io.Copy(w, &buf)
+	})
+
+	ts := httptest.NewServer(MaxBytesHandler(echo, maxSize))
+	defer ts.Close()
+
+	c := ts.Client()
+	var buf strings.Builder
+	body := strings.NewReader(strings.Repeat("a", int(requestSize)))
+	res, err := c.Post(ts.URL, "text/plain", body)
+	if err != nil {
+		t.Errorf("unexpected connection error: %v", err)
+	} else {
+		_, err = io.Copy(&buf, res.Body)
+		res.Body.Close()
+		if err != nil {
+			t.Errorf("unexpected read error: %v", err)
+		}
+	}
+	if handlerN > maxSize {
+		t.Errorf("expected max request body %d; got %d", maxSize, handlerN)
+	}
+	if requestSize > maxSize && handlerErr == nil {
+		t.Error("expected error on handler side; got nil")
+	}
+	if requestSize <= maxSize {
+		if handlerErr != nil {
+			t.Errorf("%d expected nil error on handler side; got %v", requestSize, handlerErr)
+		}
+		if handlerN != requestSize {
+			t.Errorf("expected request of size %d; got %d", requestSize, handlerN)
+		}
+	}
+	if buf.Len() != int(handlerN) {
+		t.Errorf("expected echo of size %d; got %d", handlerN, buf.Len())
+	}
+}

src/net/http/server.go

@@ -3610,3 +3610,12 @@ func tlsRecordHeaderLooksLikeHTTP(hdr [5]byte) bool {
 	}
 	return false
 }
+
+// MaxBytesHandler returns a Handler that runs h with its ResponseWriter and Request.Body wrapped by a MaxBytesReader.
+func MaxBytesHandler(h Handler, n int64) Handler {
+	return HandlerFunc(func(w ResponseWriter, r *Request) {
+		r2 := *r
+		r2.Body = MaxBytesReader(w, r.Body, n)
+		h.ServeHTTP(w, &r2)
+	})
+}