io: NopCloser forward WriterTo implementations if the reader supports it

The io_test.go don't bother to test actually reading or WritingTo of the
NopCloser because the logic is simple.
It didn't even had direct tests before.

Fixes #51566

src/io/io.go

@@ -621,7 +621,12 @@ func (discard) ReadFrom(r Reader) (n int64, err error) {
 
 // NopCloser returns a ReadCloser with a no-op Close method wrapping
 // the provided Reader r.
+// If r implements WriterTo, the returned ReadCloser will implement WriterTo
+// by forwarding calls to r.
 func NopCloser(r Reader) ReadCloser {
+	if _, ok := r.(WriterTo); ok {
+		return nopCloserWriterTo{r}
+	}
 	return nopCloser{r}
 }
 
@@ -631,6 +636,16 @@ type nopCloser struct {
 
 func (nopCloser) Close() error { return nil }
 
+type nopCloserWriterTo struct {
+	Reader
+}
+
+func (nopCloserWriterTo) Close() error { return nil }
+
+func (c nopCloserWriterTo) WriteTo(w Writer) (n int64, err error) {
+	return c.Reader.(WriterTo).WriteTo(w)
+}
+
 // ReadAll reads from r until an error or EOF and returns the data it read.
 // A successful call returns err == nil, not err == EOF. Because ReadAll is
 // defined to read from src until EOF, it does not treat an EOF from Read

src/io/io_test.go

@@ -471,3 +471,24 @@ func TestCopyLargeWriter(t *testing.T) {
 		t.Errorf("Copy error: got %v, want %v", err, want)
 	}
 }
+
+func TestNopCloserWriterToForwarding(t *testing.T) {
+	for _, tc := range [...]struct {
+		Name string
+		r    Reader
+	}{
+		{"not a WriterTo", Reader(nil)},
+		{"a WriterTo", struct {
+			Reader
+			WriterTo
+		}{}},
+	} {
+		nc := NopCloser(tc.r)
+
+		_, expected := tc.r.(WriterTo)
+		_, got := nc.(WriterTo)
+		if expected != got {
+			t.Errorf("NopCloser incorrectly forwards WriterTo for %s, got %t want %t", tc.Name, got, expected)
+		}
+	}
+}

src/net/http/transfer.go

@@ -422,8 +422,8 @@ func (t *transferWriter) doBodyCopy(dst io.Writer, src io.Reader) (n int64, err
 //
 // This function is only intended for use in writeBody.
 func (t *transferWriter) unwrapBody() io.Reader {
-	if reflect.TypeOf(t.Body) == nopCloserType {
-		return reflect.ValueOf(t.Body).Field(0).Interface().(io.Reader)
+	if r, ok := unwrapNopCloser(t.Body); ok {
+		return r
 	}
 	if r, ok := t.Body.(*readTrackingBody); ok {
 		r.didRead = true
@@ -1081,6 +1081,21 @@ func (fr finishAsyncByteRead) Read(p []byte) (n int, err error) {
 }
 
 var nopCloserType = reflect.TypeOf(io.NopCloser(nil))
+var nopCloserWriterToType = reflect.TypeOf(io.NopCloser(struct {
+	io.Reader
+	io.WriterTo
+}{}))
+
+// unwrapNopCloser return the underlying reader and true if r is a NopCloser
+// else it return false
+func unwrapNopCloser(r io.Reader) (underlyingReader io.Reader, isNopCloser bool) {
+	switch reflect.TypeOf(r) {
+	case nopCloserType, nopCloserWriterToType:
+		return reflect.ValueOf(r).Field(0).Interface().(io.Reader), true
+	default:
+		return nil, false
+	}
+}
 
 // isKnownInMemoryReader reports whether r is a type known to not
 // block on Read. Its caller uses this as an optional optimization to
@@ -1090,8 +1105,8 @@ func isKnownInMemoryReader(r io.Reader) bool {
 	case *bytes.Reader, *bytes.Buffer, *strings.Reader:
 		return true
 	}
-	if reflect.TypeOf(r) == nopCloserType {
-		return isKnownInMemoryReader(reflect.ValueOf(r).Field(0).Interface().(io.Reader))
+	if r, ok := unwrapNopCloser(r); ok {
+		return isKnownInMemoryReader(r)
 	}
 	if r, ok := r.(*readTrackingBody); ok {
 		return isKnownInMemoryReader(r.ReadCloser)