io: NopCloser forward WriterTo implementations if the reader supports it

This patch also include related fixes to net/http.

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

Fixes #51566

Change-Id: I1943ee2c20d0fe749f4d04177342ce6eca443efe
GitHub-Last-Rev: a6b9af4e945a6903735a74aa185e2d1c4c2e2cef
GitHub-Pull-Request: golang/go#52340
Reviewed-on: https://go-review.googlesource.com/c/go/+/400236
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

2 files changed, 36 insertions, 0 deletions

diff --git a/src/io/io.go b/src/io/io.go
index 1ea01d5d63..db88125f50 100644
--- a/src/io/io.go
+++ b/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
diff --git a/src/io/io_test.go b/src/io/io_test.go
index 3088460480..a51a1fa160 100644
--- a/src/io/io_test.go
+++ b/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)
+		}
+	}
+}