net/http: don't write HEAD response body in ResponseWriter.ReadFrom

Responses to HEAD requests don't have a body.

The ResponseWriter automatically discards writes to the response body
when responding to a HEAD request. ResponseWriter.ReadFrom was failing
to discard writes under some circumstances; fix it to do so.

Fixes #68609

Change-Id: I912f6b2b2a535df28ae37b875fcf15b10da1af2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/601475
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/net/http/serve_test.go

@@ -1510,7 +1510,7 @@ func testHeadResponses(t *testing.T, mode testMode) {
 		}
 
 		// Also exercise the ReaderFrom path
-		_, err = io.Copy(w, strings.NewReader("789a"))
+		_, err = io.Copy(w, struct{ io.Reader }{strings.NewReader("789a")})
 		if err != nil {
 			t.Errorf("Copy(ResponseWriter, ...): %v", err)
 		}
@@ -1537,6 +1537,34 @@ func testHeadResponses(t *testing.T, mode testMode) {
 	}
 }
 
+// Ensure ResponseWriter.ReadFrom doesn't write a body in response to a HEAD request.
+// https://go.dev/issue/68609
+func TestHeadReaderFrom(t *testing.T) { run(t, testHeadReaderFrom, []testMode{http1Mode}) }
+func testHeadReaderFrom(t *testing.T, mode testMode) {
+	// Body is large enough to exceed the content-sniffing length.
+	wantBody := strings.Repeat("a", 4096)
+	cst := newClientServerTest(t, mode, HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.(io.ReaderFrom).ReadFrom(strings.NewReader(wantBody))
+	}))
+	res, err := cst.c.Head(cst.ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+	res, err = cst.c.Get(cst.ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	gotBody, err := io.ReadAll(res.Body)
+	res.Body.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(gotBody) != wantBody {
+		t.Errorf("got unexpected body len=%v, want %v", len(gotBody), len(wantBody))
+	}
+}
+
 func TestTLSHandshakeTimeout(t *testing.T) {
 	run(t, testTLSHandshakeTimeout, []testMode{https1Mode, http2Mode})
 }

src/net/http/server.go

@@ -611,7 +611,7 @@ func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
 	w.cw.flush() // make sure Header is written; flush data to rwc
 
 	// Now that cw has been flushed, its chunking field is guaranteed initialized.
-	if !w.cw.chunking && w.bodyAllowed() {
+	if !w.cw.chunking && w.bodyAllowed() && w.req.Method != "HEAD" {
 		n0, err := rf.ReadFrom(src)
 		n += n0
 		w.written += n0