[release-branch.go1.21] Revert "net/http: use Copy in ServeContent if CopyN not needed"

This reverts CL 446276.

Reason for revert: Causing surprising performance regression.

Fixes #61530

Change-Id: Ic970f2e05d875b606ce274ea621f7e4c8c337481
Reviewed-on: https://go-review.googlesource.com/c/go/+/512615
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit df0a1297899aff1c46b66523e75aa12b0ff5049f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/515795
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

2 files changed, 4 insertions, 48 deletions

diff --git a/src/net/http/fs.go b/src/net/http/fs.go
index 55094400ac..41e0b43ac8 100644
--- a/src/net/http/fs.go
+++ b/src/net/http/fs.go
@@ -349,13 +349,8 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 
 	w.WriteHeader(code)
 
-	if r.Method != MethodHead {
-		if sendSize == size {
-			// use Copy in the non-range case to make use of WriterTo if available
-			io.Copy(w, sendContent)
-		} else {
-			io.CopyN(w, sendContent, sendSize)
-		}
+	if r.Method != "HEAD" {
+		io.CopyN(w, sendContent, sendSize)
 	}
 }
 
diff --git a/src/net/http/fs_test.go b/src/net/http/fs_test.go
index e37e0f04c9..3fb9e01235 100644
--- a/src/net/http/fs_test.go
+++ b/src/net/http/fs_test.go
@@ -924,7 +924,6 @@ func testServeContent(t *testing.T, mode testMode) {
 		wantContentType  string
 		wantContentRange string
 		wantStatus       int
-		wantContent      []byte
 	}
 	htmlModTime := mustStat(t, "testdata/index.html").ModTime()
 	tests := map[string]testCase{
@@ -1140,24 +1139,6 @@ func testServeContent(t *testing.T, mode testMode) {
 			wantStatus:  412,
 			wantLastMod: htmlModTime.UTC().Format(TimeFormat),
 		},
-		"uses_writeTo_if_available_and_non-range": {
-			content:          &panicOnNonWriterTo{seekWriterTo: strings.NewReader("foobar")},
-			serveContentType: "text/plain; charset=utf-8",
-			wantContentType:  "text/plain; charset=utf-8",
-			wantStatus:       StatusOK,
-			wantContent:      []byte("foobar"),
-		},
-		"do_not_use_writeTo_for_range_requests": {
-			content:          &panicOnWriterTo{ReadSeeker: strings.NewReader("foobar")},
-			serveContentType: "text/plain; charset=utf-8",
-			reqHeader: map[string]string{
-				"Range": "bytes=0-4",
-			},
-			wantContentType:  "text/plain; charset=utf-8",
-			wantContentRange: "bytes 0-4/6",
-			wantStatus:       StatusPartialContent,
-			wantContent:      []byte("fooba"),
-		},
 	}
 	for testName, tt := range tests {
 		var content io.ReadSeeker
@@ -1171,8 +1152,7 @@ func testServeContent(t *testing.T, mode testMode) {
 		} else {
 			content = tt.content
 		}
-		contentOut := &strings.Builder{}
-		for _, method := range []string{MethodGet, MethodHead} {
+		for _, method := range []string{"GET", "HEAD"} {
 			//restore content in case it is consumed by previous method
 			if content, ok := content.(*strings.Reader); ok {
 				content.Seek(0, io.SeekStart)
@@ -1198,8 +1178,7 @@ func testServeContent(t *testing.T, mode testMode) {
 			if err != nil {
 				t.Fatal(err)
 			}
-			contentOut.Reset()
-			io.Copy(contentOut, res.Body)
+			io.Copy(io.Discard, res.Body)
 			res.Body.Close()
 			if res.StatusCode != tt.wantStatus {
 				t.Errorf("test %q using %q: got status = %d; want %d", testName, method, res.StatusCode, tt.wantStatus)
@@ -1213,28 +1192,10 @@ func testServeContent(t *testing.T, mode testMode) {
 			if g, e := res.Header.Get("Last-Modified"), tt.wantLastMod; g != e {
 				t.Errorf("test %q using %q: got last-modified = %q, want %q", testName, method, g, e)
 			}
-			if g, e := contentOut.String(), tt.wantContent; e != nil && method == MethodGet && g != string(e) {
-				t.Errorf("test %q using %q: got unexpected content %q, want %q", testName, method, g, e)
-			}
 		}
 	}
 }
 
-type seekWriterTo interface {
-	io.Seeker
-	io.WriterTo
-}
-
-type panicOnNonWriterTo struct {
-	io.Reader
-	seekWriterTo
-}
-
-type panicOnWriterTo struct {
-	io.ReadSeeker
-	io.WriterTo
-}
-
 // Issue 12991
 func TestServerFileStatError(t *testing.T) {
 	rec := httptest.NewRecorder()