From 2a3b16e184331501eaa2e3ed626642f1edf9a410 Mon Sep 17 00:00:00 2001
From: Leo Antunes <leo@costela.net>
Date: Sun, 30 Oct 2022 10:15:27 +0000
Subject: [PATCH] net/http: use Copy in ServeContent if CopyN not needed

This small PR allows optimizations made in io.Copy (like the use of
io.WriterTo) to be used in one possible path of http.ServeContent
(in case of a non-Range request).
This, in turn, allows us to skip the buffer allocation in io.Copy.

Change-Id: Ifa2ece206ecd4556aaaed15d663b65e95e00bb0a
GitHub-Last-Rev: 94fc0318145ba1bd48502564f6488aade871c301
GitHub-Pull-Request: golang/go#56480
Reviewed-on: https://go-review.googlesource.com/c/go/+/446276
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
---
 src/net/http/fs.go      |  9 +++++++--
 src/net/http/fs_test.go | 43 +++++++++++++++++++++++++++++++++++++++--
 2 files changed, 48 insertions(+), 4 deletions(-)

diff --git a/src/net/http/fs.go b/src/net/http/fs.go
index 83459046bf..7f302491ab 100644
--- a/src/net/http/fs.go
+++ b/src/net/http/fs.go
@@ -349,8 +349,13 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 
 	w.WriteHeader(code)
 
-	if r.Method != "HEAD" {
-		io.CopyN(w, sendContent, sendSize)
+	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)
+		}
 	}
 }
 
diff --git a/src/net/http/fs_test.go b/src/net/http/fs_test.go
index 74f7a80e27..ce42920123 100644
--- a/src/net/http/fs_test.go
+++ b/src/net/http/fs_test.go
@@ -937,6 +937,7 @@ 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{
@@ -1152,6 +1153,24 @@ 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
@@ -1165,7 +1184,8 @@ func testServeContent(t *testing.T, mode testMode) {
 		} else {
 			content = tt.content
 		}
-		for _, method := range []string{"GET", "HEAD"} {
+		contentOut := &strings.Builder{}
+		for _, method := range []string{MethodGet, MethodHead} {
 			//restore content in case it is consumed by previous method
 			if content, ok := content.(*strings.Reader); ok {
 				content.Seek(0, io.SeekStart)
@@ -1191,7 +1211,8 @@ func testServeContent(t *testing.T, mode testMode) {
 			if err != nil {
 				t.Fatal(err)
 			}
-			io.Copy(io.Discard, res.Body)
+			contentOut.Reset()
+			io.Copy(contentOut, 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)
@@ -1205,10 +1226,28 @@ 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()
-- 
2.50.0