net/http: use Copy in ServeContent if CopyN not needed

src/net/http/fs.go

@@ -374,8 +374,13 @@ 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)
+		}
 	}
 }
 

src/net/http/fs_test.go

@@ -928,6 +928,7 @@ 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{
@@ -1143,6 +1144,24 @@ 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
@@ -1156,7 +1175,8 @@ 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)
@@ -1182,7 +1202,8 @@ 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)
@@ -1196,10 +1217,28 @@ 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()

src/net/sendfile_linux.go

@@ -7,7 +7,7 @@ package net
 import (
 	"internal/poll"
 	"io"
-	"os"
+	"syscall"
 )
 
 // sendFile copies the contents of r to c using the sendfile
@@ -27,7 +27,10 @@ func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
 			return 0, nil, true
 		}
 	}
-	f, ok := r.(*os.File)
+	f, ok := r.(interface {
+		Fd() uintptr // not used, but limits the type to *os.File
+		SyscallConn() (syscall.RawConn, error)
+	})
 	if !ok {
 		return 0, nil, false
 	}

src/net/sendfile_unix_alt.go

@@ -9,7 +9,8 @@ package net
 import (
 	"internal/poll"
 	"io"
-	"os"
+	"io/fs"
+	"syscall"
 )
 
 // sendFile copies the contents of r to c using the sendfile
@@ -34,7 +35,12 @@ func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
 			return 0, nil, true
 		}
 	}
-	f, ok := r.(*os.File)
+	f, ok := r.(interface {
+		io.Seeker
+		Fd() uintptr // not used, but limits the type to *os.File
+		Stat() (fs.FileInfo, error)
+		SyscallConn() (syscall.RawConn, error)
+	})
 	if !ok {
 		return 0, nil, false
 	}

src/net/sendfile_windows.go

@@ -7,7 +7,6 @@ package net
 import (
 	"internal/poll"
 	"io"
-	"os"
 	"syscall"
 )
 
@@ -29,7 +28,9 @@ func sendFile(fd *netFD, r io.Reader) (written int64, err error, handled bool) {
 		}
 	}
 
-	f, ok := r.(*os.File)
+	f, ok := r.(interface {
+		Fd() uintptr
+	})
 	if !ok {
 		return 0, nil, false
 	}