net/http, strings, bytes: fix http race, revert part of Reader behavior change

I fixed this data race regression in two ways: in net/http itself, and also
partially reverting the change from https://golang.org/cl/77580046 .
Previously a Read from a strings.Reader or bytes.Reader returning 0 bytes
would not be a memory write. After 77580046 it was. This reverts that back
in case others depended on that. Also adds tests.

Fixes #7856

LGTM=ruiu, iant
R=iant, ruiu
CC=golang-codereviews, gri
https://golang.org/cl/94740044

src/pkg/bytes/reader.go

@@ -30,13 +30,13 @@ func (r *Reader) Len() int {
 }
 
 func (r *Reader) Read(b []byte) (n int, err error) {
-	r.prevRune = -1
 	if len(b) == 0 {
 		return 0, nil
 	}
 	if r.i >= int64(len(r.s)) {
 		return 0, io.EOF
 	}
+	r.prevRune = -1
 	n = copy(b, r.s[r.i:])
 	r.i += int64(n)
 	return

src/pkg/bytes/reader_test.go

@@ -115,6 +115,27 @@ func TestReaderAtConcurrent(t *testing.T) {
 	wg.Wait()
 }
 
+func TestEmptyReaderConcurrent(t *testing.T) {
+	// Test for the race detector, to verify a Read that doesn't yield any bytes
+	// is okay to use from multiple goroutines. This was our historic behavior.
+	// See golang.org/issue/7856
+	r := NewReader([]byte{})
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			var buf [1]byte
+			r.Read(buf[:])
+		}()
+		go func() {
+			defer wg.Done()
+			r.Read(nil)
+		}()
+	}
+	wg.Wait()
+}
+
 func TestReaderWriteTo(t *testing.T) {
 	for i := 0; i < 30; i += 3 {
 		var l int
@@ -164,7 +185,7 @@ var UnreadRuneErrorTests = []struct {
 	name string
 	f    func(*Reader)
 }{
-	{"Read", func(r *Reader) { r.Read([]byte{}) }},
+	{"Read", func(r *Reader) { r.Read([]byte{0}) }},
 	{"ReadByte", func(r *Reader) { r.ReadByte() }},
 	{"UnreadRune", func(r *Reader) { r.UnreadRune() }},
 	{"Seek", func(r *Reader) { r.Seek(0, 1) }},

src/pkg/net/http/serve_test.go

@@ -2461,6 +2461,39 @@ func TestServerKeepAlivesEnabled(t *testing.T) {
 	}
 }
 
+// golang.org/issue/7856
+func TestServerEmptyBodyRace(t *testing.T) {
+	defer afterTest(t)
+	var n int32
+	ts := httptest.NewServer(HandlerFunc(func(rw ResponseWriter, req *Request) {
+		atomic.AddInt32(&n, 1)
+	}))
+	defer ts.Close()
+	var wg sync.WaitGroup
+	const reqs = 20
+	for i := 0; i < reqs; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			res, err := Get(ts.URL)
+			if err != nil {
+				t.Error(err)
+				return
+			}
+			defer res.Body.Close()
+			_, err = io.Copy(ioutil.Discard, res.Body)
+			if err != nil {
+				t.Error(err)
+				return
+			}
+		}()
+	}
+	wg.Wait()
+	if got := atomic.LoadInt32(&n); got != reqs {
+		t.Errorf("handler ran %d times; want %d", got, reqs)
+	}
+}
+
 func TestServerConnStateNew(t *testing.T) {
 	sawNew := false // if the test is buggy, we'll race on this variable.
 	srv := &Server{

src/pkg/net/http/server.go

@@ -1971,17 +1971,24 @@ func (globalOptionsHandler) ServeHTTP(w ResponseWriter, r *Request) {
 	}
 }
 
+type eofReaderWithWriteTo struct{}
+
+func (eofReaderWithWriteTo) WriteTo(io.Writer) (int64, error) { return 0, nil }
+func (eofReaderWithWriteTo) Read([]byte) (int, error)         { return 0, io.EOF }
+
 // eofReader is a non-nil io.ReadCloser that always returns EOF.
-// It embeds a *strings.Reader so it still has a WriteTo method
-// and io.Copy won't need a buffer.
+// It has a WriteTo method so io.Copy won't need a buffer.
 var eofReader = &struct {
-	*strings.Reader
+	eofReaderWithWriteTo
 	io.Closer
 }{
-	strings.NewReader(""),
+	eofReaderWithWriteTo{},
 	ioutil.NopCloser(nil),
 }
 
+// Verify that an io.Copy from an eofReader won't require a buffer.
+var _ io.WriterTo = eofReader
+
 // initNPNRequest is an HTTP handler that initializes certain
 // uninitialized fields in its *Request. Such partially-initialized
 // Requests come from NPN protocol handlers.

src/pkg/strings/reader.go

@@ -29,13 +29,13 @@ func (r *Reader) Len() int {
 }
 
 func (r *Reader) Read(b []byte) (n int, err error) {
-	r.prevRune = -1
 	if len(b) == 0 {
 		return 0, nil
 	}
 	if r.i >= int64(len(r.s)) {
 		return 0, io.EOF
 	}
+	r.prevRune = -1
 	n = copy(b, r.s[r.i:])
 	r.i += int64(n)
 	return

src/pkg/strings/reader_test.go

@@ -115,6 +115,27 @@ func TestReaderAtConcurrent(t *testing.T) {
 	wg.Wait()
 }
 
+func TestEmptyReaderConcurrent(t *testing.T) {
+	// Test for the race detector, to verify a Read that doesn't yield any bytes
+	// is okay to use from multiple goroutines. This was our historic behavior.
+	// See golang.org/issue/7856
+	r := strings.NewReader("")
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			var buf [1]byte
+			r.Read(buf[:])
+		}()
+		go func() {
+			defer wg.Done()
+			r.Read(nil)
+		}()
+	}
+	wg.Wait()
+}
+
 func TestWriteTo(t *testing.T) {
 	const str = "0123456789"
 	for i := 0; i <= len(str); i++ {

src/pkg/strings/strings_test.go

@@ -862,7 +862,7 @@ var UnreadRuneErrorTests = []struct {
 	name string
 	f    func(*Reader)
 }{
-	{"Read", func(r *Reader) { r.Read([]byte{}) }},
+	{"Read", func(r *Reader) { r.Read([]byte{0}) }},
 	{"ReadByte", func(r *Reader) { r.ReadByte() }},
 	{"UnreadRune", func(r *Reader) { r.UnreadRune() }},
 	{"Seek", func(r *Reader) { r.Seek(0, 1) }},