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

3 files changed, 23 insertions, 2 deletions

diff --git a/src/pkg/strings/reader.go b/src/pkg/strings/reader.go
index ee83ceb505..82df974398 100644
--- a/src/pkg/strings/reader.go
+++ b/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
diff --git a/src/pkg/strings/reader_test.go b/src/pkg/strings/reader_test.go
index 4d95355af7..bee90eb258 100644
--- a/src/pkg/strings/reader_test.go
+++ b/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++ {
diff --git a/src/pkg/strings/strings_test.go b/src/pkg/strings/strings_test.go
index 95a42019a3..e40a18015e 100644
--- a/src/pkg/strings/strings_test.go
+++ b/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) }},