os: do not interpret 0-length read as EOF

Fixes #2402.

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5298081

src/pkg/os/file.go

@@ -69,7 +69,7 @@ func (file *File) Read(b []byte) (n int, err Error) {
 	if n < 0 {
 		n = 0
 	}
-	if n == 0 && !iserror(e) {
+	if n == 0 && len(b) > 0 && !iserror(e) {
 		return 0, EOF
 	}
 	if iserror(e) {

src/pkg/os/os_test.go

@@ -165,6 +165,27 @@ func TestLstat(t *testing.T) {
 	}
 }
 
+// Read with length 0 should not return EOF.
+func TestRead0(t *testing.T) {
+	path := sfdir + "/" + sfname
+	f, err := Open(path)
+	if err != nil {
+		t.Fatal("open failed:", err)
+	}
+	defer f.Close()
+
+	b := make([]byte, 0)
+	n, err := f.Read(b)
+	if n != 0 || err != nil {
+		t.Errorf("Read(0) = %d, %v, want 0, nil", n, err)
+	}
+	b = make([]byte, 100)
+	n, err = f.Read(b)
+	if n <= 0 || err != nil {
+		t.Errorf("Read(100) = %d, %v, want >0, nil", n, err)
+	}
+}
+
 func testReaddirnames(dir string, contents []string, t *testing.T) {
 	file, err := Open(dir)
 	defer file.Close()