os: return io.EOF from windows ReadAt as documented

Fixes #5619.

R=golang-dev, r, peter.armitage, go.peter.90
CC=golang-dev
https://golang.org/cl/9952044

src/pkg/os/file_windows.go

@@ -312,6 +312,10 @@ func (f *File) pread(b []byte, off int64) (n int, err error) {
 	var done uint32
 	e = syscall.ReadFile(syscall.Handle(f.fd), b, &done, &o)
 	if e != nil {
+		if e == syscall.ERROR_HANDLE_EOF {
+			// end of file
+			return 0, nil
+		}
 		return 0, e
 	}
 	return int(done), nil

src/pkg/os/os_test.go

@@ -1114,3 +1114,19 @@ func TestStatDirModeExec(t *testing.T) {
 		t.Errorf("Stat %q: mode %#o want %#o", path, dir.Mode()&mode, mode)
 	}
 }
+
+func TestReadAtEOF(t *testing.T) {
+	f := newFile("TestReadAtEOF", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	_, err := f.ReadAt(make([]byte, 10), 0)
+	switch err {
+	case io.EOF:
+		// all good
+	case nil:
+		t.Fatalf("ReadAt succeeded")
+	default:
+		t.Fatalf("ReadAt failed: %s", err)
+	}
+}

src/pkg/syscall/ztypes_windows.go

@@ -10,6 +10,7 @@ const (
 	ERROR_PATH_NOT_FOUND      Errno = 3
 	ERROR_ACCESS_DENIED       Errno = 5
 	ERROR_NO_MORE_FILES       Errno = 18
+	ERROR_HANDLE_EOF          Errno = 38
 	ERROR_FILE_EXISTS         Errno = 80
 	ERROR_BROKEN_PIPE         Errno = 109
 	ERROR_BUFFER_OVERFLOW     Errno = 111