os: another attempt to handle OpenFile flag parameter properly on Windows

Fixes #1791.

R=rsc, r, r, iant
CC=golang-dev
https://golang.org/cl/4551046

src/pkg/os/os_test.go

@@ -918,7 +918,15 @@ func TestAppend(t *testing.T) {
 	}
 	s = writeFile(t, f, O_CREATE|O_APPEND|O_RDWR, "new&append")
 	if s != "new&append" {
-		t.Fatalf("writeFile: have %q want %q", s, "new&append")
+		t.Fatalf("writeFile: after append have %q want %q", s, "new&append")
+	}
+	s = writeFile(t, f, O_CREATE|O_RDWR, "old")
+	if s != "old&append" {
+		t.Fatalf("writeFile: after create have %q want %q", s, "old&append")
+	}
+	s = writeFile(t, f, O_CREATE|O_TRUNC|O_RDWR, "new")
+	if s != "new" {
+		t.Fatalf("writeFile: after truncate have %q want %q", s, "new")
 	}
 }
 

src/pkg/syscall/syscall_windows.go

@@ -230,16 +230,13 @@ func Open(path string, mode int, perm uint32) (fd int, errno int) {
 	}
 	var createmode uint32
 	switch {
-	case mode&O_CREAT != 0:
-		switch {
-		case mode&O_EXCL != 0:
+	case mode&(O_CREAT|O_EXCL) == (O_CREAT | O_EXCL):
 		createmode = CREATE_NEW
-		case mode&O_APPEND != 0:
-			createmode = OPEN_ALWAYS
-		default:
+	case mode&(O_CREAT|O_TRUNC) == (O_CREAT | O_TRUNC):
 		createmode = CREATE_ALWAYS
-		}
-	case mode&O_TRUNC != 0:
+	case mode&O_CREAT == O_CREAT:
+		createmode = OPEN_ALWAYS
+	case mode&O_TRUNC == O_TRUNC:
 		createmode = TRUNCATE_EXISTING
 	default:
 		createmode = OPEN_EXISTING