os: Open with O_APPEND|O_CREATE to append to the end of file on Windows

Credit for the fix goes to Hector, test by PeterGo.

Fixes #1655.

R=golang-dev, rsc1, peterGo
CC=golang-dev, hector
https://golang.org/cl/4436051

src/pkg/os/os_test.go

@@ -886,6 +886,18 @@ func TestAppend(t *testing.T) {
 	if s != "new|append" {
 		t.Fatalf("writeFile: have %q want %q", s, "new|append")
 	}
+	s = writeFile(t, f, O_CREATE|O_APPEND|O_RDWR, "|append")
+	if s != "new|append|append" {
+		t.Fatalf("writeFile: have %q want %q", s, "new|append|append")
+	}
+	err := Remove(f)
+	if err != nil {
+		t.Fatalf("Remove: %v", err)
+	}
+	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")
+	}
 }
 
 func TestStatDirWithTrailingSlash(t *testing.T) {

src/pkg/syscall/syscall_windows.go

@@ -220,9 +220,12 @@ func Open(path string, mode int, perm uint32) (fd int, errno int) {
 	var createmode uint32
 	switch {
 	case mode&O_CREAT != 0:
-		if mode&O_EXCL != 0 {
+		switch {
+		case mode&O_EXCL != 0:
 			createmode = CREATE_NEW
-		} else {
+		case mode&O_APPEND != 0:
+			createmode = OPEN_ALWAYS
+		default:
 			createmode = CREATE_ALWAYS
 		}
 	case mode&O_TRUNC != 0: