tests: add OpenAt test

Tests that openat(2) works as expected when the parent
directory is renamed (it does).

Originally written to get a FUSE log about what happens
when userspace calls openat, but it might make sense to
just keep it as a test.

Change-Id: I3fac815a7c0f713b4f859f6876d60380df9bc953

posixtest/test.go

@@ -37,6 +37,7 @@ var All = map[string]func(*testing.T, string){
 	"ReadDir":                    ReadDir,
 	"ReadDirPicksUpCreate":       ReadDirPicksUpCreate,
 	"DirectIO":                   DirectIO,
+	"OpenAt":                     OpenAt,
 }
 
 func DirectIO(t *testing.T, mnt string) {
@@ -505,3 +506,34 @@ func AppendWrite(t *testing.T, mnt string) {
 		t.Errorf("got %q want %q", got, want)
 	}
 }
+
+// OpenAt tests syscall.Openat().
+//
+// Hint:
+// $ go test ./fs -run TestPosix/OpenAt -v
+func OpenAt(t *testing.T, mnt string) {
+	dir1 := mnt + "/dir1"
+	err := os.Mkdir(dir1, 0777)
+	if err != nil {
+		t.Fatal(err)
+	}
+	dirfd, err := syscall.Open(dir1, syscall.O_RDONLY, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer syscall.Close(dirfd)
+	dir2 := mnt + "/dir2"
+	err = os.Rename(dir1, dir2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	fd, err := syscall.Openat(dirfd, "file1", syscall.O_CREAT, 0700)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer syscall.Close(fd)
+	_, err = os.Stat(dir2 + "/file1")
+	if err != nil {
+		t.Error(err)
+	}
+}