Extract checkPerm to package-level function, resolve perms in open

- checkPerm no longer needs *Server: it takes uid, perm, and inGroup
  directly. The caller (open) resolves uid/gid via fileOwnerGroup,
  perm via makeStat, and group membership via InGroup.
- checkPerm is now a pure function: (uname, mode, uid, perm, inGroup)
  → returns only on permissions, needs nothing from Server state.
- open is the remaining method that resolves these values.

server.go: 10 remaining Server methods.

cmd/olliesrv/server.go

@@ -144,14 +144,11 @@ func fileOwnerGroup(root *fs.Tree, path string) (uid, gid string) {
 	return "ollie", "agent"
 }
 
-func (s *Server) checkPerm(uname, path string, mode uint8) error {
-	uid, gid := fileOwnerGroup(s.rootTree, path)
-	dir := makeStat(s.rootTree, path)
-	perm := uint32(dir.Mode) & 0777
+func checkPerm(uname string, mode uint8, uid string, perm uint32, inGroup bool) error {
 	var bits uint32
 	if uname == uid {
 		bits = (perm >> 6) & 7
-	} else if s.InGroup(gid, uname) {
+	} else if inGroup {
 		bits = (perm >> 3) & 7
 	} else {
 		bits = perm & 7
@@ -415,7 +412,8 @@ func (s *Server) open(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
 	if !ok {
 		return errFcall(fc, "bad fid")
 	}
-	if err := s.checkPerm(cs.uname, f.path, fc.Mode); err != nil {
+	uid, gid := fileOwnerGroup(s.rootTree, f.path)
+	if err := checkPerm(cs.uname, fc.Mode, uid, uint32(makeStat(s.rootTree, f.path).Mode)&0777, s.InGroup(gid, cs.uname)); err != nil {
 		return errFcall(fc, err.Error())
 	}
 	f.mode = fc.Mode