etcdserver: Check whether node is a directory

If dir=true and prevExist=true PUT only if the target node is a
directory. Otherwise return "Not a directory".

Addresses etcd-io#518 on github.

etcdserver/server.go

@@ -30,6 +30,7 @@ import (
 	"time"
 
 	"github.com/coreos/etcd/discovery"
+	etcdErr "github.com/coreos/etcd/error"
 	"github.com/coreos/etcd/etcdserver/etcdhttp/httptypes"
 	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
 	"github.com/coreos/etcd/etcdserver/stats"
@@ -695,6 +696,9 @@ func (s *EtcdServer) applyRequest(r pb.Request) Response {
 		switch {
 		case existsSet:
 			if exists {
+				if r.Dir && !s.store.IsKeyDir(r.Path) {
+					return f(nil, etcdErr.NewError(etcdErr.EcodeNotDir, r.Path, s.store.Index()))
+				}
 				return f(s.store.Update(r.Path, r.Val, expr))
 			}
 			return f(s.store.Create(r.Path, r.Dir, r.Val, false, expr))