zetcd: increment zxid on path validation failures

This commit implements the PerfectZXidMode functionality for failed path
validation, causing the ZXid of the server to be incremented on path validation
failures.

This brings the server in-line with upstream Zookeeper, which increments the
ZXid *before* potentially failing the request due to a bad path.

Building with PerfectZXidMode set to false turns the behavior into a noop.

zketcd.go

@@ -47,7 +47,11 @@ func (z *zkEtcd) Create(xid Xid, op *CreateRequest) ZKResponse {
 		zkPath = fmt.Sprintf("%s1", zkPath)
 	}
 	if err := validatePath(zkPath); err != nil {
-		return mkZKErr(xid, z.s.ZXid(), errBadArguments)
+		zxid, err := z.incrementAndGetZxid()
+		if err != nil {
+			return mkErr(err)
+		}
+		return mkZKErr(xid, zxid, errBadArguments)
 	}
 
 	opts := []etcd.OpOption{}
@@ -323,7 +327,11 @@ func (z *zkEtcd) GetData(xid Xid, op *GetDataRequest) ZKResponse {
 
 func (z *zkEtcd) SetData(xid Xid, op *SetDataRequest) ZKResponse {
 	if err := validatePath(op.Path); err != nil {
-		return mkZKErr(xid, z.s.ZXid(), errBadArguments)
+		zxid, err := z.incrementAndGetZxid()
+		if err != nil {
+			return mkErr(err)
+		}
+		return mkZKErr(xid, zxid, errBadArguments)
 	}
 
 	p := mkPath(op.Path)
@@ -401,7 +409,11 @@ func (z *zkEtcd) GetAcl(xid Xid, op *GetAclRequest) ZKResponse {
 
 func (z *zkEtcd) SetAcl(xid Xid, op *SetAclRequest) ZKResponse {
 	if err := validatePath(op.Path); err != nil {
-		return mkZKErr(xid, z.s.ZXid(), errBadArguments)
+		zxid, err := z.incrementAndGetZxid()
+		if err != nil {
+			return mkErr(err)
+		}
+		return mkZKErr(xid, zxid, errBadArguments)
 	}
 	panic("setAcl")
 }
@@ -546,6 +558,23 @@ func (z *zkEtcd) doSTM(applyf func(s v3sync.STM) error) (*etcd.TxnResponse, erro
 	return v3sync.NewSTMSerializable(z.c.Ctx(), z.c, applyf)
 }
 
+// incrementAndGetZxid forces a write to the err-node to increment the Zxid
+// to keep the numbers aligned with Zookeeper's semantics. It is gated on the
+// PerfectZxid global at the moment (which is hardcoded true).
+func (z *zkEtcd) incrementAndGetZxid() (ZXid, error) {
+	applyf := func(s v3sync.STM) (err error) {
+		if PerfectZXidMode {
+			s.Put("/zk/err-node", "1")
+		}
+		return nil
+	}
+	resp, err := z.doSTM(applyf)
+	if err != nil {
+		return -1, err
+	}
+	return ZXid(resp.Header.Revision), nil
+}
+
 func encodeACLs(acls []ACL) string {
 	var b bytes.Buffer
 	gob.NewEncoder(&b).Encode(acls)