Issue #2905: Add check-not-exists to TXN endpoint

This patch adds support for asserting that a given
key does not exist in the KV store.

Fixes #2905

api/kv.go

@@ -60,6 +60,7 @@ const (
 	KVGetTree        KVOp = "get-tree"
 	KVCheckSession   KVOp = "check-session"
 	KVCheckIndex     KVOp = "check-index"
+	KVCheckNotExists KVOp = "check-not-exists"
 )
 
 // KVTxnOp defines a single operation inside a transaction.

consul/state/txn.go

@@ -79,6 +79,12 @@ func (s *StateStore) txnKVS(tx *memdb.Txn, idx uint64, op *structs.TxnKVOp) (str
 	case api.KVCheckIndex:
 		entry, err = s.kvsCheckIndexTxn(tx, op.DirEnt.Key, op.DirEnt.ModifyIndex)
 
+	case api.KVCheckNotExists:
+		_, entry, err = s.kvsGetTxn(tx, nil, op.DirEnt.Key)
+		if entry != nil && err == nil {
+			err = fmt.Errorf("key %q exists", op.DirEnt.Key)
+		}
+
 	default:
 		err = fmt.Errorf("unknown KV verb %q", op.Verb)
 	}

consul/txn_endpoint_test.go

@@ -14,6 +14,57 @@ import (
 	"github.com/hashicorp/net-rpc-msgpackrpc"
 )
 
+func TestTxn_CheckNotExists(t *testing.T) {
+	dir1, s1 := testServer(t)
+	defer os.RemoveAll(dir1)
+	defer s1.Shutdown()
+	codec := rpcClient(t, s1)
+	defer codec.Close()
+
+	testrpc.WaitForLeader(t, s1.RPC, "dc1")
+
+	apply := func(arg *structs.TxnRequest) (*structs.TxnResponse, error) {
+		out := new(structs.TxnResponse)
+		err := msgpackrpc.CallWithCodec(codec, "Txn.Apply", arg, out)
+		return out, err
+	}
+
+	checkKeyNotExists := &structs.TxnRequest{
+		Datacenter: "dc1",
+		Ops: structs.TxnOps{
+			{
+				KV: &structs.TxnKVOp{
+					Verb:   api.KVCheckNotExists,
+					DirEnt: structs.DirEntry{Key: "test"},
+				},
+			},
+		},
+	}
+
+	createKey := &structs.TxnRequest{
+		Datacenter: "dc1",
+		Ops: structs.TxnOps{
+			{
+				KV: &structs.TxnKVOp{
+					Verb:   api.KVSet,
+					DirEnt: structs.DirEntry{Key: "test"},
+				},
+			},
+		},
+	}
+
+	if _, err := apply(checkKeyNotExists); err != nil {
+		t.Fatalf("testing for non-existent key failed: %s", err)
+	}
+	if _, err := apply(createKey); err != nil {
+		t.Fatalf("creating new key failed: %s", err)
+	}
+	out, err := apply(checkKeyNotExists)
+	if err != nil || out == nil || len(out.Errors) != 1 || out.Errors[0].Error() != `op 0: key "test" exists` {
+		t.Fatalf("testing for existent key failed: %#v", out)
+	}
+}
+
 func TestTxn_Apply(t *testing.T) {
 	dir1, s1 := testServer(t)
 	defer os.RemoveAll(dir1)
@@ -234,6 +285,14 @@ func TestTxn_Apply_ACLDeny(t *testing.T) {
 					},
 				},
 			},
+			&structs.TxnOp{
+				KV: &structs.TxnKVOp{
+					Verb: api.KVCheckNotExists,
+					DirEnt: structs.DirEntry{
+						Key: "nope",
+					},
+				},
+			},
 		},
 		WriteRequest: structs.WriteRequest{
 			Token: id,

website/source/api/txn.markdown

@@ -150,7 +150,7 @@ The following table summarizes the available verbs and the fields that apply to
 that operation ("X" means a field is required and "O" means it is optional):
 
 | Verb               | Operation                                    | Key  | Value | Flags | Index | Session |
-| --------------- | -------------------------------------------- | :--: | :---: | :---: | :---: | :-----: |
+| ------------------ | -------------------------------------------- | :--: | :---: | :---: | :---: | :-----: |
 | `set`              | Sets the `Key` to the given `Value`          | `x`  | `x`   | `o`   |       |         |  
 | `cas`              | Sets, but with CAS semantics                 | `x`  | `x`   | `o`   | `x`   |         |  
 | `lock`             | Lock with the given `Session`                | `x`  | `x`   | `o`   |       | `x`     |  
@@ -159,6 +159,7 @@ that operation ("X" means a field is required and "O" means it is optional):
 | `get-tree`         | Gets all keys with the prefix                | `x`  |       |       |       |         |  
 | `check-index`      | Fail if modify index != index                | `x`  |       |       | `x`   |         |  
 | `check-session`    | Fail if not locked by session                | `x`  |       |       |       | `x`     |  
+| `check-not-exists` | Fail if key exists                           | `x`  |       |       |       |         |  
 | `delete`           | Delete the key                               | `x`  |       |       |       |         |  
 | `delete-tree`      | Delete all keys with a prefix                | `x`  |       |       |       |         |  
 | `delete-cas`       | Delete, but with CAS semantics               | `x`  |       |       | `x`   |         |