Add kv txn get-not-exists operation.

.changelog/14474.txt

@@ -0,0 +1,3 @@
+```release-note:feature
+http: Add new `get-or-empty` operation to the txn api. Refer to the [API docs](https://www.consul.io/api-docs/txn#kv-operations) for more information.
+```

agent/consul/kvs_endpoint.go

@@ -49,7 +49,7 @@ func kvsPreApply(logger hclog.Logger, srv *Server, authz resolver.Result, op api
 			return false, err
 		}
 
-	case api.KVGet, api.KVGetTree:
+	case api.KVGet, api.KVGetTree, api.KVGetOrEmpty:
 		// Filtering for GETs is done on the output side.
 
 	case api.KVCheckSession, api.KVCheckIndex:

agent/consul/state/txn.go

@@ -60,6 +60,13 @@ func (s *Store) txnKVS(tx WriteTxn, idx uint64, op *structs.TxnKVOp) (structs.Tx
 			err = fmt.Errorf("key %q doesn't exist", op.DirEnt.Key)
 		}
 
+	case api.KVGetOrEmpty:
+		_, entry, err = kvsGetTxn(tx, nil, op.DirEnt.Key, op.DirEnt.EnterpriseMeta)
+		if entry == nil && err == nil {
+			entry = &op.DirEnt
+			entry.Value = nil
+		}
+
 	case api.KVGetTree:
 		var entries structs.DirEntries
 		_, entries, err = s.kvsListTxn(tx, nil, op.DirEnt.Key, op.DirEnt.EnterpriseMeta)
@@ -95,7 +102,7 @@ func (s *Store) txnKVS(tx WriteTxn, idx uint64, op *structs.TxnKVOp) (structs.Tx
 	// value (we have to clone so we don't modify the entry being used by
 	// the state store).
 	if entry != nil {
-		if op.Verb == api.KVGet {
+		if op.Verb == api.KVGet || op.Verb == api.KVGetOrEmpty {
 			result := structs.TxnResult{KV: entry}
 			return structs.TxnResults{&result}, nil
 		}

agent/consul/state/txn_test.go

@@ -577,6 +577,22 @@ func TestStateStore_Txn_KVS(t *testing.T) {
 				},
 			},
 		},
+		&structs.TxnOp{
+			KV: &structs.TxnKVOp{
+				Verb: api.KVGetOrEmpty,
+				DirEnt: structs.DirEntry{
+					Key: "foo/update",
+				},
+			},
+		},
+		&structs.TxnOp{
+			KV: &structs.TxnKVOp{
+				Verb: api.KVGetOrEmpty,
+				DirEnt: structs.DirEntry{
+					Key: "foo/not-exists",
+				},
+			},
+		},
 		&structs.TxnOp{
 			KV: &structs.TxnKVOp{
 				Verb: api.KVCheckIndex,
@@ -702,6 +718,22 @@ func TestStateStore_Txn_KVS(t *testing.T) {
 				},
 			},
 		},
+		&structs.TxnResult{
+			KV: &structs.DirEntry{
+				Key:   "foo/update",
+				Value: []byte("stale"),
+				RaftIndex: structs.RaftIndex{
+					CreateIndex: 5,
+					ModifyIndex: 5,
+				},
+			},
+		},
+		&structs.TxnResult{
+			KV: &structs.DirEntry{
+				Key:   "foo/not-exists",
+				Value: nil,
+			},
+		},
 		&structs.TxnResult{
 			KV: &structs.DirEntry{
 

api/txn.go

@@ -67,6 +67,7 @@ const (
 	KVLock           KVOp = "lock"
 	KVUnlock         KVOp = "unlock"
 	KVGet            KVOp = "get"
+	KVGetOrEmpty     KVOp = "get-or-empty"
 	KVGetTree        KVOp = "get-tree"
 	KVCheckSession   KVOp = "check-session"
 	KVCheckIndex     KVOp = "check-index"

website/content/api-docs/txn.mdx

@@ -267,12 +267,13 @@ The following tables summarize the available verbs and the fields that apply to
 those operations ("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`   |
 | `unlock`           | Unlock with the given `Session`           | `x` |  `x`  |  `o`  |       |   `x`   |
 | `get`              | Get the key, fails if it does not exist   | `x` |       |       |       |         |
+| `get-or-empty`     | Get the key, or null if it does not exist | `x` |       |       |       |         |
 | `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`   |