state: use an error to indicate compare failed

Errors are values. We can use the error value to identify the 'comparison failed' case which makes the function easier to use and should make it harder to miss handle the error case

agent/consul/state/catalog.go

@@ -1,6 +1,7 @@
 package state
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
 	"strings"
@@ -736,34 +737,31 @@ func (s *Store) EnsureService(idx uint64, node string, svc *structs.NodeService)
 	return nil
 }
 
+var errCASCompareFailed = errors.New("compare-and-set: comparison failed")
+
 // ensureServiceCASTxn updates a service only if the existing index matches the given index.
 // Returns a bool indicating if a write happened and any error.
-func (s *Store) ensureServiceCASTxn(tx *memdb.Txn, idx uint64, node string, svc *structs.NodeService) (bool, error) {
+func (s *Store) ensureServiceCASTxn(tx *memdb.Txn, idx uint64, node string, svc *structs.NodeService) error {
 	// Retrieve the existing service.
 	_, existing, err := firstWatchCompoundWithTxn(tx, "services", "id", &svc.EnterpriseMeta, node, svc.ID)
 	if err != nil {
-		return false, fmt.Errorf("failed service lookup: %s", err)
+		return fmt.Errorf("failed service lookup: %s", err)
 	}
 
 	// Check if the we should do the set. A ModifyIndex of 0 means that
 	// we are doing a set-if-not-exists.
 	if svc.ModifyIndex == 0 && existing != nil {
-		return false, nil
+		return errCASCompareFailed
 	}
 	if svc.ModifyIndex != 0 && existing == nil {
-		return false, nil
+		return errCASCompareFailed
 	}
 	e, ok := existing.(*structs.ServiceNode)
 	if ok && svc.ModifyIndex != 0 && svc.ModifyIndex != e.ModifyIndex {
-		return false, nil
+		return errCASCompareFailed
 	}
 
-	// Perform the update.
-	if err := s.ensureServiceTxn(tx, idx, node, svc); err != nil {
-		return false, err
-	}
-
-	return true, nil
+	return s.ensureServiceTxn(tx, idx, node, svc)
 }
 
 // ensureServiceTxn is used to upsert a service registration within an

agent/consul/state/catalog_test.go

@@ -2,6 +2,11 @@ package state
 
 import (
 	"fmt"
+	"reflect"
+	"sort"
+	"strings"
+	"testing"
+
 	"github.com/hashicorp/consul/agent/structs"
 	"github.com/hashicorp/consul/api"
 	"github.com/hashicorp/consul/lib"
@@ -11,10 +16,6 @@ import (
 	"github.com/pascaldekloe/goe/verify"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
-	"reflect"
-	"sort"
-	"strings"
-	"testing"
 )
 
 func makeRandomNodeID(t *testing.T) types.NodeID {
@@ -4395,9 +4396,8 @@ func TestStateStore_ensureServiceCASTxn(t *testing.T) {
 
 	// attempt to update with a 0 index
 	tx := s.db.Txn(true)
-	update, err := s.ensureServiceCASTxn(tx, 3, "node1", &ns)
-	require.False(t, update)
-	require.NoError(t, err)
+	err := s.ensureServiceCASTxn(tx, 3, "node1", &ns)
+	require.Equal(t, err, errCASCompareFailed)
 	tx.Commit()
 
 	// ensure no update happened
@@ -4411,9 +4411,8 @@ func TestStateStore_ensureServiceCASTxn(t *testing.T) {
 	ns.ModifyIndex = 99
 	// attempt to update with a non-matching index
 	tx = s.db.Txn(true)
-	update, err = s.ensureServiceCASTxn(tx, 4, "node1", &ns)
-	require.False(t, update)
-	require.NoError(t, err)
+	err = s.ensureServiceCASTxn(tx, 4, "node1", &ns)
+	require.Equal(t, err, errCASCompareFailed)
 	tx.Commit()
 
 	// ensure no update happened
@@ -4427,8 +4426,7 @@ func TestStateStore_ensureServiceCASTxn(t *testing.T) {
 	ns.ModifyIndex = 2
 	// update with the matching modify index
 	tx = s.db.Txn(true)
-	update, err = s.ensureServiceCASTxn(tx, 7, "node1", &ns)
-	require.True(t, update)
+	err = s.ensureServiceCASTxn(tx, 7, "node1", &ns)
 	require.NoError(t, err)
 	tx.Commit()
 

agent/consul/state/txn.go

@@ -230,11 +230,13 @@ func (s *Store) txnService(tx *memdb.Txn, idx uint64, op *structs.TxnServiceOp)
 		return newTxnResultFromNodeServiceEntry(entry), err
 
 	case api.ServiceCAS:
-		ok, err := s.ensureServiceCASTxn(tx, idx, op.Node, &op.Service)
-		// TODO: err != nil case is ignored
-		if !ok && err == nil {
+		err := s.ensureServiceCASTxn(tx, idx, op.Node, &op.Service)
+		switch {
+		case err == errCASCompareFailed:
 			err := fmt.Errorf("failed to set service %q on node %q, index is stale", op.Service.ID, op.Node)
 			return nil, err
+		case err != nil:
+			return nil, err
 		}
 
 		entry, err := s.getNodeServiceTxn(tx, op.Node, op.Service.ID, &op.Service.EnterpriseMeta)