consul: Adding support for lock-delay in sessions

consul/kvs_endpoint.go

@@ -25,6 +25,23 @@ func (k *KVS) Apply(args *structs.KVSRequest, reply *bool) error {
 		return fmt.Errorf("Must provide key")
 	}
 
+	// If this is a lock, we must check for a lock-delay. Since lock-delay
+	// is based on wall-time, each peer expire the lock-delay at a slightly
+	// different time. This means the enforcement of lock-delay cannot be done
+	// after the raft log is committed as it would lead to inconsistent FSMs.
+	// Instead, the lock-delay must be enforced before commit. This means that
+	// only the wall-time of the leader node is used, preventing any inconsistencies.
+	if args.Op == structs.KVSLock {
+		state := k.srv.fsm.State()
+		expires := state.KVSLockDelay(args.DirEnt.Key)
+		if expires.After(time.Now()) {
+			k.srv.logger.Printf("[WARN] consul.kvs: Rejecting lock of %s due to lock-delay until %v",
+				args.DirEnt.Key, expires)
+			*reply = false
+			return nil
+		}
+	}
+
 	// Apply the update
 	resp, err := k.srv.raftApply(structs.KVSRequestType, args)
 	if err != nil {

consul/kvs_endpoint_test.go

@@ -5,6 +5,7 @@ import (
 	"github.com/hashicorp/consul/testutil"
 	"os"
 	"testing"
+	"time"
 )
 
 func TestKVS_Apply(t *testing.T) {
@@ -224,5 +225,72 @@ func TestKVSEndpoint_ListKeys(t *testing.T) {
 	if dirent.Keys[2] != "/test/sub/" {
 		t.Fatalf("Bad: %v", dirent.Keys)
 	}
-
+}
+
+func TestKVS_Apply_LockDelay(t *testing.T) {
+	dir1, s1 := testServer(t)
+	defer os.RemoveAll(dir1)
+	defer s1.Shutdown()
+	client := rpcClient(t, s1)
+	defer client.Close()
+
+	testutil.WaitForLeader(t, client.Call, "dc1")
+
+	// Create and invalidate a session with a lock
+	state := s1.fsm.State()
+	if err := state.EnsureNode(1, structs.Node{"foo", "127.0.0.1"}); err != nil {
+		t.Fatalf("err: %v")
+	}
+	session := &structs.Session{
+		Node:      "foo",
+		LockDelay: 50 * time.Millisecond,
+	}
+	if err := state.SessionCreate(2, session); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	id := session.ID
+	d := &structs.DirEntry{
+		Key:     "test",
+		Session: id,
+	}
+	if ok, err := state.KVSLock(3, d); err != nil || !ok {
+		t.Fatalf("err: %v", err)
+	}
+	if err := state.SessionDestroy(4, id); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Make a new session that is valid
+	if err := state.SessionCreate(5, session); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	validId := session.ID
+
+	// Make a lock request
+	arg := structs.KVSRequest{
+		Datacenter: "dc1",
+		Op:         structs.KVSLock,
+		DirEnt: structs.DirEntry{
+			Key:     "test",
+			Session: validId,
+		},
+	}
+	var out bool
+	if err := client.Call("KVS.Apply", &arg, &out); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if out != false {
+		t.Fatalf("should not acquire")
+	}
+
+	// Wait for lock-delay
+	time.Sleep(50 * time.Millisecond)
+
+	// Should acquire
+	if err := client.Call("KVS.Apply", &arg, &out); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if out != true {
+		t.Fatalf("should acquire")
+	}
 }

consul/state_store.go

@@ -10,6 +10,8 @@ import (
 	"os"
 	"runtime"
 	"strings"
+	"sync"
+	"time"
 )
 
 const (
@@ -54,6 +56,22 @@ type StateStore struct {
 	tables            MDBTables
 	watch             map[*MDBTable]*NotifyGroup
 	queryTables       map[string]MDBTables
+
+	// lockDelay is used to mark certain locks as unacquirable.
+	// When a lock is forcefully released (failing health
+	// check, destroyed session, etc), it is subject to the LockDelay
+	// impossed by the session. This prevents another session from
+	// acquiring the lock for some period of time as a protection against
+	// split-brains. This is inspired by the lock-delay in Chubby.
+	// Because this relies on wall-time, we cannot assume all peers
+	// perceive time as flowing uniformly. This means KVSLock MUST ignore
+	// lockDelay, since the lockDelay may have expired on the leader,
+	// but not on the follower. Rejecting the lock could result in
+	// inconsistencies in the FSMs due to the rate time progresses. Instead,
+	// only the opinion of the leader is respected, and the Raft log
+	// is never questioned.
+	lockDelay     map[string]time.Time
+	lockDelayLock sync.RWMutex
 }
 
 // StateSnapshot is used to provide a point-in-time snapshot
@@ -98,6 +116,7 @@ func NewStateStore(logOutput io.Writer) (*StateStore, error) {
 		path:      path,
 		env:       env,
 		watch:     make(map[*MDBTable]*NotifyGroup),
+		lockDelay: make(map[string]time.Time),
 	}
 
 	// Ensure we can initialize
@@ -1076,6 +1095,17 @@ func (s *StateStore) KVSUnlock(index uint64, d *structs.DirEntry) (bool, error)
 	return s.kvsSet(index, d, kvUnlock)
 }
 
+// KVSLockDelay returns the expiration time of a key lock delay. A key may
+// have a lock delay if it was unlocked due to a session invalidation instead
+// of a graceful unlock. This must be checked on the leader node, and not in
+// KVSLock due to the variability of clocks.
+func (s *StateStore) KVSLockDelay(key string) time.Time {
+	s.lockDelayLock.RLock()
+	expires := s.lockDelay[key]
+	s.lockDelayLock.RUnlock()
+	return expires
+}
+
 // kvsSet is the internal setter
 func (s *StateStore) kvsSet(
 	index uint64,
@@ -1367,8 +1397,14 @@ func (s *StateStore) invalidateSession(index uint64, tx *MDBTxn, id string) erro
 	}
 	session := res[0].(*structs.Session)
 
+	// Enforce the MaxLockDelay
+	delay := session.LockDelay
+	if delay > structs.MaxLockDelay {
+		delay = structs.MaxLockDelay
+	}
+
 	// Invalidate any held locks
-	if err := s.invalidateLocks(index, tx, id); err != nil {
+	if err := s.invalidateLocks(index, tx, delay, id); err != nil {
 		return err
 	}
 
@@ -1395,11 +1431,20 @@ func (s *StateStore) invalidateSession(index uint64, tx *MDBTxn, id string) erro
 
 // invalidateLocks is used to invalidate all the locks held by a session
 // within a given txn. All tables should be locked in the tx.
-func (s *StateStore) invalidateLocks(index uint64, tx *MDBTxn, id string) error {
+func (s *StateStore) invalidateLocks(index uint64, tx *MDBTxn,
+	lockDelay time.Duration, id string) error {
 	pairs, err := s.kvsTable.GetTxn(tx, "session", id)
 	if err != nil {
 		return err
 	}
+
+	var expires time.Time
+	if lockDelay > 0 {
+		s.lockDelayLock.Lock()
+		defer s.lockDelayLock.Unlock()
+		expires = time.Now().Add(lockDelay)
+	}
+
 	for _, pair := range pairs {
 		kv := pair.(*structs.DirEntry)
 		kv.Session = ""        // Clear the lock
@@ -1407,6 +1452,16 @@ func (s *StateStore) invalidateLocks(index uint64, tx *MDBTxn, id string) error
 		if err := s.kvsTable.InsertTxn(tx, kv); err != nil {
 			return err
 		}
+		// If there is a lock delay, prevent acquisition
+		// for at least lockDelay period
+		if lockDelay > 0 {
+			s.lockDelay[kv.Key] = expires
+			time.AfterFunc(lockDelay, func() {
+				s.lockDelayLock.Lock()
+				delete(s.lockDelay, kv.Key)
+				s.lockDelayLock.Unlock()
+			})
+		}
 	}
 	if len(pairs) > 0 {
 		if err := s.kvsTable.SetLastIndexTxn(tx, index); err != nil {

consul/state_store_test.go

@@ -6,6 +6,7 @@ import (
 	"reflect"
 	"sort"
 	"testing"
+	"time"
 )
 
 func testStateStore() (*StateStore, error) {
@@ -2059,7 +2060,7 @@ func TestSessionInvalidate_KeyUnlock(t *testing.T) {
 	if err := store.EnsureNode(3, structs.Node{"foo", "127.0.0.1"}); err != nil {
 		t.Fatalf("err: %v")
 	}
-	session := &structs.Session{Node: "foo"}
+	session := &structs.Session{Node: "foo", LockDelay: 50 * time.Millisecond}
 	if err := store.SessionCreate(4, session); err != nil {
 		t.Fatalf("err: %v", err)
 	}
@@ -2095,4 +2096,10 @@ func TestSessionInvalidate_KeyUnlock(t *testing.T) {
 	if d2.Session != "" {
 		t.Fatalf("bad: %v", *d2)
 	}
+
+	// Key should have a lock delay
+	expires := store.KVSLockDelay("/foo")
+	if expires.Before(time.Now().Add(30 * time.Millisecond)) {
+		t.Fatalf("Bad: %v", expires)
+	}
 }

consul/structs/structs.go

@@ -29,6 +29,12 @@ const (
 	HealthCritical = "critical"
 )
 
+const (
+	// MaxLockDelay provides a maximum LockDelay value for
+	// a session. Any value above this will not be respected.
+	MaxLockDelay = 60 * time.Second
+)
+
 // RPCInfo is used to describe common information about query
 type RPCInfo interface {
 	RequestDatacenter() string
@@ -343,10 +349,11 @@ type IndexedKeyList struct {
 // Session is used to represent an open session in the KV store.
 // This issued to associate node checks with acquired locks.
 type Session struct {
+	CreateIndex uint64
 	ID          string
 	Node        string
 	Checks      []string
-	CreateIndex uint64
+	LockDelay   time.Duration
 }
 type Sessions []*Session