clientv3/concurrency: Mutex

clientv3/concurrency/mutex.go

@@ -0,0 +1,111 @@
+// Copyright 2016 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package concurrency
+
+import (
+	"sync"
+
+	"github.com/coreos/etcd/Godeps/_workspace/src/golang.org/x/net/context"
+	v3 "github.com/coreos/etcd/clientv3"
+)
+
+// Mutex implements the sync Locker interface with etcd
+type Mutex struct {
+	client *v3.Client
+	kv     v3.KV
+	ctx    context.Context
+
+	pfx   string
+	myKey string
+	myRev int64
+}
+
+func NewMutex(ctx context.Context, client *v3.Client, pfx string) *Mutex {
+	return &Mutex{client, v3.NewKV(client), ctx, pfx, "", -1}
+}
+
+// Lock locks the mutex with a cancellable context. If the context is cancelled
+// while trying to acquire the lock, the mutex tries to clean its stale lock entry.
+func (m *Mutex) Lock(ctx context.Context) error {
+	s, err := NewSession(m.client)
+	if err != nil {
+		return err
+	}
+	// put self in lock waiters via myKey; oldest waiter holds lock
+	m.myKey, m.myRev, err = NewUniqueKey(ctx, m.kv, m.pfx, v3.WithLease(s.Lease()))
+	// wait for lock to become available
+	for err == nil {
+		// find oldest element in waiters via revision of insertion
+		var resp *v3.GetResponse
+		resp, err = m.kv.Get(ctx, m.pfx, v3.WithFirstRev()...)
+		if err != nil {
+			break
+		}
+		if m.myRev == resp.Kvs[0].CreateRevision {
+			// myKey is oldest in waiters; myKey holds the lock now
+			return nil
+		}
+		// otherwise myKey isn't lowest, so there must be a pfx prior to myKey
+		opts := append(v3.WithLastRev(), v3.WithRev(m.myRev-1))
+		resp, err = m.kv.Get(ctx, m.pfx, opts...)
+		if err != nil {
+			break
+		}
+		lastKey := string(resp.Kvs[0].Key)
+		// wait for release on prior pfx
+		err = waitUpdate(ctx, m.client, lastKey, v3.WithRev(m.myRev))
+		// try again in case lastKey left the wait list before acquiring the lock;
+		// myKey can only hold the lock if it's the oldest in the list
+	}
+
+	// release lock key if cancelled
+	select {
+	case <-ctx.Done():
+		m.Unlock()
+	default:
+	}
+	return err
+}
+
+func (m *Mutex) Unlock() error {
+	if _, err := m.kv.Delete(m.ctx, m.myKey); err != nil {
+		return err
+	}
+	m.myKey = "\x00"
+	m.myRev = -1
+	return nil
+}
+
+func (m *Mutex) IsOwner() v3.Cmp {
+	return v3.Compare(v3.CreatedRevision(m.myKey), "=", m.myRev)
+}
+
+type lockerMutex struct{ *Mutex }
+
+func (lm *lockerMutex) Lock() {
+	if err := lm.Mutex.Lock(lm.ctx); err != nil {
+		panic(err)
+	}
+}
+func (lm *lockerMutex) Unlock() {
+	if err := lm.Mutex.Unlock(); err != nil {
+		panic(err)
+	}
+}
+
+// NewLocker creates a sync.Locker backed by an etcd mutex.
+func NewLocker(ctx context.Context, client *v3.Client, pfx string) sync.Locker {
+	return &lockerMutex{NewMutex(ctx, client, pfx)}
+}