client: Add a package or managing leases on arbitrary kv keys

Built to initially be used by the migration code since our existing
lease mechanisms for ranges and SQL schemas aren't very generalizable.

pkg/internal/client/lease.go

@@ -0,0 +1,156 @@
+// Copyright 2016 The Cockroach Authors.
+//
+// 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 client
+
+import (
+	"fmt"
+	"time"
+
+	"golang.org/x/net/context"
+
+	"github.com/cockroachdb/cockroach/pkg/roachpb"
+	"github.com/cockroachdb/cockroach/pkg/util/hlc"
+	"github.com/cockroachdb/cockroach/pkg/util/uuid"
+	"github.com/pkg/errors"
+)
+
+// DefaultLeaseDuration is the duration a lease will be acquired for if no
+// duration was specified in a LeaseManager's options.
+// Exported for testing purposes.
+const DefaultLeaseDuration = time.Minute
+
+// LeaseNotAvailableError indicates that the lease the caller attempted to
+// acquire is currently held by a different client.
+type LeaseNotAvailableError struct {
+	key        roachpb.Key
+	expiration hlc.Timestamp
+}
+
+func (e *LeaseNotAvailableError) Error() string {
+	return fmt.Sprintf("lease %q is not available until at least %s", e.key, e.expiration)
+}
+
+// LeaseManager provides functionality for acquiring and managing leases
+// via the kv api.
+type LeaseManager struct {
+	db            *DB
+	clock         *hlc.Clock
+	clientID      string
+	leaseDuration time.Duration
+}
+
+// Lease contains the state of a lease on a particular key.
+type Lease struct {
+	key roachpb.Key
+	val *LeaseVal
+}
+
+// LeaseManagerOptions are used to configure a new LeaseManager.
+type LeaseManagerOptions struct {
+	// ClientID must be unique to this LeaseManager instance.
+	ClientID      string
+	LeaseDuration time.Duration
+}
+
+// NewLeaseManager allocates a new LeaseManager.
+func NewLeaseManager(db *DB, clock *hlc.Clock, options LeaseManagerOptions) *LeaseManager {
+	if options.ClientID == "" {
+		options.ClientID = uuid.MakeV4().String()
+	}
+	if options.LeaseDuration <= 0 {
+		options.LeaseDuration = DefaultLeaseDuration
+	}
+	return &LeaseManager{
+		db:            db,
+		clock:         clock,
+		clientID:      options.ClientID,
+		leaseDuration: options.LeaseDuration,
+	}
+}
+
+// AcquireLease attempts to grab a lease on the provided key. Returns a non-nil
+// lease object if it was successful, or an error if it failed to acquire the
+// lease for any reason.
+//
+// NB: Acquiring a non-expired lease is allowed if this LeaseManager's clientID
+// matches the lease owner's ID. This behavior allows a process to re-grab
+// leases without having to wait if it restarts and uses the same ID.
+func (m *LeaseManager) AcquireLease(ctx context.Context, key roachpb.Key) (*Lease, error) {
+	lease := &Lease{
+		key: key,
+	}
+	if err := m.db.Txn(ctx, func(txn *Txn) error {
+		var val LeaseVal
+		err := txn.GetProto(key, &val)
+		if err != nil {
+			return err
+		}
+		if !m.leaseAvailable(&val) {
+			return &LeaseNotAvailableError{key: key, expiration: val.Expiration}
+		}
+		lease.val = &LeaseVal{
+			Owner:      m.clientID,
+			Expiration: m.clock.Now().Add(m.leaseDuration.Nanoseconds(), 0),
+		}
+		return txn.Put(key, lease.val)
+	}); err != nil {
+		return nil, err
+	}
+	return lease, nil
+}
+
+func (m *LeaseManager) leaseAvailable(val *LeaseVal) bool {
+	return val.Owner == m.clientID || m.timeRemaining(val) <= 0
+}
+
+// TimeRemaining returns the amount of time left on the given lease.
+func (m *LeaseManager) TimeRemaining(l *Lease) time.Duration {
+	return m.timeRemaining(l.val)
+}
+
+func (m *LeaseManager) timeRemaining(val *LeaseVal) time.Duration {
+	return val.Expiration.GoTime().Sub(m.clock.Now().GoTime()) - m.clock.MaxOffset()
+}
+
+// ExtendLease attempts to push the expiration time of the lease farther out
+// into the future.
+func (m *LeaseManager) ExtendLease(ctx context.Context, l *Lease) error {
+	if m.TimeRemaining(l) < 0 {
+		return errors.Errorf("can't extend lease that expired at time %s", l.val.Expiration)
+	}
+
+	newVal := &LeaseVal{
+		Owner:      m.clientID,
+		Expiration: m.clock.Now().Add(m.leaseDuration.Nanoseconds(), 0),
+	}
+
+	err := m.db.CPut(ctx, l.key, newVal, l.val)
+	if err != nil {
+		if _, ok := err.(*roachpb.ConditionFailedError); ok {
+			// Something is wrong - immediately expire the local lease state.
+			l.val.Expiration = hlc.ZeroTimestamp
+			return errors.Wrap(err, "local lease state %v out of sync with DB state")
+		}
+		return err
+	}
+	l.val = newVal
+	return nil
+}
+
+// ReleaseLease attempts to release the given lease so that another process can
+// grab it.
+func (m *LeaseManager) ReleaseLease(ctx context.Context, l *Lease) error {
+	return m.db.CPut(ctx, l.key, nil, l.val)
+}