tee.go

// Copyright 2013, Google Inc. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package helpers

import (
	"fmt"
	"sync"

	log "github.com/golang/glog"
	"github.com/youtube/vitess/go/vt/topo"
	"golang.org/x/net/context"

	topodatapb "github.com/youtube/vitess/go/vt/proto/topodata"
	vschemapb "github.com/youtube/vitess/go/vt/proto/vschema"
)

// Tee is an implementation of topo.Server that uses a primary
// underlying topo.Server for all changes, but also duplicates the
// changes to a secondary topo.Server. It also locks both topo servers
// when needed.  It is meant to be used during transitions from one
// topo.Server to another.
//
// - primary: we read everything from it, and write to it. We also create
//     MasterParticipation from it.
// - secondary: we write to it as well, but we usually don't fail.
// - we lock primary/secondary if reverseLockOrder is False,
// or secondary/primary if reverseLockOrder is True.
type Tee struct {
	primary   topo.Impl
	secondary topo.Impl

	readFrom       topo.Impl
	readFromSecond topo.Impl

	lockFirst  topo.Impl
	lockSecond topo.Impl

	// protects the variables below this point
	mu sync.Mutex

	keyspaceVersionMapping map[string]versionMapping
	shardVersionMapping    map[string]versionMapping
	tabletVersionMapping   map[topodatapb.TabletAlias]versionMapping

	keyspaceLockPaths map[string]string
	shardLockPaths    map[string]string
}

// When reading a version from 'readFrom', we also read another version
// from 'readFromSecond', and save the mapping to this map. We only keep one
// mapping for a given object, no need to overdo it.
// FIXME(alainjobart) remove this when topo API is all converted to Backend.
type versionMapping struct {
	readFromVersion       int64
	readFromSecondVersion int64
}

// NewTee returns a new topo.Impl object
func NewTee(primary, secondary topo.Impl, reverseLockOrder bool) *Tee {
	lockFirst := primary
	lockSecond := secondary
	if reverseLockOrder {
		lockFirst = secondary
		lockSecond = primary
	}
	return &Tee{
		primary:                primary,
		secondary:              secondary,
		readFrom:               primary,
		readFromSecond:         secondary,
		lockFirst:              lockFirst,
		lockSecond:             lockSecond,
		keyspaceVersionMapping: make(map[string]versionMapping),
		shardVersionMapping:    make(map[string]versionMapping),
		tabletVersionMapping:   make(map[topodatapb.TabletAlias]versionMapping),
		keyspaceLockPaths:      make(map[string]string),
		shardLockPaths:         make(map[string]string),
	}
}

//
// topo.Server management interface.
//

// Close is part of the topo.Server interface
func (tee *Tee) Close() {
	tee.primary.Close()
	tee.secondary.Close()
}

//
// Backend API
//

// ListDir is part of the topo.Backend interface.
func (tee *Tee) ListDir(ctx context.Context, cell, dirPath string) ([]string, error) {
	return tee.primary.ListDir(ctx, cell, dirPath)
}

// Create is part of the topo.Backend interface.
func (tee *Tee) Create(ctx context.Context, cell, filePath string, contents []byte) (topo.Version, error) {
	primaryVersion, err := tee.primary.Create(ctx, cell, filePath, contents)
	if err != nil {
		return nil, err
	}

	// This is critical enough that we want to fail. However, we support
	// an unconditional update if the file already exists.
	_, err = tee.secondary.Create(ctx, cell, filePath, contents)
	if err == topo.ErrNodeExists {
		_, err = tee.secondary.Update(ctx, cell, filePath, contents, nil)
	}
	if err != nil {
		return nil, err
	}

	return primaryVersion, nil
}

// Update is part of the topo.Backend interface.
func (tee *Tee) Update(ctx context.Context, cell, filePath string, contents []byte, version topo.Version) (topo.Version, error) {
	primaryVersion, err := tee.primary.Update(ctx, cell, filePath, contents, version)
	if err != nil {
		// failed on primary, not updating secondary
		return nil, err
	}

	// Always do an unconditional update on secondary.
	if _, err = tee.secondary.Update(ctx, cell, filePath, contents, nil); err != nil {
		log.Warningf("secondary.Update(%v,%v,unconditonal) failed: %v", cell, filePath, err)
	}
	return primaryVersion, nil
}

// Get is part of the topo.Backend interface.
func (tee *Tee) Get(ctx context.Context, cell, filePath string) ([]byte, topo.Version, error) {
	return tee.primary.Get(ctx, cell, filePath)
}

// Delete is part of the topo.Backend interface.
func (tee *Tee) Delete(ctx context.Context, cell, filePath string, version topo.Version) error {
	// If primary fails, no need to go further.
	if err := tee.primary.Delete(ctx, cell, filePath, version); err != nil {
		return err
	}

	// Always do an unconditonal delete on secondary.
	if err := tee.secondary.Delete(ctx, cell, filePath, nil); err != nil && err != topo.ErrNoNode {
		// Secondary didn't work, and the node wasn't gone already.
		log.Warningf("secondary.Delete(%v,%v) failed: %v", cell, filePath, err)
	}

	return nil
}

// Watch is part of the topo.Backend interface
func (tee *Tee) Watch(ctx context.Context, cell, filePath string) (*topo.WatchData, <-chan *topo.WatchData, topo.CancelFunc) {
	return tee.primary.Watch(ctx, cell, filePath)
}

//
// Cell management, global
//

// GetKnownCells is part of the topo.Server interface
func (tee *Tee) GetKnownCells(ctx context.Context) ([]string, error) {
	return tee.readFrom.GetKnownCells(ctx)
}

//
// Keyspace management, global.
//

// CreateKeyspace is part of the topo.Server interface
func (tee *Tee) CreateKeyspace(ctx context.Context, keyspace string, value *topodatapb.Keyspace) error {
	if err := tee.primary.CreateKeyspace(ctx, keyspace, value); err != nil {
		return err
	}

	// this is critical enough that we want to fail
	if err := tee.secondary.CreateKeyspace(ctx, keyspace, value); err != nil {
		return err
	}
	return nil
}

// UpdateKeyspace is part of the topo.Server interface
func (tee *Tee) UpdateKeyspace(ctx context.Context, keyspace string, value *topodatapb.Keyspace, existingVersion int64) (newVersion int64, err error) {
	if newVersion, err = tee.primary.UpdateKeyspace(ctx, keyspace, value, existingVersion); err != nil {
		// failed on primary, not updating secondary
		return
	}

	// if we have a mapping between keyspace version in first topo
	// and keyspace version in second topo, replace the version number.
	// if not, this will probably fail and log.
	tee.mu.Lock()
	kvm, ok := tee.keyspaceVersionMapping[keyspace]
	if ok && kvm.readFromVersion == existingVersion {
		existingVersion = kvm.readFromSecondVersion
		delete(tee.keyspaceVersionMapping, keyspace)
	}
	tee.mu.Unlock()
	if newVersion2, serr := tee.secondary.UpdateKeyspace(ctx, keyspace, value, existingVersion); serr != nil {
		// not critical enough to fail
		if serr == topo.ErrNoNode {
			// the keyspace doesn't exist on the secondary, let's
			// just create it
			if serr = tee.secondary.CreateKeyspace(ctx, keyspace, value); serr != nil {
				log.Warningf("secondary.CreateKeyspace(%v) failed (after UpdateKeyspace returned ErrNoNode): %v", keyspace, serr)
			} else {
				log.Infof("secondary.UpdateKeyspace(%v) failed with ErrNoNode, CreateKeyspace then worked.", keyspace)
				_, secondaryVersion, gerr := tee.secondary.GetKeyspace(ctx, keyspace)
				if gerr != nil {
					log.Warningf("Failed to re-read keyspace(%v) after creating it on secondary: %v", keyspace, gerr)
				} else {
					tee.mu.Lock()
					tee.keyspaceVersionMapping[keyspace] = versionMapping{
						readFromVersion:       newVersion,
						readFromSecondVersion: secondaryVersion,
					}
					tee.mu.Unlock()
				}
			}
		} else {
			log.Warningf("secondary.UpdateKeyspace(%v) failed: %v", keyspace, serr)
		}
	} else {
		tee.mu.Lock()
		tee.keyspaceVersionMapping[keyspace] = versionMapping{
			readFromVersion:       newVersion,
			readFromSecondVersion: newVersion2,
		}
		tee.mu.Unlock()
	}
	return
}

// DeleteKeyspace is part of the topo.Server interface
func (tee *Tee) DeleteKeyspace(ctx context.Context, keyspace string) error {
	err := tee.primary.DeleteKeyspace(ctx, keyspace)
	if err != nil && err != topo.ErrNoNode {
		return err
	}

	if err := tee.secondary.DeleteKeyspace(ctx, keyspace); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.DeleteKeyspace(%v) failed: %v", keyspace, err)
	}
	return err
}

// GetKeyspace is part of the topo.Server interface
func (tee *Tee) GetKeyspace(ctx context.Context, keyspace string) (*topodatapb.Keyspace, int64, error) {
	k, version, err := tee.readFrom.GetKeyspace(ctx, keyspace)
	if err != nil {
		return nil, 0, err
	}

	_, version2, err := tee.readFromSecond.GetKeyspace(ctx, keyspace)
	if err != nil {
		// can't read from secondary, so we can's keep version map
		return k, version, nil
	}

	tee.mu.Lock()
	tee.keyspaceVersionMapping[keyspace] = versionMapping{
		readFromVersion:       version,
		readFromSecondVersion: version2,
	}
	tee.mu.Unlock()
	return k, version, nil
}

// GetKeyspaces is part of the topo.Server interface
func (tee *Tee) GetKeyspaces(ctx context.Context) ([]string, error) {
	return tee.readFrom.GetKeyspaces(ctx)
}

//
// Shard management, global.
//

// CreateShard is part of the topo.Server interface
func (tee *Tee) CreateShard(ctx context.Context, keyspace, shard string, value *topodatapb.Shard) error {
	err := tee.primary.CreateShard(ctx, keyspace, shard, value)
	if err != nil && err != topo.ErrNodeExists {
		return err
	}

	serr := tee.secondary.CreateShard(ctx, keyspace, shard, value)
	if serr != nil && serr != topo.ErrNodeExists {
		// not critical enough to fail
		log.Warningf("secondary.CreateShard(%v,%v) failed: %v", keyspace, shard, err)
	}
	return err
}

// UpdateShard is part of the topo.Server interface
func (tee *Tee) UpdateShard(ctx context.Context, keyspace, shard string, value *topodatapb.Shard, existingVersion int64) (newVersion int64, err error) {
	if newVersion, err = tee.primary.UpdateShard(ctx, keyspace, shard, value, existingVersion); err != nil {
		// failed on primary, not updating secondary
		return
	}

	// if we have a mapping between shard version in first topo
	// and shard version in second topo, replace the version number.
	// if not, this will probably fail and log.
	tee.mu.Lock()
	svm, ok := tee.shardVersionMapping[keyspace+"/"+shard]
	if ok && svm.readFromVersion == existingVersion {
		existingVersion = svm.readFromSecondVersion
		delete(tee.shardVersionMapping, keyspace+"/"+shard)
	}
	tee.mu.Unlock()
	if newVersion2, serr := tee.secondary.UpdateShard(ctx, keyspace, shard, value, existingVersion); serr != nil {
		// not critical enough to fail
		if serr == topo.ErrNoNode {
			// the shard doesn't exist on the secondary, let's
			// just create it
			if serr = tee.secondary.CreateShard(ctx, keyspace, shard, value); serr != nil {
				log.Warningf("secondary.CreateShard(%v,%v) failed (after UpdateShard returned ErrNoNode): %v", keyspace, shard, serr)
			} else {
				log.Infof("secondary.UpdateShard(%v, %v) failed with ErrNoNode, CreateShard then worked.", keyspace, shard)
				_, v, gerr := tee.secondary.GetShard(ctx, keyspace, shard)
				if gerr != nil {
					log.Warningf("Failed to re-read shard(%v, %v) after creating it on secondary: %v", keyspace, shard, gerr)
				} else {
					tee.mu.Lock()
					tee.shardVersionMapping[keyspace+"/"+shard] = versionMapping{
						readFromVersion:       newVersion,
						readFromSecondVersion: v,
					}
					tee.mu.Unlock()
				}
			}
		} else {
			log.Warningf("secondary.UpdateShard(%v, %v) failed: %v", keyspace, shard, serr)
		}
	} else {
		tee.mu.Lock()
		tee.shardVersionMapping[keyspace+"/"+shard] = versionMapping{
			readFromVersion:       newVersion,
			readFromSecondVersion: newVersion2,
		}
		tee.mu.Unlock()
	}
	return
}

// GetShard is part of the topo.Server interface
func (tee *Tee) GetShard(ctx context.Context, keyspace, shard string) (*topodatapb.Shard, int64, error) {
	s, v, err := tee.readFrom.GetShard(ctx, keyspace, shard)
	if err != nil {
		return nil, 0, err
	}

	_, v2, err := tee.readFromSecond.GetShard(ctx, keyspace, shard)
	if err != nil {
		// can't read from secondary, so we can's keep version map
		return s, v, nil
	}

	tee.mu.Lock()
	tee.shardVersionMapping[keyspace+"/"+shard] = versionMapping{
		readFromVersion:       v,
		readFromSecondVersion: v2,
	}
	tee.mu.Unlock()
	return s, v, nil
}

// GetShardNames is part of the topo.Server interface
func (tee *Tee) GetShardNames(ctx context.Context, keyspace string) ([]string, error) {
	return tee.readFrom.GetShardNames(ctx, keyspace)
}

// DeleteShard is part of the topo.Server interface
func (tee *Tee) DeleteShard(ctx context.Context, keyspace, shard string) error {
	err := tee.primary.DeleteShard(ctx, keyspace, shard)
	if err != nil && err != topo.ErrNoNode {
		return err
	}

	if err := tee.secondary.DeleteShard(ctx, keyspace, shard); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.DeleteShard(%v, %v) failed: %v", keyspace, shard, err)
	}
	return err
}

//
// Tablet management, per cell.
//

// CreateTablet is part of the topo.Server interface
func (tee *Tee) CreateTablet(ctx context.Context, tablet *topodatapb.Tablet) error {
	err := tee.primary.CreateTablet(ctx, tablet)
	if err != nil && err != topo.ErrNodeExists {
		return err
	}

	if err := tee.primary.CreateTablet(ctx, tablet); err != nil && err != topo.ErrNodeExists {
		// not critical enough to fail
		log.Warningf("secondary.CreateTablet(%v) failed: %v", tablet.Alias, err)
	}
	return err
}

// UpdateTablet is part of the topo.Server interface
func (tee *Tee) UpdateTablet(ctx context.Context, tablet *topodatapb.Tablet, existingVersion int64) (newVersion int64, err error) {
	if newVersion, err = tee.primary.UpdateTablet(ctx, tablet, existingVersion); err != nil {
		// failed on primary, not updating secondary
		return
	}

	// if we have a mapping between tablet version in first topo
	// and tablet version in second topo, replace the version number.
	// if not, this will probably fail and log.
	tee.mu.Lock()
	tvm, ok := tee.tabletVersionMapping[*tablet.Alias]
	if ok && tvm.readFromVersion == existingVersion {
		existingVersion = tvm.readFromSecondVersion
		delete(tee.tabletVersionMapping, *tablet.Alias)
	}
	tee.mu.Unlock()
	if newVersion2, serr := tee.secondary.UpdateTablet(ctx, tablet, existingVersion); serr != nil {
		// not critical enough to fail
		if serr == topo.ErrNoNode {
			// the tablet doesn't exist on the secondary, let's
			// just create it
			if serr = tee.secondary.CreateTablet(ctx, tablet); serr != nil {
				log.Warningf("secondary.CreateTablet(%v) failed (after UpdateTablet returned ErrNoNode): %v", tablet.Alias, serr)
			} else {
				log.Infof("secondary.UpdateTablet(%v) failed with ErrNoNode, CreateTablet then worked.", tablet.Alias)
				_, v, gerr := tee.secondary.GetTablet(ctx, tablet.Alias)
				if gerr != nil {
					log.Warningf("Failed to re-read tablet(%v) after creating it on secondary: %v", tablet.Alias, gerr)
				} else {
					tee.mu.Lock()
					tee.tabletVersionMapping[*tablet.Alias] = versionMapping{
						readFromVersion:       newVersion,
						readFromSecondVersion: v,
					}
					tee.mu.Unlock()
				}
			}
		} else {
			log.Warningf("secondary.UpdateTablet(%v) failed: %v", tablet.Alias, serr)
		}
	} else {
		tee.mu.Lock()
		tee.tabletVersionMapping[*tablet.Alias] = versionMapping{
			readFromVersion:       newVersion,
			readFromSecondVersion: newVersion2,
		}
		tee.mu.Unlock()
	}
	return
}

// DeleteTablet is part of the topo.Server interface
func (tee *Tee) DeleteTablet(ctx context.Context, alias *topodatapb.TabletAlias) error {
	if err := tee.primary.DeleteTablet(ctx, alias); err != nil {
		return err
	}

	if err := tee.secondary.DeleteTablet(ctx, alias); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.DeleteTablet(%v) failed: %v", alias, err)
	}
	return nil
}

// GetTablet is part of the topo.Server interface
func (tee *Tee) GetTablet(ctx context.Context, alias *topodatapb.TabletAlias) (*topodatapb.Tablet, int64, error) {
	t, v, err := tee.readFrom.GetTablet(ctx, alias)
	if err != nil {
		return nil, 0, err
	}

	_, v2, err := tee.readFromSecond.GetTablet(ctx, alias)
	if err != nil {
		// can't read from secondary, so we can's keep version map
		return t, v, nil
	}

	tee.mu.Lock()
	tee.tabletVersionMapping[*alias] = versionMapping{
		readFromVersion:       v,
		readFromSecondVersion: v2,
	}
	tee.mu.Unlock()
	return t, v, nil
}

// GetTabletsByCell is part of the topo.Server interface
func (tee *Tee) GetTabletsByCell(ctx context.Context, cell string) ([]*topodatapb.TabletAlias, error) {
	return tee.readFrom.GetTabletsByCell(ctx, cell)
}

//
// Shard replication graph management, local.
//

// UpdateShardReplicationFields is part of the topo.Server interface
func (tee *Tee) UpdateShardReplicationFields(ctx context.Context, cell, keyspace, shard string, update func(*topodatapb.ShardReplication) error) error {
	if err := tee.primary.UpdateShardReplicationFields(ctx, cell, keyspace, shard, update); err != nil {
		// failed on primary, not updating secondary
		return err
	}

	if err := tee.secondary.UpdateShardReplicationFields(ctx, cell, keyspace, shard, update); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.UpdateShardReplicationFields(%v, %v, %v) failed: %v", cell, keyspace, shard, err)
	}
	return nil
}

// GetShardReplication is part of the topo.Server interface
func (tee *Tee) GetShardReplication(ctx context.Context, cell, keyspace, shard string) (*topo.ShardReplicationInfo, error) {
	return tee.readFrom.GetShardReplication(ctx, cell, keyspace, shard)
}

// DeleteShardReplication is part of the topo.Server interface
func (tee *Tee) DeleteShardReplication(ctx context.Context, cell, keyspace, shard string) error {
	if err := tee.primary.DeleteShardReplication(ctx, cell, keyspace, shard); err != nil {
		return err
	}

	if err := tee.secondary.DeleteShardReplication(ctx, cell, keyspace, shard); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.DeleteShardReplication(%v, %v, %v) failed: %v", cell, keyspace, shard, err)
	}
	return nil
}

// DeleteKeyspaceReplication is part of the topo.Server interface
func (tee *Tee) DeleteKeyspaceReplication(ctx context.Context, cell, keyspace string) error {
	if err := tee.primary.DeleteKeyspaceReplication(ctx, cell, keyspace); err != nil {
		return err
	}

	if err := tee.secondary.DeleteKeyspaceReplication(ctx, cell, keyspace); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.DeleteKeyspaceReplication(%v, %v) failed: %v", cell, keyspace, err)
	}
	return nil
}

//
// Serving Graph management, per cell.
//

// GetSrvKeyspaceNames is part of the topo.Server interface
func (tee *Tee) GetSrvKeyspaceNames(ctx context.Context, cell string) ([]string, error) {
	return tee.readFrom.GetSrvKeyspaceNames(ctx, cell)
}

// UpdateSrvKeyspace is part of the topo.Server interface
func (tee *Tee) UpdateSrvKeyspace(ctx context.Context, cell, keyspace string, srvKeyspace *topodatapb.SrvKeyspace) error {
	if err := tee.primary.UpdateSrvKeyspace(ctx, cell, keyspace, srvKeyspace); err != nil {
		return err
	}

	if err := tee.secondary.UpdateSrvKeyspace(ctx, cell, keyspace, srvKeyspace); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.UpdateSrvKeyspace(%v, %v) failed: %v", cell, keyspace, err)
	}
	return nil
}

// DeleteSrvKeyspace is part of the topo.Server interface
func (tee *Tee) DeleteSrvKeyspace(ctx context.Context, cell, keyspace string) error {
	err := tee.primary.DeleteSrvKeyspace(ctx, cell, keyspace)
	if err != nil && err != topo.ErrNoNode {
		return err
	}

	if err := tee.secondary.DeleteSrvKeyspace(ctx, cell, keyspace); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.DeleteSrvKeyspace(%v, %v) failed: %v", cell, keyspace, err)
	}
	return err
}

// GetSrvKeyspace is part of the topo.Server interface
func (tee *Tee) GetSrvKeyspace(ctx context.Context, cell, keyspace string) (*topodatapb.SrvKeyspace, error) {
	return tee.readFrom.GetSrvKeyspace(ctx, cell, keyspace)
}

// UpdateSrvVSchema is part of the topo.Server interface
func (tee *Tee) UpdateSrvVSchema(ctx context.Context, cell string, srvVSchema *vschemapb.SrvVSchema) error {
	if err := tee.primary.UpdateSrvVSchema(ctx, cell, srvVSchema); err != nil {
		return err
	}

	if err := tee.secondary.UpdateSrvVSchema(ctx, cell, srvVSchema); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.UpdateSrvVSchema(%v) failed: %v", cell, err)
	}
	return nil
}

// GetSrvVSchema is part of the topo.Server interface
func (tee *Tee) GetSrvVSchema(ctx context.Context, cell string) (*vschemapb.SrvVSchema, error) {
	return tee.readFrom.GetSrvVSchema(ctx, cell)
}

//
// Keyspace and Shard locks for actions, global.
//

// LockKeyspaceForAction is part of the topo.Server interface
func (tee *Tee) LockKeyspaceForAction(ctx context.Context, keyspace, contents string) (string, error) {
	// lock lockFirst
	pLockPath, err := tee.lockFirst.LockKeyspaceForAction(ctx, keyspace, contents)
	if err != nil {
		return "", err
	}

	// lock lockSecond
	sLockPath, err := tee.lockSecond.LockKeyspaceForAction(ctx, keyspace, contents)
	if err != nil {
		if err := tee.lockFirst.UnlockKeyspaceForAction(ctx, keyspace, pLockPath, "{}"); err != nil {
			log.Warningf("Failed to unlock lockFirst keyspace after failed lockSecond lock for %v", keyspace)
		}
		return "", err
	}

	// remember both locks, keyed by lockFirst lock path
	tee.mu.Lock()
	tee.keyspaceLockPaths[pLockPath] = sLockPath
	tee.mu.Unlock()
	return pLockPath, nil
}

// UnlockKeyspaceForAction is part of the topo.Server interface
func (tee *Tee) UnlockKeyspaceForAction(ctx context.Context, keyspace, lockPath, results string) error {
	// get from map
	tee.mu.Lock() // not using defer for unlock, to minimize lock time
	sLockPath, ok := tee.keyspaceLockPaths[lockPath]
	if !ok {
		tee.mu.Unlock()
		return fmt.Errorf("no lockPath %v in keyspaceLockPaths", lockPath)
	}
	delete(tee.keyspaceLockPaths, lockPath)
	tee.mu.Unlock()

	// unlock lockSecond, then lockFirst
	serr := tee.lockSecond.UnlockKeyspaceForAction(ctx, keyspace, sLockPath, results)
	perr := tee.lockFirst.UnlockKeyspaceForAction(ctx, keyspace, lockPath, results)

	if serr != nil {
		if perr != nil {
			log.Warningf("Secondary UnlockKeyspaceForAction(%v, %v) failed: %v", keyspace, sLockPath, serr)
		}
		return serr
	}
	return perr
}

// LockShardForAction is part of the topo.Server interface
func (tee *Tee) LockShardForAction(ctx context.Context, keyspace, shard, contents string) (string, error) {
	// lock lockFirst
	pLockPath, err := tee.lockFirst.LockShardForAction(ctx, keyspace, shard, contents)
	if err != nil {
		return "", err
	}

	// lock lockSecond
	sLockPath, err := tee.lockSecond.LockShardForAction(ctx, keyspace, shard, contents)
	if err != nil {
		if err := tee.lockFirst.UnlockShardForAction(ctx, keyspace, shard, pLockPath, "{}"); err != nil {
			log.Warningf("Failed to unlock lockFirst shard after failed lockSecond lock for %v/%v", keyspace, shard)
		}
		return "", err
	}

	// remember both locks, keyed by lockFirst lock path
	tee.mu.Lock()
	tee.shardLockPaths[pLockPath] = sLockPath
	tee.mu.Unlock()
	return pLockPath, nil
}

// UnlockShardForAction is part of the topo.Server interface
func (tee *Tee) UnlockShardForAction(ctx context.Context, keyspace, shard, lockPath, results string) error {
	// get from map
	tee.mu.Lock() // not using defer for unlock, to minimize lock time
	sLockPath, ok := tee.shardLockPaths[lockPath]
	if !ok {
		tee.mu.Unlock()
		return fmt.Errorf("no lockPath %v in shardLockPaths", lockPath)
	}
	delete(tee.shardLockPaths, lockPath)
	tee.mu.Unlock()

	// unlock lockSecond, then lockFirst
	serr := tee.lockSecond.UnlockShardForAction(ctx, keyspace, shard, sLockPath, results)
	perr := tee.lockFirst.UnlockShardForAction(ctx, keyspace, shard, lockPath, results)

	if serr != nil {
		if perr != nil {
			log.Warningf("Secondary UnlockShardForAction(%v/%v, %v) failed: %v", keyspace, shard, sLockPath, serr)
		}
		return serr
	}
	return perr
}

// SaveVSchema is part of the topo.Server interface
func (tee *Tee) SaveVSchema(ctx context.Context, keyspace string, contents *vschemapb.Keyspace) error {
	err := tee.primary.SaveVSchema(ctx, keyspace, contents)
	if err != nil {
		return err
	}

	if err := tee.secondary.SaveVSchema(ctx, keyspace, contents); err != nil {
		// not critical enough to fail
		log.Warningf("secondary.SaveVSchema() failed: %v", err)
	}
	return err
}

// GetVSchema is part of the topo.Server interface
func (tee *Tee) GetVSchema(ctx context.Context, keyspace string) (*vschemapb.Keyspace, error) {
	return tee.readFrom.GetVSchema(ctx, keyspace)
}

// NewMasterParticipation is part of the topo.Server interface
func (tee *Tee) NewMasterParticipation(name, id string) (topo.MasterParticipation, error) {
	return tee.primary.NewMasterParticipation(name, id)
}

var _ topo.Impl = (*Tee)(nil) // compile-time interface check