concurrency: support returning response header for mutex

clientv3/concurrency/election.go

@@ -69,7 +69,7 @@ func (e *Election) Campaign(ctx context.Context, val string) error {
 		}
 	}
 
-	err = waitDeletes(ctx, client, e.keyPrefix, e.leaderRev-1)
+	_, err = waitDeletes(ctx, client, e.keyPrefix, e.leaderRev-1)
 	if err != nil {
 		// clean up in case of context cancel
 		select {

clientv3/concurrency/key.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 
 	v3 "github.com/coreos/etcd/clientv3"
+	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
 	"github.com/coreos/etcd/mvcc/mvccpb"
 	"golang.org/x/net/context"
 )
@@ -46,19 +47,19 @@ func waitDelete(ctx context.Context, client *v3.Client, key string, rev int64) e
 
 // waitDeletes efficiently waits until all keys matching the prefix and no greater
 // than the create revision.
-func waitDeletes(ctx context.Context, client *v3.Client, pfx string, maxCreateRev int64) error {
+func waitDeletes(ctx context.Context, client *v3.Client, pfx string, maxCreateRev int64) (*pb.ResponseHeader, error) {
 	getOpts := append(v3.WithLastCreate(), v3.WithMaxCreateRev(maxCreateRev))
 	for {
 		resp, err := client.Get(ctx, pfx, getOpts...)
 		if err != nil {
-			return err
+			return nil, err
 		}
 		if len(resp.Kvs) == 0 {
-			return nil
+			return resp.Header, nil
 		}
 		lastKey := string(resp.Kvs[0].Key)
 		if err = waitDelete(ctx, client, lastKey, resp.Header.Revision); err != nil {
-			return err
+			return nil, err
 		}
 	}
 }

clientv3/concurrency/mutex.go

@@ -19,6 +19,7 @@ import (
 	"sync"
 
 	v3 "github.com/coreos/etcd/clientv3"
+	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
 	"golang.org/x/net/context"
 )
 
@@ -29,13 +30,14 @@ type Mutex struct {
 	pfx   string
 	myKey string
 	myRev int64
+	hdr   *pb.ResponseHeader
 }
 
 func NewMutex(s *Session, pfx string) *Mutex {
-	return &Mutex{s, pfx + "/", "", -1}
+	return &Mutex{s, pfx + "/", "", -1, nil}
 }
 
-// Lock locks the mutex with a cancellable context. If the context is cancelled
+// Lock locks the mutex with a cancelable context. If the context is canceled
 // while trying to acquire the lock, the mutex tries to clean its stale lock entry.
 func (m *Mutex) Lock(ctx context.Context) error {
 	s := m.s
@@ -57,14 +59,15 @@ func (m *Mutex) Lock(ctx context.Context) error {
 	}
 
 	// wait for deletion revisions prior to myKey
-	err = waitDeletes(ctx, client, m.pfx, m.myRev-1)
+	hdr, werr := waitDeletes(ctx, client, m.pfx, m.myRev-1)
 	// release lock key if cancelled
 	select {
 	case <-ctx.Done():
 		m.Unlock(client.Ctx())
 	default:
+		m.hdr = hdr
 	}
-	return err
+	return werr
 }
 
 func (m *Mutex) Unlock(ctx context.Context) error {
@@ -83,6 +86,9 @@ func (m *Mutex) IsOwner() v3.Cmp {
 
 func (m *Mutex) Key() string { return m.myKey }
 
+// Header is the response header received from etcd on acquiring the lock.
+func (m *Mutex) Header() *pb.ResponseHeader { return m.hdr }
+
 type lockerMutex struct{ *Mutex }
 
 func (lm *lockerMutex) Lock() {