Merge pull request #8107 from heyitsanthony/lock-faster

concurrency: fetch current lock holder when creating waitlist key

clientv3/concurrency/mutex.go

@@ -49,7 +49,9 @@ func (m *Mutex) Lock(ctx context.Context) error {
 	put := v3.OpPut(m.myKey, "", v3.WithLease(s.Lease()))
 	// reuse key in case this session already holds the lock
 	get := v3.OpGet(m.myKey)
-	resp, err := client.Txn(ctx).If(cmp).Then(put).Else(get).Commit()
+	// fetch current holder to complete uncontended path with only one RPC
+	getOwner := v3.OpGet(m.pfx, v3.WithFirstCreate()...)
+	resp, err := client.Txn(ctx).If(cmp).Then(put, getOwner).Else(get, getOwner).Commit()
 	if err != nil {
 		return err
 	}
@@ -57,6 +59,12 @@ func (m *Mutex) Lock(ctx context.Context) error {
 	if !resp.Succeeded {
 		m.myRev = resp.Responses[0].GetResponseRange().Kvs[0].CreateRevision
 	}
+	// if no key on prefix / the minimum rev is key, already hold the lock
+	ownerKey := resp.Responses[1].GetResponseRange().Kvs
+	if len(ownerKey) == 0 || ownerKey[0].CreateRevision == m.myRev {
+		m.hdr = resp.Header
+		return nil
+	}
 
 	// wait for deletion revisions prior to myKey
 	hdr, werr := waitDeletes(ctx, client, m.pfx, m.myRev-1)