clientv3/concurrency/mutex.go
func (m *Mutex) Lock(ctx context.Context) error {
	s := m.s
	client := m.s.Client()

	m.myKey = fmt.Sprintf("%s%x", m.pfx, s.Lease())
	cmp := v3.Compare(v3.CreateRevision(m.myKey), "=", 0)
	// put self in lock waiters via myKey; oldest waiter holds lock
	put := v3.OpPut(m.myKey, "", v3.WithLease(s.Lease()))
	// reuse key in case this session already holds the lock
	get := v3.OpGet(m.myKey)
	// 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
	}
	m.myRev = resp.Header.Revision
	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)
	// release lock key if wait failed
	if werr != nil {
		m.Unlock(client.Ctx())
	} else {
		m.hdr = hdr
	}
	return werr
}
func (m *Mutex) Lock(ctx context.Context) error {
	s := m.s
	client := m.s.Client()

	m.myKey = fmt.Sprintf("%s%x", m.pfx, s.Lease())
	cmp := v3.Compare(v3.CreateRevision(m.myKey), "=", 0)
	// put self in lock waiters via myKey; oldest waiter holds lock
	put := v3.OpPut(m.myKey, "", v3.WithLease(s.Lease()))
	// reuse key in case this session already holds the lock
	get := v3.OpGet(m.myKey)
	// 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
	}
	m.myRev = resp.Header.Revision
	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)
	// release lock key if wait failed
	if werr != nil {
		m.Unlock(client.Ctx())
	} else {
		m.hdr = hdr
	}
	return werr
}
// 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.