Nil-safe topo.lock

Signed-off-by: Florent Poinsard <[email protected]>

go/vt/topo/locks.go

@@ -19,6 +19,7 @@ package topo
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"os"
 	"os/user"
 	"sync"
@@ -158,9 +159,6 @@ type iTopoLock interface {
 
 // perform the topo lock operation
 func (l *Lock) lock(ctx context.Context, ts *Server, lt iTopoLock, opts ...LockOption) (LockDescriptor, error) {
-	if err := ctx.Err(); err != nil {
-		return nil, err
-	}
 	for _, o := range opts {
 		o.apply(&l.Options)
 	}
@@ -178,6 +176,12 @@ func (l *Lock) lock(ctx context.Context, ts *Server, lt iTopoLock, opts ...LockO
 		return nil, err
 	}
 
+	if err := ctx.Err(); err != nil {
+		return nil, err
+	}
+	if ts.globalCell == nil {
+		return nil, errors.New("no global cell connection on the topo server")
+	}
 	switch l.Options.lockType {
 	case NonBlocking:
 		return ts.globalCell.TryLock(ctx, lt.Path(), j)