Support specifying CurrentPrimary in ERS

Signed-off-by: Tim Vaillancourt <[email protected]>

go/vt/vtctl/grpcvtctldserver/server.go

@@ -1233,6 +1233,7 @@ func (s *VtctldServer) EmergencyReparentShard(ctx context.Context, req *vtctldat
 			WaitReplicasTimeout:       waitReplicasTimeout,
 			WaitAllTablets:            req.WaitForAllTablets,
 			PreventCrossCellPromotion: req.PreventCrossCellPromotion,
+			CurrentPrimaryAlias:       req.CurrentPrimary,
 		},
 	)
 

go/vt/vtctl/reparentutil/emergency_reparenter.go

@@ -60,6 +60,7 @@ type EmergencyReparentOptions struct {
 	WaitAllTablets            bool
 	WaitReplicasTimeout       time.Duration
 	PreventCrossCellPromotion bool
+	CurrentPrimaryAlias       *topodatapb.TabletAlias
 
 	// Private options managed internally. We use value passing to avoid leaking
 	// these details back out.
@@ -165,6 +166,10 @@ func (erp *EmergencyReparenter) reparentShardLocked(ctx context.Context, ev *eve
 	}
 	ev.ShardInfo = *shardInfo
 
+	if opts.CurrentPrimaryAlias != nil && opts.CurrentPrimaryAlias != shardInfo.PrimaryAlias {
+		return vterrors.Errorf(vtrpc.Code_FAILED_PRECONDITION, "CurrentPrimary %q is not equal to %q", shardInfo.PrimaryAlias, opts.CurrentPrimaryAlias)
+	}
+
 	keyspaceDurability, err := erp.ts.GetKeyspaceDurability(ctx, keyspace)
 	if err != nil {
 		return err

proto/vtctldata.proto

@@ -680,6 +680,8 @@ message EmergencyReparentShardRequest {
   // WaitForAllTablets makes ERS wait for a response from all the tablets before proceeding.
   // Useful when all the tablets are up and reachable.
   bool wait_for_all_tablets = 7;
+  // CurrentPrimary is the alias we expect to be the current primary.
+  topodata.TabletAlias current_primary = 8;
 }
 
 message EmergencyReparentShardResponse {