Respect tolerable replication lag even when the new primary has been provided in PRS

go/vt/vtctl/reparentutil/planned_reparenter.go

@@ -170,6 +170,7 @@
 		return true, vterrors.Errorf(vtrpc.Code_FAILED_PRECONDITION, "primary-elect tablet %v is the same as the tablet to avoid", topoproto.TabletAliasString(opts.NewPrimaryAlias))
 	}
 
+	toCheckReplicationLag := true
 	if opts.NewPrimaryAlias == nil {
 		// We don't want to fail when both ShardInfo.PrimaryAlias and AvoidPrimaryAlias are nil.
 		// This happens when we are using PRS to initialize the cluster without specifying the NewPrimaryAlias
@@ -191,6 +192,8 @@
 
 		pr.logger.Infof("elected new primary candidate %v", topoproto.TabletAliasString(opts.NewPrimaryAlias))
 		event.DispatchUpdate(ev, "elected new primary candidate")
+		// ChooseNewPrimary already takes into account the replication lag and doesn't require any further verification.
+		toCheckReplicationLag = false
 	}
 
 	primaryElectAliasStr := topoproto.TabletAliasString(opts.NewPrimaryAlias)
@@ -200,6 +203,20 @@
 		return true, vterrors.Errorf(vtrpc.Code_FAILED_PRECONDITION, "primary-elect tablet %v is not in the shard", primaryElectAliasStr)
 	}
 
+	// We check for replication lag if the primary was provided to us and tolerable replication lag is defined.
+	if toCheckReplicationLag && opts.TolerableReplLag > 0 {
+		// Find the replication lag for the primary-elect tablet.
+		_, replLag, err := findPositionAndLagForTablet(ctx, newPrimaryTabletInfo.Tablet, pr.logger, pr.tmc, opts.WaitReplicasTimeout)
+		if err != nil {
+			return true, err
+		}
+
+		// Check if the replication lag is more than that we can tolerate.
+		if replLag > opts.TolerableReplLag {
+			return true, vterrors.Errorf(vtrpc.Code_FAILED_PRECONDITION, "primary-elect tablet %v has %v replication lag which is more than tolerable", primaryElectAliasStr, replLag)
+		}
+	}
+
 	// PRS is only meant to be called when all the tablets are healthy.
 	// So we assume that all the tablets are reachable and check if the primary elect will be able
 	// to make progress if it is promoted. This is needed because sometimes users may ask to promote

go/vt/vtctl/reparentutil/planned_reparenter_flaky_test.go

@@ -616,6 +616,114 @@ func TestPlannedReparenter_preflightChecks(t *testing.T) {
 			},
 			shouldErr: false,
 		},
+		{
+			name: "new primary provided - replication lag is tolerable",
+			ev: &events.Reparent{
+				ShardInfo: *topo.NewShardInfo("testkeyspace", "-", &topodatapb.Shard{
+					PrimaryAlias: &topodatapb.TabletAlias{
+						Cell: "zone1",
+						Uid:  500,
+					},
+				}, nil),
+			},
+			tmc: &testutil.TabletManagerClient{
+				ReplicationStatusResults: map[string]struct {
+					Position *replicationdatapb.Status
+					Error    error
+				}{
+					"zone1-0000000100": {
+						Position: &replicationdatapb.Status{
+							Position:              "MySQL56/3E11FA47-71CA-11E1-9E33-C80AA9429562:1-2",
+							ReplicationLagSeconds: 2,
+						},
+					},
+				},
+			},
+			tabletMap: map[string]*topo.TabletInfo{
+				"zone1-0000000100": {
+					Tablet: &topodatapb.Tablet{
+						Alias: &topodatapb.TabletAlias{
+							Cell: "zone1",
+							Uid:  100,
+						},
+					},
+				},
+			},
+			opts: &PlannedReparentOptions{
+				NewPrimaryAlias: &topodatapb.TabletAlias{
+					Cell: "zone1",
+					Uid:  100,
+				},
+				TolerableReplLag: 10 * time.Second,
+			},
+			expectedIsNoop: false,
+			expectedEvent: &events.Reparent{
+				ShardInfo: *topo.NewShardInfo("testkeyspace", "-", &topodatapb.Shard{
+					PrimaryAlias: &topodatapb.TabletAlias{
+						Cell: "zone1",
+						Uid:  500,
+					},
+				}, nil),
+				NewPrimary: &topodatapb.Tablet{
+					Alias: &topodatapb.TabletAlias{
+						Cell: "zone1",
+						Uid:  100,
+					},
+				},
+			},
+			shouldErr: false,
+		},
+		{
+			name: "new primary provided - replication lag is not tolerable",
+			ev: &events.Reparent{
+				ShardInfo: *topo.NewShardInfo("testkeyspace", "-", &topodatapb.Shard{
+					PrimaryAlias: &topodatapb.TabletAlias{
+						Cell: "zone1",
+						Uid:  500,
+					},
+				}, nil),
+			},
+			tmc: &testutil.TabletManagerClient{
+				ReplicationStatusResults: map[string]struct {
+					Position *replicationdatapb.Status
+					Error    error
+				}{
+					"zone1-0000000100": {
+						Position: &replicationdatapb.Status{
+							Position:              "MySQL56/3E11FA47-71CA-11E1-9E33-C80AA9429562:1-2",
+							ReplicationLagSeconds: 25,
+						},
+					},
+				},
+			},
+			tabletMap: map[string]*topo.TabletInfo{
+				"zone1-0000000100": {
+					Tablet: &topodatapb.Tablet{
+						Alias: &topodatapb.TabletAlias{
+							Cell: "zone1",
+							Uid:  100,
+						},
+					},
+				},
+			},
+			opts: &PlannedReparentOptions{
+				NewPrimaryAlias: &topodatapb.TabletAlias{
+					Cell: "zone1",
+					Uid:  100,
+				},
+				TolerableReplLag: 10 * time.Second,
+			},
+			expectedIsNoop: true,
+			expectedEvent: &events.Reparent{
+				ShardInfo: *topo.NewShardInfo("testkeyspace", "-", &topodatapb.Shard{
+					PrimaryAlias: &topodatapb.TabletAlias{
+						Cell: "zone1",
+						Uid:  500,
+					},
+				}, nil),
+			},
+			shouldErr: true,
+		},
 		{
 			name: "invariants hold with primary selection",
 			tmc: &testutil.TabletManagerClient{