[release-20.0] Tablet picker: Handle the case where a primary tablet is not setup for a shard (#17573) (#17575)

Signed-off-by: Rohit Nayak <rohit@planetscale.com>
Co-authored-by: vitess-bot[bot] <108069721+vitess-bot[bot]@users.noreply.github.com>
Co-authored-by: Rohit Nayak <rohit@planetscale.com>

Author: vitess-bot[bot]

go/vt/discovery/tablet_picker.go

@@ -388,8 +388,13 @@ func (tp *TabletPicker) GetMatchingTablets(ctx context.Context) []*topo.TabletIn
 			log.Errorf("Error getting shard %s/%s: %v", tp.keyspace, tp.shard, err)
 			return nil
 		}
-		if _, ignore := tp.ignoreTablets[si.PrimaryAlias.String()]; !ignore {
-			aliases = append(aliases, si.PrimaryAlias)
+
+		// It is possible that there is a cluster event (ERS/PRS, for example) due to which
+		// there is no primary elected for the shard at the moment.
+		if si.PrimaryAlias != nil {
+			if _, ignore := tp.ignoreTablets[si.PrimaryAlias.String()]; !ignore {
+				aliases = append(aliases, si.PrimaryAlias)
+			}
 		}
 	} else {
 		actualCells := make([]string, 0)
@@ -426,6 +431,9 @@ func (tp *TabletPicker) GetMatchingTablets(ctx context.Context) []*topo.TabletIn
 				continue
 			}
 			for _, node := range sri.Nodes {
+				if node.TabletAlias == nil {
+					continue
+				}
 				if _, ignore := tp.ignoreTablets[node.TabletAlias.String()]; !ignore {
 					aliases = append(aliases, node.TabletAlias)
 				}

go/vt/discovery/tablet_picker_test.go

@@ -62,6 +62,29 @@ func TestPickPrimary(t *testing.T) {
 	assert.True(t, proto.Equal(want, tablet), "Pick: %v, want %v", tablet, want)
 }
 
+// TestPickNoPrimary confirms that if the picker was setup only for primary tablets but
+// there is no primary setup for the shard we correctly return an error.
+func TestPickNoPrimary(t *testing.T) {
+	defer utils.EnsureNoLeaks(t)
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	te := newPickerTestEnv(t, ctx, []string{"cell", "otherCell"})
+	want := addTablet(ctx, te, 100, topodatapb.TabletType_PRIMARY, "cell", true, true)
+	defer deleteTablet(t, te, want)
+	_, err := te.topoServ.UpdateShardFields(ctx, te.keyspace, te.shard, func(si *topo.ShardInfo) error {
+		si.PrimaryAlias = nil // force a missing primary
+		return nil
+	})
+	require.NoError(t, err)
+
+	tp, err := NewTabletPicker(ctx, te.topoServ, []string{"otherCell"}, "cell", te.keyspace, te.shard, "primary", TabletPickerOptions{})
+	require.NoError(t, err)
+
+	_, err = tp.PickForStreaming(ctx)
+	require.Errorf(t, err, "No healthy serving tablet")
+}
+
 func TestPickLocalPreferences(t *testing.T) {
 	defer utils.EnsureNoLeaks(t)
 	type tablet struct {

go/vt/topo/tablet.go

@@ -489,6 +489,9 @@ func (ts *Server) GetTabletMap(ctx context.Context, tabletAliases []*topodatapb.
 	var sem = semaphore.NewWeighted(int64(concurrency))
 
 	for _, tabletAlias := range tabletAliases {
+		if tabletAlias == nil {
+			return nil, vterrors.Errorf(vtrpc.Code_INVALID_ARGUMENT, "nil tablet alias in list")
+		}
 		wg.Add(1)
 		go func(tabletAlias *topodatapb.TabletAlias) {
 			defer wg.Done()