guard against NPE when setting precomputed data

If a precomputed call returns a nil Row result somehow, that could
cause a nil pointer exception when handling the result in
handlePreCall.

In this particular case, A Distinct call on a BSI field with a filter
which returned no results could return a SignedRow{} with nil *Rows
inside of it. This only manifested if there was data in a single shard
as otherwise the reduce logic created a SignedRow with empty *Row
objects rather than nil ones. Isn't that fun?

Extra fun: the reason the filter was returning no results was not
because it was actually empty, but because of another bug where
constructing the Distinct calls to compute the aggregate of a GroupBy
doesn't take into account that the group might include an integer
field which means that the call needs to be constructed
differently. That bug is not fixed in this commit, hence the tests are
still failing, but not panicking.

executor.go

@@ -441,9 +441,11 @@ func (e *executor) handlePreCalls(ctx context.Context, qcx *Qcx, index string, c
 	// shards if the query has to go to them
 	opt.EmbeddedData = append(opt.EmbeddedData, row)
 	// and stash a copy locally, so local calls can use it
-	c.Precomputed = make(map[uint64]interface{}, len(row.segments))
-	for _, segment := range row.segments {
-		c.Precomputed[segment.shard] = &Row{segments: []rowSegment{segment}}
+	if row != nil {
+		c.Precomputed = make(map[uint64]interface{}, len(row.segments))
+		for _, segment := range row.segments {
+			c.Precomputed[segment.shard] = &Row{segments: []rowSegment{segment}}
+		}
 	}
 	return nil
 }

executor_test.go

@@ -7144,6 +7144,88 @@ pangolin,1,100
 	}
 }
 
+// TestVariousSingleShardQueries tests queries on a dataset which
+// consists of an unkeyed index, and data only in the first
+// shard. Turns out that there are some interesting failure modes
+// which only crop up with one shard. An example is that the mapReduce
+// logic does its first reduce call with a nil interface{} value, and
+// an actual result value. If this is the only reduce that gets done
+// (because there's only one shard), the result might never go through
+// normal merge/reduction logic and might e.g. be nil instead of an
+// empty struct resulting in an NPE later on.
+func TestVariousSingleShardQueries(t *testing.T) {
+	for _, clusterSize := range []int{1, 4} {
+		t.Run(fmt.Sprintf("%d-node", clusterSize), func(t *testing.T) {
+			variousSingleShardQueries(t, clusterSize)
+		})
+	}
+}
+
+func variousSingleShardQueries(t *testing.T, clusterSize int) {
+	c := test.MustRunCluster(t, clusterSize)
+	defer c.Close()
+
+	// Create and populate "likenums" similar to "likes", but without keys on the field.
+	c.CreateField(t, "events", pilosa.IndexOptions{Keys: false, TrackExistence: true}, "lostcount", pilosa.OptFieldTypeInt(0, 1000000000))
+	c.ImportIntID(t, "events", "lostcount", []test.IntID{
+		{Val: 0, ID: 1},
+		{Val: 1, ID: 2},
+		{Val: 0, ID: 3},
+		{Val: 2, ID: 4},
+		{Val: 2, ID: 5},
+		{Val: 0, ID: 6},
+		{Val: 3, ID: 7},
+		{Val: 3, ID: 8},
+		{Val: 3, ID: 9},
+		{Val: 0, ID: 10},
+	})
+
+	c.CreateField(t, "events", pilosa.IndexOptions{Keys: false, TrackExistence: true}, "jittermax", pilosa.OptFieldTypeInt(0, 1000000000))
+	c.ImportIntID(t, "events", "jittermax", []test.IntID{
+		{Val: 17, ID: 1},
+		{Val: 3, ID: 2},
+		{Val: 42, ID: 3},
+		{Val: 9, ID: 4},
+		{Val: 17, ID: 5},
+		{Val: 3, ID: 6},
+		{Val: 42, ID: 7},
+		{Val: 9, ID: 8},
+		{Val: 17, ID: 9},
+		{Val: 3, ID: 10},
+	})
+
+	tests := []struct {
+		query       string
+		csvVerifier string
+	}{
+		{
+			query: "GroupBy(Rows(lostcount), aggregate=Count(Distinct(field=jittermax)))",
+			csvVerifier: `0,4,3
+1,1,1
+2,2,2
+3,3,3
+`,
+		},
+	}
+
+	for i, tst := range tests {
+		t.Run(fmt.Sprintf("%d-%s", i, tst.query), func(t *testing.T) {
+			tr := c.QueryGRPC(t, "events", tst.query)
+			csvString, err := tableResponseToCSVString(tr)
+			if err != nil {
+				t.Fatal(err)
+			}
+			// verify everything after header
+			got := csvString[strings.Index(csvString, "\n")+1:]
+			if got != tst.csvVerifier {
+				t.Errorf("expected:\n%s\ngot:\n%s", tst.csvVerifier, got)
+			}
+
+		})
+	}
+
+}
+
 // tableResponseToCSV converts a generic TableResponse to a CSV format
 // and writes it to the writer.
 func tableResponseToCSV(m *proto.TableResponse, w io.Writer) error {

test/cluster.go

@@ -63,7 +63,7 @@ func (c *Cluster) QueryHTTP(t testing.TB, index, query string) (string, error) {
 	if len(c.Nodes) == 0 {
 		t.Fatal("must have at least one node in cluster to query")
 	}
-	
+
 	return c.Nodes[0].Query(t, index, "", query)
 }
 
@@ -189,6 +189,30 @@ func (c *Cluster) ImportIntKey(t testing.TB, index, field string, pairs []IntKey
 	}
 }
 
+type IntID struct {
+	Val int64
+	ID  uint64
+}
+
+// ImportIntID imports data into an int field in an unkeyed index.
+func (c *Cluster) ImportIntID(t testing.TB, index, field string, pairs []IntID) {
+	t.Helper()
+	importRequest := &pilosa.ImportValueRequest{
+		Index:     index,
+		Field:     field,
+		Shard:     math.MaxUint64,
+		ColumnIDs: make([]uint64, len(pairs)),
+		Values:    make([]int64, len(pairs)),
+	}
+	for i, pair := range pairs {
+		importRequest.Values[i] = pair.Val
+		importRequest.ColumnIDs[i] = pair.ID
+	}
+	if err := c.Nodes[0].API.ImportValue(context.Background(), nil, importRequest); err != nil {
+		t.Fatalf("importing IntID data: %v", err)
+	}
+}
+
 // KeyID represents a key and an ID for importing data into an index
 // and field where one uses string keys and the other does not.
 type KeyID struct {