Fix `getFromThrough()` function for ShortRefs (#11386)

**What this PR does / why we need it**:

Since chunks don't necessarily have the same duration, we cannot assume
that the last item of a slice sorted by `From` time also has the
greatest `Through` time.

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

pkg/bloomgateway/util.go

@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"github.com/prometheus/common/model"
+	"golang.org/x/exp/slices"
 
 	"github.com/grafana/loki/pkg/logproto"
 	v1 "github.com/grafana/loki/pkg/storage/bloom/v1"
@@ -76,15 +77,22 @@ func getDayTime(ts model.Time) time.Time {
 	return time.Date(ts.Time().Year(), ts.Time().Month(), ts.Time().Day(), 0, 0, 0, 0, time.UTC)
 }
 
-// TODO(chaudum): Fix Through time calculation
 // getFromThrough assumes a list of ShortRefs sorted by From time
-// However, it does also assume that the last item has the highest
-// Through time, which might not be the case!
 func getFromThrough(refs []*logproto.ShortRef) (model.Time, model.Time) {
 	if len(refs) == 0 {
 		return model.Earliest, model.Latest
 	}
-	return refs[0].From, refs[len(refs)-1].Through
+
+	maxItem := slices.MaxFunc(refs, func(a, b *logproto.ShortRef) int {
+		if a.Through > b.Through {
+			return 1
+		} else if a.Through < b.Through {
+			return -1
+		}
+		return 0
+	})
+
+	return refs[0].From, maxItem.Through
 }
 
 // convertToSearches converts a list of line filter expressions to a list of

pkg/bloomgateway/util_test.go

@@ -3,6 +3,7 @@ package bloomgateway
 import (
 	"testing"
 
+	"github.com/prometheus/common/model"
 	"github.com/stretchr/testify/require"
 
 	"github.com/grafana/loki/pkg/logproto"
@@ -37,6 +38,23 @@ func TestSliceIterWithIndex(t *testing.T) {
 	})
 }
 
+func TestGetFromThrough(t *testing.T) {
+	chunks := []*logproto.ShortRef{
+		{From: 0, Through: 6},
+		{From: 1, Through: 5},
+		{From: 2, Through: 9},
+		{From: 3, Through: 8},
+		{From: 4, Through: 7},
+	}
+	from, through := getFromThrough(chunks)
+	require.Equal(t, model.Time(0), from)
+	require.Equal(t, model.Time(9), through)
+
+	// assert that slice order did not change
+	require.Equal(t, model.Time(0), chunks[0].From)
+	require.Equal(t, model.Time(4), chunks[len(chunks)-1].From)
+}
+
 func mkBlockRef(minFp, maxFp uint64) bloomshipper.BlockRef {
 	return bloomshipper.BlockRef{
 		Ref: bloomshipper.Ref{