federate: Fix PeekBack usage

In most cases, there is no sample at `maxt`, so `PeekBack` has to be
used. So far, `PeekBack` did not return a float histogram, and we
disregarded even any returned normal histogram. This fixes both, and
also tweaks the unit test to discover the problem (by using an earlier
timestamp than "now" for the samples in the TSDB).

Signed-off-by: beorn7 <beorn@grafana.com>

storage/buffer.go

@@ -68,9 +68,11 @@ func (b *BufferedSeriesIterator) ReduceDelta(delta int64) bool {
 
 // PeekBack returns the nth previous element of the iterator. If there is none buffered,
 // ok is false.
-func (b *BufferedSeriesIterator) PeekBack(n int) (t int64, v float64, h *histogram.Histogram, ok bool) {
+func (b *BufferedSeriesIterator) PeekBack(n int) (
+	t int64, v float64, h *histogram.Histogram, fh *histogram.FloatHistogram, ok bool,
+) {
 	s, ok := b.buf.nthLast(n)
-	return s.t, s.v, s.h, ok
+	return s.t, s.v, s.h, s.fh, ok
 }
 
 // Buffer returns an iterator over the buffered data. Invalidates previously

storage/buffer_test.go

@@ -107,7 +107,7 @@ func TestBufferedSeriesIterator(t *testing.T) {
 		require.Equal(t, ev, v, "value mismatch")
 	}
 	prevSampleEq := func(ets int64, ev float64, eok bool) {
-		ts, v, _, ok := it.PeekBack(1)
+		ts, v, _, _, ok := it.PeekBack(1)
 		require.Equal(t, eok, ok, "exist mismatch")
 		require.Equal(t, ets, ts, "timestamp mismatch")
 		require.Equal(t, ev, v, "value mismatch")

web/federate.go

@@ -116,7 +116,8 @@ Loop:
 		var (
 			t  int64
 			v  float64
-			h  *histogram.FloatHistogram
+			h  *histogram.Histogram
+			fh *histogram.FloatHistogram
 			ok bool
 		)
 		valueType := it.Seek(maxt)
@@ -124,12 +125,15 @@ Loop:
 		case chunkenc.ValFloat:
 			t, v = it.At()
 		case chunkenc.ValFloatHistogram, chunkenc.ValHistogram:
-			t, h = it.AtFloatHistogram()
+			t, fh = it.AtFloatHistogram()
 		default:
-			t, v, _, ok = it.PeekBack(1)
+			t, v, h, fh, ok = it.PeekBack(1)
 			if !ok {
 				continue Loop
 			}
+			if h != nil {
+				fh = h.ToFloat()
+			}
 		}
 		// The exposition formats do not support stale markers, so drop them. This
 		// is good enough for staleness handling of federated data, as the
@@ -141,7 +145,7 @@ Loop:
 
 		vec = append(vec, promql.Sample{
 			Metric: s.Labels(),
-			Point:  promql.Point{T: t, V: v, H: h},
+			Point:  promql.Point{T: t, V: v, H: fh},
 		})
 	}
 	if ws := set.Warnings(); len(ws) > 0 {

web/federate_test.go

@@ -340,16 +340,16 @@ func TestFederationWithNativeHistograms(t *testing.T) {
 		l := labels.FromStrings("__name__", "test_metric", "foo", fmt.Sprintf("%d", i))
 		expL := labels.FromStrings("__name__", "test_metric", "instance", "", "foo", fmt.Sprintf("%d", i))
 		if i%3 == 0 {
-			_, err = app.Append(0, l, 101*60*1000, float64(i*100))
+			_, err = app.Append(0, l, 100*60*1000, float64(i*100))
 			expVec = append(expVec, promql.Sample{
-				Point:  promql.Point{T: 101 * 60 * 1000, V: float64(i * 100)},
+				Point:  promql.Point{T: 100 * 60 * 1000, V: float64(i * 100)},
 				Metric: expL,
 			})
 		} else {
 			hist.ZeroCount++
-			_, err = app.AppendHistogram(0, l, 101*60*1000, hist.Copy(), nil)
+			_, err = app.AppendHistogram(0, l, 100*60*1000, hist.Copy(), nil)
 			expVec = append(expVec, promql.Sample{
-				Point:  promql.Point{T: 101 * 60 * 1000, H: hist.ToFloat()},
+				Point:  promql.Point{T: 100 * 60 * 1000, H: hist.ToFloat()},
 				Metric: expL,
 			})
 		}