TSDB: Always drop 'quiet zeros' if out-of-order

"Quiet Zeros" are used to insert the starting sample for a counter.
If the zero is out-of-order then we already have some data for the
series, and we should have already handled the start value, so just drop
the quiet zero.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/head_append.go

@@ -505,6 +505,10 @@ func (s *memSeries) appendable(t int64, v float64, headMaxt, minValidTime, oooTi
 		}
 	}
 
+	if math.Float64bits(v) == value.QuietZeroNaN { // Say it's allowed; it will be dropped later in commitSamples.
+		return true, 0, nil
+	}
+
 	// The sample cannot go in the in-order chunk. Check if it can go in the out-of-order chunk.
 	if oooTimeWindow > 0 && t >= headMaxt-oooTimeWindow {
 		return true, headMaxt - t, nil
@@ -1141,13 +1145,11 @@ func (a *headAppender) commitSamples(acc *appenderCommitContext) {
 			handleAppendableError(err, &acc.floatsAppended, &acc.floatOOORejected, &acc.floatOOBRejected, &acc.floatTooOldRejected)
 		}
 
-		if math.Float64bits(s.V) == value.QuietZeroNaN {
-			s.V = 0
-		}
-
 		switch {
 		case err != nil:
 			// Do nothing here.
+		case oooSample && math.Float64bits(s.V) == value.QuietZeroNaN:
+			// No-op: we don't store quiet zeros out-of-order.
 		case oooSample:
 			// Sample is OOO and OOO handling is enabled
 			// and the delta is within the OOO tolerance.
@@ -1194,7 +1196,11 @@ func (a *headAppender) commitSamples(acc *appenderCommitContext) {
 				acc.floatsAppended--
 			}
 		default:
-			ok, chunkCreated = series.append(s.T, s.V, a.appendID, acc.appendChunkOpts)
+			v := s.V
+			if math.Float64bits(v) == value.QuietZeroNaN {
+				v = 0
+			}
+			ok, chunkCreated = series.append(s.T, v, a.appendID, acc.appendChunkOpts)
 			if ok {
 				if s.T < acc.inOrderMint {
 					acc.inOrderMint = s.T

tsdb/head_test.go

@@ -6123,6 +6123,15 @@ func TestAppendDuplicates(t *testing.T) {
 		sample{t: ts + 10, f: 0},
 	}
 	require.Equal(t, expectedSamples, result[`{foo="bar"}`])
+
+	a = h.Appender(context.Background())
+	_, err = a.Append(0, lbls, ts+5, math.Float64frombits(value.QuietZeroNaN)) // This is out-of-order, so should be dropped.
+	require.NoError(t, err)
+	require.NoError(t, a.Commit())
+
+	result, err = queryHead(t, h, math.MinInt64, math.MaxInt64, labels.Label{Name: "foo", Value: "bar"})
+	require.NoError(t, err)
+	require.Equal(t, expectedSamples, result[`{foo="bar"}`]) // Same expectedSamples as before.
 }
 
 // TestHeadDetectsDuplicateSampleAtSizeLimit tests a regression where a duplicate sample