Don't assume delta has at least one sample per vector element.

rules/ast/functions.go

@@ -85,6 +85,12 @@ func deltaImpl(timestamp time.Time, view *viewAdapter, args []Node) interface{}
 		matrixValue = matrixNode.EvalBoundaries(timestamp, view)
 	}
 	for _, samples := range matrixValue {
+		// No sense in trying to compute a delta without at least two points. Drop
+		// this vector element.
+		if len(samples.Values) < 2 {
+			continue
+		}
+
 		counterCorrection := model.SampleValue(0)
 		lastValue := model.SampleValue(0)
 		for _, sample := range samples.Values {

rules/ast/functions_test.go

@@ -0,0 +1,56 @@
+// Copyright 2013 Prometheus Team
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ast
+
+import (
+	"github.com/prometheus/prometheus/model"
+	"testing"
+	"time"
+)
+
+type emptyRangeNode struct{}
+
+func (node emptyRangeNode) Type() ExprType             { return MATRIX }
+func (node emptyRangeNode) NodeTreeToDotGraph() string { return "" }
+func (node emptyRangeNode) Children() []Node           { return []Node{} }
+
+func (node emptyRangeNode) Eval(timestamp time.Time, view *viewAdapter) Matrix {
+	return Matrix{
+		model.SampleSet{
+			Metric: model.Metric{model.MetricNameLabel: "empty_metric"},
+			Values: model.Values{},
+		},
+	}
+}
+
+func (node emptyRangeNode) EvalBoundaries(timestamp time.Time, view *viewAdapter) Matrix {
+	return Matrix{
+		model.SampleSet{
+			Metric: model.Metric{model.MetricNameLabel: "empty_metric"},
+			Values: model.Values{},
+		},
+	}
+}
+
+func TestDeltaWithEmptyElementDoesNotCrash(t *testing.T) {
+	now := time.Now()
+	vector := deltaImpl(now, nil, []Node{emptyRangeNode{}, &ScalarLiteral{value: 0}}).(Vector)
+	if len(vector) != 0 {
+		t.Fatalf("Expected empty result vector, got: %v", vector)
+	}
+	vector = deltaImpl(now, nil, []Node{emptyRangeNode{}, &ScalarLiteral{value: 1}}).(Vector)
+	if len(vector) != 0 {
+		t.Fatalf("Expected empty result vector, got: %v", vector)
+	}
+}