Add absent() function.

A common problem in Prometheus alerting is to detect when no timeseries
exist for a given metric name and label combination. Unfortunately,
Prometheus alert expressions need to be of vector type, and
"count(nonexistent_metric)" results in an empty vector, yielding no
output vector elements to base an alert on. The newly introduced
absent() function solves this issue:

  ALERT FooAbsent IF absent(foo{job="myjob"}) [...]

absent() has the following behavior:

- if the vector passed to it has any elements, it returns an empty
  vector.

- if the vector passed to it has no elements, it returns a 1-element
  vector with the value 1.

In the second case, absent() tries to be smart about deriving labels of
the 1-element output vector from the input vector:

  absent(nonexistent{job="myjob"})                   => {job="myjob"}
  absent(nonexistent{job="myjob",instance=~".*"})    => {job="myjob"}
  absent(sum(nonexistent{job="myjob"}))              => {}

That is, if the passed vector is a literal vector selector, it takes all
"=" label matchers as the basis for the output labels, but ignores all
non-equals or regex matchers. Also, if the passed vector results from a
non-selector expression, no labels can be derived.

Change-Id: I948505a1488d50265ab5692a3286bd7c8c70cd78

rules/ast/functions.go

@@ -454,6 +454,29 @@ func absImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
 	return vector
 }
 
+// === absent(vector VectorNode) Vector ===
+func absentImpl(timestamp clientmodel.Timestamp, args []Node) interface{} {
+	n := args[0].(VectorNode)
+	if len(n.Eval(timestamp)) > 0 {
+		return Vector{}
+	}
+	m := clientmodel.Metric{}
+	if vs, ok := n.(*VectorSelector); ok {
+		for _, matcher := range vs.labelMatchers {
+			if matcher.Type == metric.Equal && matcher.Name != clientmodel.MetricNameLabel {
+				m[matcher.Name] = matcher.Value
+			}
+		}
+	}
+	return Vector{
+		&clientmodel.Sample{
+			Metric:    m,
+			Value:     1,
+			Timestamp: timestamp,
+		},
+	}
+}
+
 var functions = map[string]*Function{
 	"abs": {
 		name:       "abs",
@@ -461,6 +484,12 @@ var functions = map[string]*Function{
 		returnType: VECTOR,
 		callFn:     absImpl,
 	},
+	"absent": {
+		name:       "absent",
+		argTypes:   []ExprType{VECTOR},
+		returnType: VECTOR,
+		callFn:     absentImpl,
+	},
 	"avg_over_time": {
 		name:       "avg_over_time",
 		argTypes:   []ExprType{MATRIX},

rules/rules_test.go

@@ -596,6 +596,46 @@ func TestExpressions(t *testing.T) {
 			fullRanges:     0,
 			intervalRanges: 4,
 		},
+		{
+			expr: `absent(nonexistent)`,
+			output: []string{
+				`{} => 1 @[%v]`,
+			},
+			fullRanges:     0,
+			intervalRanges: 0,
+		},
+		{
+			expr: `absent(nonexistent{job="testjob", instance="testinstance", method=~".*"})`,
+			output: []string{
+				`{instance="testinstance", job="testjob"} => 1 @[%v]`,
+			},
+			fullRanges:     0,
+			intervalRanges: 0,
+		},
+		{
+			expr: `count_scalar(absent(http_requests))`,
+			output: []string{
+				`scalar: 0 @[%v]`,
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		},
+		{
+			expr: `count_scalar(absent(sum(http_requests)))`,
+			output: []string{
+				`scalar: 0 @[%v]`,
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
+		},
+		{
+			expr: `absent(sum(nonexistent{job="testjob", instance="testinstance"}))`,
+			output: []string{
+				`{} => 1 @[%v]`,
+			},
+			fullRanges:     0,
+			intervalRanges: 0,
+		},
 	}
 
 	storage, closer := newTestStorage(t)