make sure frequency cannot be zero

frequency set to zero causes division by zero
panics in the alert schedular.

closes #14810

pkg/services/alerting/rule.go

@@ -1,16 +1,21 @@
 package alerting
 
 import (
+	"errors"
 	"fmt"
 	"regexp"
 	"strconv"
 	"time"
 
 	"github.com/grafana/grafana/pkg/components/simplejson"
-
 	m "github.com/grafana/grafana/pkg/models"
 )
 
+var (
+	ErrFrequencyCannotBeZeroOrLess = errors.New(`"evaluate every" cannot be zero or below`)
+	ErrFrequencyCouldNotBeParsed   = errors.New(`"evaluate every" field could not be parsed`)
+)
+
 type Rule struct {
 	Id                  int64
 	OrgId               int64
@@ -76,7 +81,7 @@ func getTimeDurationStringToSeconds(str string) (int64, error) {
 	matches := ValueFormatRegex.FindAllString(str, 1)
 
 	if len(matches) <= 0 {
-		return 0, fmt.Errorf("Frequency could not be parsed")
+		return 0, ErrFrequencyCouldNotBeParsed
 	}
 
 	value, err := strconv.Atoi(matches[0])
@@ -84,6 +89,10 @@ func getTimeDurationStringToSeconds(str string) (int64, error) {
 		return 0, err
 	}
 
+	if value == 0 {
+		return 0, ErrFrequencyCannotBeZeroOrLess
+	}
+
 	unit := UnitFormatRegex.FindAllString(str, 1)[0]
 
 	if val, ok := unitMultiplier[unit]; ok {
@@ -101,7 +110,6 @@ func NewRuleFromDBAlert(ruleDef *m.Alert) (*Rule, error) {
 	model.PanelId = ruleDef.PanelId
 	model.Name = ruleDef.Name
 	model.Message = ruleDef.Message
-	model.Frequency = ruleDef.Frequency
 	model.State = ruleDef.State
 	model.LastStateChange = ruleDef.NewStateDate
 	model.For = ruleDef.For
@@ -109,6 +117,13 @@ func NewRuleFromDBAlert(ruleDef *m.Alert) (*Rule, error) {
 	model.ExecutionErrorState = m.ExecutionErrorOption(ruleDef.Settings.Get("executionErrorState").MustString("alerting"))
 	model.StateChanges = ruleDef.StateChanges
 
+	model.Frequency = ruleDef.Frequency
+	// frequency cannot be zero since that would not execute the alert rule.
+	// so we fallback to 60 seconds if `Freqency` is missing
+	if model.Frequency == 0 {
+		model.Frequency = 60
+	}
+
 	for _, v := range ruleDef.Settings.Get("notifications").MustArray() {
 		jsonModel := simplejson.NewFromAny(v)
 		id, err := jsonModel.Get("id").Int64()