Prometheus: Min step defaults to seconds when no unit is set to prevent errors when running alerts. (#30966)

* update interval to fix no unit

* fix bug

* update with new method

* format go

* add test

* change test method to add test error nil

* add simplejson and model package

* change json format

* add parentheses

* add simplejson to function

* change mode to model

* move assert import package

* add one enter to avoid goimport error

* change to test case

pkg/tsdb/interval.go

@@ -2,6 +2,7 @@ package tsdb
 
 import (
 	"fmt"
+	"regexp"
 	"strings"
 	"time"
 
@@ -81,6 +82,13 @@ func GetIntervalFrom(dsInfo *models.DataSource, queryModel *simplejson.Json, def
 	}
 
 	interval = strings.Replace(strings.Replace(interval, "<", "", 1), ">", "", 1)
+	isPureNum, err := regexp.MatchString(`^\d+$`, interval)
+	if err != nil {
+		return time.Duration(0), err
+	}
+	if isPureNum {
+		interval += "s"
+	}
 	parsedInterval, err := time.ParseDuration(interval)
 	if err != nil {
 		return time.Duration(0), err

pkg/tsdb/interval_test.go

@@ -4,6 +4,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/grafana/grafana/pkg/components/simplejson"
+	"github.com/grafana/grafana/pkg/models"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -65,3 +67,26 @@ func TestFormatDuration(t *testing.T) {
 		})
 	}
 }
+
+func TestGetIntervalFrom(t *testing.T) {
+	testCases := []struct {
+		name            string
+		dsInfo          *models.DataSource
+		queryModel      string
+		defaultInterval time.Duration
+		expected        time.Duration
+	}{
+		{"45s", nil, `{"interval": "45s"}`, time.Second * 15, time.Second * 45},
+		{"45", nil, `{"interval": "45"}`, time.Second * 15, time.Second * 45},
+		{"2m", nil, `{"interval": "2m"}`, time.Second * 15, time.Minute * 2},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			js, _ := simplejson.NewJson([]byte(tc.queryModel))
+			actual, err := GetIntervalFrom(tc.dsInfo, js, tc.defaultInterval)
+			assert.Nil(t, err)
+			assert.Equal(t, tc.expected, actual)
+		})
+	}
+}