Alerting: Consistently return Prometheus-style responses from rules APIs. (#86600)

* Alerting: Consistently return Prometheus-style responses from rules APIs.

This commit is part refactor and part fix. The /rules API occasionally returns
error responses which are inconsistent with other error responses. This fixes
that, and adds a function to map from Prometheus error type and HTTP code.

* Fix integration tests

* Linter happiness

* Make linter more happy

* Fix up one more place returning non-Prometheus responses

pkg/services/ngalert/api/tooling/definitions/prom.go

@@ -3,6 +3,7 @@ package definitions
 import (
 	"container/heap"
 	"fmt"
+	"net/http"
 	"sort"
 	"strings"
 	"time"
@@ -98,6 +99,23 @@ type RuleGroup struct {
 	EvaluationTime float64   `json:"evaluationTime"`
 }
 
+// HTTPStatusCode returns the HTTP status code for a given Prometheus style error.
+func (d DiscoveryBase) HTTPStatusCode() int {
+	if d.Status == "success" {
+		return http.StatusOK
+	}
+
+	// Mapping taken from prometheus/web/api/v1/api.go
+	// Note this is not exhaustive as our API does not return
+	// the same spectrum of errors as Prometheus does.
+	switch d.ErrorType {
+	case v1.ErrBadData:
+		return http.StatusBadRequest
+	default:
+		return http.StatusInternalServerError
+	}
+}
+
 // RuleGroupsBy is a function that defines the ordering of Rule Groups.
 type RuleGroupsBy func(a1, a2 *RuleGroup) bool
 

pkg/services/ngalert/api/tooling/definitions/prom_test.go

@@ -1,12 +1,53 @@
 package definitions
 
 import (
+	"net/http"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/assert"
 )
 
+func TestDiscoveryBaseHTTPStatusCode(t *testing.T) {
+	tc := []struct {
+		name     string
+		input    DiscoveryBase
+		expected int
+	}{{
+		name: "OK when status is success",
+		input: DiscoveryBase{
+			Status: "success",
+		},
+		expected: http.StatusOK,
+	}, {
+		name: "InternalServerError when status is error but no error type",
+		input: DiscoveryBase{
+			Status: "error",
+		},
+		expected: http.StatusInternalServerError,
+	}, {
+		name: "BadRequest when status is error and type is bad_data",
+		input: DiscoveryBase{
+			Status:    "error",
+			ErrorType: "bad_data",
+		},
+		expected: http.StatusBadRequest,
+	}, {
+		name: "InternalServerError when status is error and type is server_error",
+		input: DiscoveryBase{
+			Status:    "error",
+			ErrorType: "server_error",
+		},
+		expected: http.StatusInternalServerError,
+	}}
+
+	for _, tt := range tc {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.EqualValues(t, tt.expected, tt.input.HTTPStatusCode())
+		})
+	}
+}
+
 func TestSortAlertsByImportance(t *testing.T) {
 	tm1, tm2 := time.Now(), time.Now().Add(time.Second)
 	tc := []struct {
@@ -66,7 +107,6 @@ func TestSortAlertsByImportance(t *testing.T) {
 }
 
 func TestTopKAlertsByImportance(t *testing.T) {
-	//	tm1, tm2 := time.Now(), time.Now().Add(time.Second)
 	tc := []struct {
 		name     string
 		k        int