Alerting: rule backtesting API (#57318)

* Implement backtesting engine that can process regular rule specification (with queries to datasource) as well as special kind of rules that have data frame instead of query.
* declare a new API endpoint and model
* add feature toggle `alertingBacktesting`

pkg/services/ngalert/backtesting/eval_query_test.go

@@ -0,0 +1,89 @@
+package backtesting
+
+import (
+	"context"
+	"errors"
+	"math/rand"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/mock"
+	"github.com/stretchr/testify/require"
+
+	"github.com/grafana/grafana/pkg/services/ngalert/eval"
+	"github.com/grafana/grafana/pkg/services/ngalert/eval/eval_mocks"
+)
+
+func TestQueryEvaluator_Eval(t *testing.T) {
+	ctx := context.Background()
+	interval := time.Duration(rand.Int63n(9)+1) * time.Second
+	times := rand.Intn(11) + 5
+	to := time.Now()
+	from := to.Add(-time.Duration(times) * interval)
+
+	t.Run("should evaluate query", func(t *testing.T) {
+		m := &eval_mocks.ConditionEvaluatorMock{}
+		expectedResults := eval.Results{}
+		m.EXPECT().Evaluate(mock.Anything, mock.Anything).Return(expectedResults, nil)
+		evaluator := queryEvaluator{
+			eval: m,
+		}
+
+		intervals := make([]time.Time, 0, times)
+
+		err := evaluator.Eval(ctx, from, to, interval, func(now time.Time, results eval.Results) error {
+			intervals = append(intervals, now)
+			return nil
+		})
+		require.NoError(t, err)
+		require.Len(t, intervals, times)
+
+		m.AssertNumberOfCalls(t, "Evaluate", times)
+		for _, now := range intervals {
+			m.AssertCalled(t, "Evaluate", ctx, now)
+		}
+	})
+
+	t.Run("should stop evaluation if error", func(t *testing.T) {
+		t.Run("when evaluation fails", func(t *testing.T) {
+			m := &eval_mocks.ConditionEvaluatorMock{}
+			expectedResults := eval.Results{}
+			expectedError := errors.New("test")
+			m.EXPECT().Evaluate(mock.Anything, mock.Anything).Return(expectedResults, nil).Times(3)
+			m.EXPECT().Evaluate(mock.Anything, mock.Anything).Return(nil, expectedError).Once()
+			evaluator := queryEvaluator{
+				eval: m,
+			}
+
+			intervals := make([]time.Time, 0, times)
+
+			err := evaluator.Eval(ctx, from, to, interval, func(now time.Time, results eval.Results) error {
+				intervals = append(intervals, now)
+				return nil
+			})
+			require.ErrorIs(t, err, expectedError)
+			require.Len(t, intervals, 3)
+		})
+
+		t.Run("when callback fails", func(t *testing.T) {
+			m := &eval_mocks.ConditionEvaluatorMock{}
+			expectedResults := eval.Results{}
+			expectedError := errors.New("test")
+			m.EXPECT().Evaluate(mock.Anything, mock.Anything).Return(expectedResults, nil)
+			evaluator := queryEvaluator{
+				eval: m,
+			}
+
+			intervals := make([]time.Time, 0, times)
+
+			err := evaluator.Eval(ctx, from, to, interval, func(now time.Time, results eval.Results) error {
+				if len(intervals) > 3 {
+					return expectedError
+				}
+				intervals = append(intervals, now)
+				return nil
+			})
+			require.ErrorIs(t, err, expectedError)
+		})
+	})
+}