Alerting: Detach condition validator from condition evaluator (#91150)

* Detach validator from evaluator

* Drop unnecessary interface and type

pkg/services/ngalert/eval/validate.go

@@ -0,0 +1,65 @@
+package eval
+
+import (
+	"fmt"
+
+	"github.com/grafana/grafana/pkg/expr"
+	"github.com/grafana/grafana/pkg/plugins"
+	"github.com/grafana/grafana/pkg/services/datasources"
+	"github.com/grafana/grafana/pkg/services/ngalert/models"
+	"github.com/grafana/grafana/pkg/services/pluginsintegration/pluginstore"
+)
+
+type ConditionValidator struct {
+	dataSourceCache   datasources.CacheService
+	pluginsStore      pluginstore.Store
+	expressionService expressionBuilder
+}
+
+func NewConditionValidator(datasourceCache datasources.CacheService, expressionService *expr.Service, pluginsStore pluginstore.Store) *ConditionValidator {
+	return &ConditionValidator{
+		dataSourceCache:   datasourceCache,
+		expressionService: expressionService,
+		pluginsStore:      pluginsStore,
+	}
+}
+
+func (e *ConditionValidator) Validate(ctx EvaluationContext, condition models.Condition) error {
+	req, err := getExprRequest(ctx, condition, e.dataSourceCache, ctx.AlertingResultsReader)
+	if err != nil {
+		return err
+	}
+	for _, query := range req.Queries {
+		if query.DataSource == nil {
+			continue
+		}
+		switch expr.NodeTypeFromDatasourceUID(query.DataSource.UID) {
+		case expr.TypeDatasourceNode:
+			p, found := e.pluginsStore.Plugin(ctx.Ctx, query.DataSource.Type)
+			if !found { // technically this should fail earlier during datasource resolution phase.
+				return fmt.Errorf("datasource refID %s could not be found: %w", query.RefID, plugins.ErrPluginUnavailable)
+			}
+			if !p.Backend {
+				return fmt.Errorf("datasource refID %s is not a backend datasource", query.RefID)
+			}
+		case expr.TypeMLNode:
+			_, found := e.pluginsStore.Plugin(ctx.Ctx, query.DataSource.Type)
+			if !found {
+				return fmt.Errorf("datasource refID %s could not be found: %w", query.RefID, plugins.ErrPluginUnavailable)
+			}
+		case expr.TypeCMDNode:
+		}
+	}
+	pipeline, err := e.expressionService.BuildPipeline(req)
+	if err != nil {
+		return err
+	}
+	refIDs := make([]string, 0, len(pipeline))
+	for _, node := range pipeline {
+		if node.RefID() == condition.Condition {
+			return nil
+		}
+		refIDs = append(refIDs, node.RefID())
+	}
+	return models.ErrConditionNotExist(condition.Condition, refIDs)
+}