Chore: Remove SQL engine cache (#39254)

Fix bug with engine cache not updating after data source updated, introduced in #36635.
Removes the SQL engine cache because since #36635 we already have instance caching in place using the SDK instance manager.
This also adds support for disposing cached SQL/xorm engines before a query happens to a data source that has been updated. Done by letting the SQL engine implementing the InstanceDisposer interface.

Fixes #37299

pkg/tsdb/sqleng/sql_engine.go

@@ -41,17 +41,6 @@ type SqlQueryResultTransformer interface {
 	GetConverterList() []sqlutil.StringConverter
 }
 
-type engineCacheType struct {
-	cache   map[int64]*xorm.Engine
-	updates map[int64]time.Time
-	sync.Mutex
-}
-
-var engineCache = engineCacheType{
-	cache:   make(map[int64]*xorm.Engine),
-	updates: make(map[int64]time.Time),
-}
-
 var sqlIntervalCalculator = intervalv2.NewCalculator()
 
 // NewXormEngine is an xorm.Engine factory, that can be stubbed by tests.
@@ -129,6 +118,11 @@ func (e *DataSourceHandler) transformQueryError(err error) error {
 
 func NewQueryDataHandler(config DataPluginConfiguration, queryResultTransformer SqlQueryResultTransformer,
 	macroEngine SQLMacroEngine, log log.Logger) (*DataSourceHandler, error) {
+	log.Debug("Creating engine...")
+	defer func() {
+		log.Debug("Engine created")
+	}()
+
 	queryDataHandler := DataSourceHandler{
 		queryResultTransformer: queryResultTransformer,
 		macroEngine:            macroEngine,
@@ -146,16 +140,6 @@ func NewQueryDataHandler(config DataPluginConfiguration, queryResultTransformer
 		queryDataHandler.metricColumnTypes = config.MetricColumnTypes
 	}
 
-	engineCache.Lock()
-	defer engineCache.Unlock()
-
-	if engine, present := engineCache.cache[config.DSInfo.ID]; present {
-		if updateTime := engineCache.updates[config.DSInfo.ID]; updateTime.Before(config.DSInfo.Updated) {
-			queryDataHandler.engine = engine
-			return &queryDataHandler, nil
-		}
-	}
-
 	engine, err := NewXormEngine(config.DriverName, config.ConnectionString)
 	if err != nil {
 		return nil, err
@@ -165,8 +149,6 @@ func NewQueryDataHandler(config DataPluginConfiguration, queryResultTransformer
 	engine.SetMaxIdleConns(config.DSInfo.JsonData.MaxIdleConns)
 	engine.SetConnMaxLifetime(time.Duration(config.DSInfo.JsonData.ConnMaxLifetime) * time.Second)
 
-	engineCache.updates[config.DSInfo.ID] = config.DSInfo.Updated
-	engineCache.cache[config.DSInfo.ID] = engine
 	queryDataHandler.engine = engine
 	return &queryDataHandler, nil
 }
@@ -176,6 +158,16 @@ type DBDataResponse struct {
 	refID        string
 }
 
+func (e *DataSourceHandler) Dispose() {
+	e.log.Debug("Disposing engine...")
+	if e.engine != nil {
+		if err := e.engine.Close(); err != nil {
+			e.log.Error("Failed to dispose engine", "error", err)
+		}
+	}
+	e.log.Debug("Engine disposed")
+}
+
 func (e *DataSourceHandler) QueryData(ctx context.Context, req *backend.QueryDataRequest) (*backend.QueryDataResponse, error) {
 	result := backend.NewQueryDataResponse()
 	ch := make(chan DBDataResponse, len(req.Queries))