Performance: add preallocation for some slice/map (#57860)

This change preallocates slices and maps where the size of the data is known before the object is created.

Co-authored-by: Joe Blubaugh <joe.blubaugh@grafana.com>

pkg/api/dtos/models.go

@@ -88,7 +88,7 @@ func (mr *MetricRequest) GetUniqueDatasourceTypes() []string {
 		}
 	}
 
-	res := make([]string, 0)
+	res := make([]string, 0, len(dsTypes))
 	for dsType := range dsTypes {
 		res = append(res, dsType)
 	}

pkg/api/frontendlogging/sentry.go

@@ -59,7 +59,7 @@ func (value *FrontendSentryExceptionValue) FmtStacktrace(store *SourceMapStore)
 }
 
 func (exception *FrontendSentryException) FmtStacktraces(store *SourceMapStore) string {
-	var stacktraces []string
+	stacktraces := make([]string, 0, len(exception.Values))
 	for _, value := range exception.Values {
 		stacktraces = append(stacktraces, value.FmtStacktrace(store))
 	}

pkg/plugins/manager/loader/initializer/initializer.go

@@ -87,7 +87,7 @@ func (i *Initializer) awsEnvVars() []string {
 type pluginSettings map[string]string
 
 func (ps pluginSettings) asEnvVar(prefix string, hostEnv []string) []string {
-	var env []string
+	env := make([]string, 0, len(ps))
 	for k, v := range ps {
 		key := fmt.Sprintf("%s_%s", prefix, strings.ToUpper(k))
 		if value := os.Getenv(key); value != "" {

pkg/services/ngalert/notifier/channels/alertmanager.go

@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-
 	"net/url"
 	"strings"
 
@@ -34,18 +33,21 @@ func NewAlertmanagerConfig(config *NotificationChannelConfig, fn GetDecryptedVal
 	if urlStr == "" {
 		return nil, errors.New("could not find url property in settings")
 	}
-	var urls []*url.URL
-	for _, uS := range strings.Split(urlStr, ",") {
+
+	urlParts := strings.Split(urlStr, ",")
+	urls := make([]*url.URL, 0, len(urlParts))
+
+	for _, uS := range urlParts {
 		uS = strings.TrimSpace(uS)
 		if uS == "" {
 			continue
 		}
 		uS = strings.TrimSuffix(uS, "/") + "/api/v1/alerts"
-		url, err := url.Parse(uS)
+		u, err := url.Parse(uS)
 		if err != nil {
 			return nil, fmt.Errorf("invalid url property in settings: %w", err)
 		}
-		urls = append(urls, url)
+		urls = append(urls, u)
 	}
 	return &AlertmanagerConfig{
 		NotificationChannelConfig: config,

pkg/services/provisioning/dashboards/types.go

@@ -74,7 +74,7 @@ func createDashboardJSON(data *simplejson.Json, lastModified time.Time, cfg *con
 }
 
 func mapV0ToDashboardsAsConfig(v0 []*configV0) ([]*config, error) {
-	var r []*config
+	r := make([]*config, 0, len(v0))
 	seen := make(map[string]bool)
 
 	for _, v := range v0 {
@@ -101,7 +101,7 @@ func mapV0ToDashboardsAsConfig(v0 []*configV0) ([]*config, error) {
 }
 
 func (dc *configV1) mapToDashboardsAsConfig() ([]*config, error) {
-	var r []*config
+	r := make([]*config, 0, len(dc.Providers))
 	seen := make(map[string]bool)
 
 	for _, v := range dc.Providers {

pkg/services/sqlstore/migrations/ualert/channel.go

@@ -212,7 +212,7 @@ func (m *migration) createNotifier(c *notificationChannel) (*PostableGrafanaRece
 
 // Create one receiver for every unique notification channel.
 func (m *migration) createReceivers(allChannels []*notificationChannel) (map[uidOrID]*PostableApiReceiver, []*PostableApiReceiver, error) {
-	var receivers []*PostableApiReceiver
+	receivers := make([]*PostableApiReceiver, 0, len(allChannels))
 	receiversMap := make(map[uidOrID]*PostableApiReceiver)
 
 	set := make(map[string]struct{}) // Used to deduplicate sanitized names.

pkg/services/updatechecker/plugins.go

@@ -146,7 +146,7 @@ func canUpdate(v1, v2 string) bool {
 }
 
 func (s *PluginsService) pluginIDsCSV(m map[string]plugins.PluginDTO) string {
-	var ids []string
+	ids := make([]string, 0, len(m))
 	for pluginID := range m {
 		ids = append(ids, pluginID)
 	}

pkg/tsdb/prometheus/querydata/request.go

@@ -185,7 +185,7 @@ func (s *QueryData) trace(ctx context.Context, q *models.Query) (context.Context
 }
 
 func sdkHeaderToHttpHeader(headers map[string]string) http.Header {
-	httpHeader := make(http.Header)
+	httpHeader := make(http.Header, len(headers))
 	for key, val := range headers {
 		httpHeader[key] = []string{val}
 	}