Performance: Add preallocation for some slices (#59593)

pkg/services/alerting/conditions/query.go

@@ -1,6 +1,7 @@
 package conditions
 
 import (
+	gocontext "context"
 	"errors"
 	"fmt"
 	"strings"
@@ -10,8 +11,6 @@ import (
 	"github.com/grafana/grafana/pkg/tsdb/legacydata/interval"
 	"github.com/grafana/grafana/pkg/tsdb/prometheus"
 
-	gocontext "context"
-
 	"github.com/grafana/grafana-plugin-sdk-go/data"
 
 	"github.com/grafana/grafana/pkg/components/null"
@@ -61,7 +60,7 @@ func (c *QueryCondition) Eval(context *alerting.EvalContext, requestHandler lega
 	// matches represents all the series that violate the alert condition
 	var matches []*alerting.EvalMatch
 	// allMatches capture all evaluation matches irregardless on whether the condition is met or not
-	var allMatches []*alerting.EvalMatch
+	allMatches := make([]*alerting.EvalMatch, 0, len(seriesList))
 
 	for _, series := range seriesList {
 		reducedValue := c.Reducer.Reduce(series)

pkg/services/alerting/notifiers/googlechat.go

@@ -144,6 +144,7 @@ func (gcn *GoogleChatNotifier) Notify(evalContext *alerting.EvalContext) error {
 	}
 
 	// add a text paragraph widget for the fields
+	//nolint:prealloc // break block
 	var fields []textParagraphWidget
 	fieldLimitCount := 4
 	for index, evt := range evalContext.EvalMatches {

pkg/services/contexthandler/authproxy/authproxy.go

@@ -110,7 +110,7 @@ func (auth *AuthProxy) IsAllowedIP(ip string) error {
 	}
 
 	proxies := strings.Split(auth.cfg.AuthProxyWhitelist, ",")
-	var proxyObjs []*net.IPNet
+	proxyObjs := make([]*net.IPNet, 0, len(proxies))
 	for _, proxy := range proxies {
 		result, err := coerceProxyAddress(proxy)
 		if err != nil {

pkg/services/live/pipeline/frame_output_remote_write.go

@@ -118,7 +118,7 @@ func (out *RemoteWriteFrameOutput) sample(timeSeries []prompb.TimeSeries) []prom
 		sample.Samples = append(sample.Samples, filteredSamples...)
 		samples[name] = sample
 	}
-	var toReturn []prompb.TimeSeries
+	toReturn := make([]prompb.TimeSeries, 0, len(samples))
 	for _, ts := range samples {
 		toReturn = append(toReturn, ts)
 	}

pkg/services/live/pipeline/rule_builder_storage.go

@@ -309,7 +309,7 @@ func (f *StorageRuleBuilder) BuildRules(ctx context.Context, orgID int64) ([]*Li
 		return nil, err
 	}
 
-	var rules []*LiveChannelRule
+	rules := make([]*LiveChannelRule, 0, len(channelRules))
 
 	for _, ruleConfig := range channelRules {
 		rule := &LiveChannelRule{

pkg/services/ngalert/notifier/alertmanager.go

@@ -430,7 +430,7 @@ func (am *Alertmanager) applyConfig(cfg *apimodels.PostableUserConfig, rawConfig
 	am.dispatcher = dispatch.NewDispatcher(am.alerts, am.route, routingStage, am.marker, am.timeoutFunc, &nilLimits{}, am.logger, am.dispatcherMetrics)
 
 	// Check which receivers are active and create the receiver stage.
-	var receivers []*notify.Receiver
+	receivers := make([]*notify.Receiver, 0, len(integrationsMap))
 	activeReceivers := am.getActiveReceiversMap(am.route)
 	for name := range integrationsMap {
 		stage := am.createReceiverStage(name, integrationsMap[name], am.waitFunc, am.notificationLog)
@@ -479,7 +479,7 @@ func (am *Alertmanager) buildIntegrationsMap(receivers []*apimodels.PostableApiR
 
 // buildReceiverIntegrations builds a list of integration notifiers off of a receiver config.
 func (am *Alertmanager) buildReceiverIntegrations(receiver *apimodels.PostableApiReceiver, tmpl *template.Template) ([]*notify.Integration, error) {
-	var integrations []*notify.Integration
+	integrations := make([]*notify.Integration, 0, len(receiver.GrafanaManagedReceivers))
 	for i, r := range receiver.GrafanaManagedReceivers {
 		n, err := am.buildReceiverIntegration(r, tmpl)
 		if err != nil {

pkg/services/ngalert/notifier/receivers.go

@@ -203,10 +203,10 @@ func (am *Alertmanager) GetReceivers(ctx context.Context) []apimodels.Receiver {
 	am.reloadConfigMtx.RLock()
 	defer am.reloadConfigMtx.RUnlock()
 
-	var apiReceivers []apimodels.Receiver
+	apiReceivers := make([]apimodels.Receiver, 0, len(am.receivers))
 	for _, rcv := range am.receivers {
 		// Build integrations slice for each receiver.
-		var integrations []*models.Integration
+		integrations := make([]*models.Integration, 0, len(rcv.Integrations()))
 		for _, integration := range rcv.Integrations() {
 			name := integration.Name()
 			sendResolved := integration.SendResolved()

pkg/services/ngalert/state/cache.go

@@ -222,7 +222,6 @@ func (c *cache) getAll(orgID int64) []*State {
 }
 
 func (c *cache) getStatesForRuleUID(orgID int64, alertRuleUID string) []*State {
-	var result []*State
 	c.mtxStates.RLock()
 	defer c.mtxStates.RUnlock()
 	orgRules, ok := c.states[orgID]
@@ -233,6 +232,7 @@ func (c *cache) getStatesForRuleUID(orgID int64, alertRuleUID string) []*State {
 	if !ok {
 		return nil
 	}
+	result := make([]*State, 0, len(rs.states))
 	for _, state := range rs.states {
 		result = append(result, state)
 	}