Fix data race in root endpoint (#2745)

There is a data race in the root endpoint when the action accesses action.App.coreVersion and other properties from action.App. These properties are updated in the App.Tick() goroutine. This commit fixes the data race by wrapping these properties in a read write lock. Whenever the properties are updated the write lock is acquired. Similarly, whenever the properties are read the read lock is acquired.

services/horizon/internal/actions_root.go

@@ -26,15 +26,16 @@ func (action *RootAction) JSON() error {
 		"strictReceivePaths": StrictReceivePathsQuery{}.URITemplate(),
 		"strictSendPaths":    FindFixedPathsQuery{}.URITemplate(),
 	}
+	coreInfo := action.App.coreSettings.get()
 	resourceadapter.PopulateRoot(
 		action.R.Context(),
 		&res,
 		ledger.CurrentState(),
 		action.App.horizonVersion,
-		action.App.coreVersion,
+		coreInfo.coreVersion,
 		action.App.config.NetworkPassphrase,
-		action.App.currentProtocolVersion,
-		action.App.coreSupportedProtocolVersion,
+		coreInfo.currentProtocolVersion,
+		coreInfo.coreSupportedProtocolVersion,
 		action.App.config.FriendbotURL,
 		templates,
 	)

services/horizon/internal/app.go

@@ -12,6 +12,7 @@ import (
 
 	metrics "github.com/rcrowley/go-metrics"
 	"github.com/stellar/go/clients/stellarcore"
+	proto "github.com/stellar/go/protocols/stellarcore"
 	horizonContext "github.com/stellar/go/services/horizon/internal/context"
 	"github.com/stellar/go/services/horizon/internal/db2/core"
 	"github.com/stellar/go/services/horizon/internal/db2/history"
@@ -30,24 +31,47 @@ import (
 	graceful "gopkg.in/tylerb/graceful.v1"
 )
 
-// App represents the root of the state of a horizon instance.
-type App struct {
-	config                       Config
-	web                          *web
-	historyQ                     *history.Q
-	coreQ                        *core.Q
-	ctx                          context.Context
-	cancel                       func()
-	coreVersion                  string
-	horizonVersion               string
+type coreSettings struct {
 	currentProtocolVersion       int32
 	coreSupportedProtocolVersion int32
-	orderBookStream              *expingest.OrderBookStream
-	submitter                    *txsub.System
-	paths                        paths.Finder
-	expingester                  *expingest.System
-	reaper                       *reap.System
-	ticks                        *time.Ticker
+	coreVersion                  string
+}
+
+type coreSettingsStore struct {
+	sync.RWMutex
+	coreSettings
+}
+
+func (c *coreSettingsStore) set(resp *proto.InfoResponse) {
+	c.Lock()
+	defer c.Unlock()
+	c.coreVersion = resp.Info.Build
+	c.currentProtocolVersion = int32(resp.Info.Ledger.Version)
+	c.coreSupportedProtocolVersion = int32(resp.Info.ProtocolVersion)
+}
+
+func (c *coreSettingsStore) get() coreSettings {
+	c.RLock()
+	defer c.RUnlock()
+	return c.coreSettings
+}
+
+// App represents the root of the state of a horizon instance.
+type App struct {
+	config          Config
+	web             *web
+	historyQ        *history.Q
+	coreQ           *core.Q
+	ctx             context.Context
+	cancel          func()
+	horizonVersion  string
+	coreSettings    coreSettingsStore
+	orderBookStream *expingest.OrderBookStream
+	submitter       *txsub.System
+	paths           paths.Finder
+	expingester     *expingest.System
+	reaper          *reap.System
+	ticks           *time.Ticker
 
 	// metrics
 	metrics                  metrics.Registry
@@ -384,9 +408,7 @@ func (a *App) UpdateStellarCoreInfo() {
 		os.Exit(1)
 	}
 
-	a.coreVersion = resp.Info.Build
-	a.currentProtocolVersion = int32(resp.Info.Ledger.Version)
-	a.coreSupportedProtocolVersion = int32(resp.Info.ProtocolVersion)
+	a.coreSettings.set(resp)
 }
 
 // UpdateMetrics triggers a refresh of several metrics gauges, such as open