[MM-28397] Fix data race in SqlSupplier (#15419)

* Fix data race in SqlSupplier

* Add comment

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/supplier.go

@@ -115,7 +115,7 @@ type SqlSupplier struct {
 	lockedToMaster bool
 	context        context.Context
 	license        *model.License
-	licenseMutex   sync.Mutex
+	licenseMutex   sync.RWMutex
 }
 
 type TraceOnAdapter struct{}
@@ -330,7 +330,10 @@ func (ss *SqlSupplier) GetMaster() *gorp.DbMap {
 }
 
 func (ss *SqlSupplier) GetSearchReplica() *gorp.DbMap {
-	if ss.license == nil {
+	ss.licenseMutex.RLock()
+	license := ss.license
+	ss.licenseMutex.RUnlock()
+	if license == nil {
 		return ss.GetMaster()
 	}
 
@@ -343,7 +346,10 @@ func (ss *SqlSupplier) GetSearchReplica() *gorp.DbMap {
 }
 
 func (ss *SqlSupplier) GetReplica() *gorp.DbMap {
-	if len(ss.settings.DataSourceReplicas) == 0 || ss.lockedToMaster || ss.license == nil {
+	ss.licenseMutex.RLock()
+	license := ss.license
+	ss.licenseMutex.RUnlock()
+	if len(ss.settings.DataSourceReplicas) == 0 || ss.lockedToMaster || license == nil {
 		return ss.GetMaster()
 	}
 

store/sqlstore/supplier_test.go

@@ -19,6 +19,36 @@ import (
 	"github.com/mattermost/mattermost-server/v5/store/storetest"
 )
 
+// This test was used to consistently reproduce the race
+// before the fix in MM-28397.
+// Keeping it here to help avoiding future regressions.
+func TestSupplierLicenseRace(t *testing.T) {
+	settings := makeSqlSettings(model.DATABASE_DRIVER_SQLITE)
+	settings.DataSourceReplicas = []string{":memory:"}
+	settings.DataSourceSearchReplicas = []string{":memory:"}
+	supplier := sqlstore.NewSqlSupplier(*settings, nil)
+
+	wg := sync.WaitGroup{}
+	wg.Add(3)
+
+	go func() {
+		supplier.UpdateLicense(&model.License{})
+		wg.Done()
+	}()
+
+	go func() {
+		supplier.GetReplica()
+		wg.Done()
+	}()
+
+	go func() {
+		supplier.GetSearchReplica()
+		wg.Done()
+	}()
+
+	wg.Wait()
+}
+
 func TestGetReplica(t *testing.T) {
 	t.Parallel()
 	testCases := []struct {