MM-56402: Initialize replica conn pool on-demand (#26410)

Previously, we would setup both pools only when
GetMasterDB was called. This was inefficient and
would waste open connections if the replica wasn't used
at all.

We fix it to initialize the pools as they are called.

https://mattermost.atlassian.net/browse/MM-56402
```release-note
NONE
```

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

server/public/pluginapi/store.go

@@ -15,10 +15,11 @@ import (
 
 // StoreService exposes the underlying database.
 type StoreService struct {
-	initialized bool
-	api         plugin.API
-	driver      plugin.Driver
-	mutex       sync.Mutex
+	initializedMaster  bool
+	initializedReplica bool
+	api                plugin.API
+	driver             plugin.Driver
+	mutex              sync.Mutex
 
 	masterDB  *sql.DB
 	replicaDB *sql.DB
@@ -31,7 +32,7 @@ func (s *StoreService) GetMasterDB() (*sql.DB, error) {
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 
-	if err := s.initialize(); err != nil {
+	if err := s.initializeMaster(); err != nil {
 		return nil, err
 	}
 
@@ -46,7 +47,7 @@ func (s *StoreService) GetReplicaDB() (*sql.DB, error) {
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 
-	if err := s.initialize(); err != nil {
+	if err := s.initializeReplica(); err != nil {
 		return nil, err
 	}
 
@@ -62,20 +63,18 @@ func (s *StoreService) Close() error {
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
 
-	if !s.initialized {
-		return nil
-	}
-
-	if err := s.masterDB.Close(); err != nil {
-		return err
-	}
-
 	if s.replicaDB != nil {
 		if err := s.replicaDB.Close(); err != nil {
 			return err
 		}
 	}
 
+	if s.masterDB != nil {
+		if err := s.masterDB.Close(); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }
 
@@ -84,8 +83,8 @@ func (s *StoreService) DriverName() string {
 	return *s.api.GetConfig().SqlSettings.DriverName
 }
 
-func (s *StoreService) initialize() error {
-	if s.initialized {
+func (s *StoreService) initializeMaster() error {
+	if s.initializedMaster {
 		return nil
 	}
 
@@ -93,25 +92,33 @@ func (s *StoreService) initialize() error {
 		return errors.New("no db driver was provided")
 	}
 
-	config := s.api.GetUnsanitizedConfig()
-
 	// Set up master db
-	db := sql.OpenDB(driver.NewConnector(s.driver, true))
+	db := sql.OpenDB(driver.NewConnector(s.driver, true /* IsMaster */))
 	if err := db.Ping(); err != nil {
 		return errors.Wrap(err, "failed to connect to master db")
 	}
 	s.masterDB = db
 
+	s.initializedMaster = true
+
+	return nil
+}
+
+func (s *StoreService) initializeReplica() error {
+	if s.initializedReplica {
+		return nil
+	}
+
+	config := s.api.GetUnsanitizedConfig()
 	// Set up replica db
 	if len(config.SqlSettings.DataSourceReplicas) > 0 {
-		db := sql.OpenDB(driver.NewConnector(s.driver, false))
+		db := sql.OpenDB(driver.NewConnector(s.driver, false /* IsMaster */))
 		if err := db.Ping(); err != nil {
 			return errors.Wrap(err, "failed to connect to replica db")
 		}
 		s.replicaDB = db
 	}
 
-	s.initialized = true
-
+	s.initializedReplica = true
 	return nil
 }

server/public/pluginapi/store_test.go

@@ -12,18 +12,10 @@ import (
 
 func TestStore(t *testing.T) {
 	t.Run("master db singleton", func(t *testing.T) {
-		config := &model.Config{
-			SqlSettings: model.SqlSettings{
-				DriverName: model.NewString("test"),
-				DataSource: model.NewString("TestStore-master-db"),
-			},
-		}
-
 		api := &plugintest.API{}
-		defer api.AssertExpectations(t)
-		api.On("GetUnsanitizedConfig").Return(config)
 
 		driver := &plugintest.Driver{}
+		defer driver.AssertExpectations(t)
 		driver.On("Conn", true).Return("test", nil)
 		driver.On("ConnPing", "test").Return(nil)
 		driver.On("ConnClose", "test").Return(nil)
@@ -52,6 +44,7 @@ func TestStore(t *testing.T) {
 		}
 
 		driver := &plugintest.Driver{}
+		defer driver.AssertExpectations(t)
 		driver.On("Conn", true).Return("test", nil)
 		driver.On("ConnPing", "test").Return(nil)
 		driver.On("ConnClose", "test").Return(nil)
@@ -88,7 +81,7 @@ func TestStore(t *testing.T) {
 		api.On("GetUnsanitizedConfig").Return(config)
 
 		driver := &plugintest.Driver{}
-		driver.On("Conn", true).Return("test", nil)
+		defer driver.AssertExpectations(t)
 		driver.On("Conn", false).Return("test", nil)
 		driver.On("ConnPing", "test").Return(nil)
 		driver.On("ConnClose", "test").Return(nil)