kvdb/postgres: make global dbConns thread safe

In this commit, a mutex is added to guard access to the global dbConns
variable in the kvdb/postgres package.

kvdb/postgres/db.go

@@ -57,11 +57,21 @@ type db struct {
 // Enforce db implements the walletdb.DB interface.
 var _ walletdb.DB = (*db)(nil)
 
-// Global set of database connections.
-var dbConns *dbConnSet
+var (
+	// dbConns is a global set of database connections.
+	dbConns   *dbConnSet
+	dbConnsMu sync.Mutex
+)
 
 // Init initializes the global set of database connections.
 func Init(maxConnections int) {
+	dbConnsMu.Lock()
+	defer dbConnsMu.Unlock()
+
+	if dbConns != nil {
+		return
+	}
+
 	dbConns = newDbConnSet(maxConnections)
 }
 
@@ -70,6 +80,9 @@ func Init(maxConnections int) {
 func newPostgresBackend(ctx context.Context, config *Config, prefix string) (
 	*db, error) {
 
+	dbConnsMu.Lock()
+	defer dbConnsMu.Unlock()
+
 	if prefix == "" {
 		return nil, errors.New("empty postgres prefix")
 	}
@@ -256,6 +269,9 @@ func (db *db) Copy(w io.Writer) error {
 // Close cleanly shuts down the database and syncs all data.
 // This function is part of the walletdb.Db interface implementation.
 func (db *db) Close() error {
+	dbConnsMu.Lock()
+	defer dbConnsMu.Unlock()
+
 	log.Infof("Closing database %v", db.prefix)
 
 	return dbConns.Close(db.cfg.Dsn)

kvdb/postgres/db_conn_set.go

@@ -19,7 +19,8 @@ type dbConnSet struct {
 	dbConn         map[string]*dbConn
 	maxConnections int
 
-	sync.Mutex
+	// mu is used to guard access to the dbConn map.
+	mu sync.Mutex
 }
 
 // newDbConnSet initializes a new set of connections.
@@ -33,8 +34,8 @@ func newDbConnSet(maxConnections int) *dbConnSet {
 // Open opens a new database connection. If a connection already exists for the
 // given dsn, the existing connection is returned.
 func (d *dbConnSet) Open(dsn string) (*sql.DB, error) {
-	d.Lock()
-	defer d.Unlock()
+	d.mu.Lock()
+	defer d.mu.Unlock()
 
 	if dbConn, ok := d.dbConn[dsn]; ok {
 		dbConn.count++
@@ -66,8 +67,8 @@ func (d *dbConnSet) Open(dsn string) (*sql.DB, error) {
 // Close closes the connection with the given dsn. If there are still other
 // users of the same connection, this function does nothing.
 func (d *dbConnSet) Close(dsn string) error {
-	d.Lock()
-	defer d.Unlock()
+	d.mu.Lock()
+	defer d.mu.Unlock()
 
 	dbConn, ok := d.dbConn[dsn]
 	if !ok {