Fix a deadlock in Session.Close() during control connection reconnect

We switched from separate mutex for closing to sessionStateMu in
312a614.
This change introduced a deadlock.

We don't need to hold the mutex for the whole duration of Close(),
we only need to update the status atomically.
Previously IsClosed() returned true only after all closing is done
(because of the deferred unlock).
We can emulate that by setting isClosed at the end of Close().
We need a new variable to ensure that Close() is only executed once.

Fixes #1687

session.go

@@ -73,8 +73,10 @@ type Session struct {
 
 	// sessionStateMu protects isClosed and isInitialized.
 	sessionStateMu sync.RWMutex
-	// isClosed is true once Session.Close is called.
+	// isClosed is true once Session.Close is finished.
 	isClosed bool
+	// isClosing bool is true once Session.Close is started.
+	isClosing bool
 	// isInitialized is true once Session.init succeeds.
 	// you can use initialized() to read the value.
 	isInitialized bool
@@ -463,11 +465,12 @@ func (s *Session) Bind(stmt string, b func(q *QueryInfo) ([]interface{}, error))
 func (s *Session) Close() {
 
 	s.sessionStateMu.Lock()
-	defer s.sessionStateMu.Unlock()
-	if s.isClosed {
+	if s.isClosing {
+		s.sessionStateMu.Unlock()
 		return
 	}
-	s.isClosed = true
+	s.isClosing = true
+	s.sessionStateMu.Unlock()
 
 	if s.pool != nil {
 		s.pool.Close()
@@ -488,6 +491,10 @@ func (s *Session) Close() {
 	if s.cancel != nil {
 		s.cancel()
 	}
+
+	s.sessionStateMu.Lock()
+	s.isClosed = true
+	s.sessionStateMu.Unlock()
 }
 
 func (s *Session) Closed() bool {