grpc: hold ac.mu while calling resetTransport to prevent concurrent c…

…onnection attempts (grpc#7390)

clientconn.go

@@ -918,9 +918,8 @@ func (ac *addrConn) connect() error {
 		ac.mu.Unlock()
 		return nil
 	}
-	ac.mu.Unlock()
 
-	ac.resetTransport()
+	ac.resetTransportAndUnlock()
 	return nil
 }
 
@@ -992,11 +991,9 @@ func (ac *addrConn) updateAddrs(addrs []resolver.Address) {
 		ac.updateConnectivityState(connectivity.Idle, nil)
 	}
 
-	ac.mu.Unlock()
-
 	// Since we were connecting/connected, we should start a new connection
 	// attempt.
-	go ac.resetTransport()
+	go ac.resetTransportAndUnlock()
 }
 
 // getServerName determines the serverName to be used in the connection
@@ -1231,8 +1228,10 @@ func (ac *addrConn) adjustParams(r transport.GoAwayReason) {
 	}
 }
 
-func (ac *addrConn) resetTransport() {
-	ac.mu.Lock()
+// resetTransportAndUnlock unconditionally connects the addrConn.
+//
+// ac.mu must be held by the caller, and this function will guarantee it is released.
+func (ac *addrConn) resetTransportAndUnlock() {
 	acCtx := ac.ctx
 	if acCtx.Err() != nil {
 		ac.mu.Unlock()