adjust max default TCP clients to 9 because of bug in LwIP

STM32lwipopts.h

@@ -29,7 +29,15 @@
 
 // we can not include our "defines.h" here
 // so we need to duplicate that define
-#define MAX_NUM_TCP_CLIENTS 20
+#define MAX_NUM_TCP_CLIENTS_HERE 9
+
+#ifdef MAX_NUM_TCP_CLIENTS
+ #if MAX_NUM_TCP_CLIENTS != MAX_NUM_TCP_CLIENTS_HERE
+  #error MAX_NUM_TCP_CLIENTS and MAX_NUM_TCP_CLIENTS_HERE must be same
+ #endif
+#else
+ #define MAX_NUM_TCP_CLIENTS MAX_NUM_TCP_CLIENTS_HERE
+#endif
 
 // increase ARP cache
 #undef  MEMP_NUM_APR_QUEUE

config.example.h

@@ -140,7 +140,10 @@ The configuration file for DCC-EX Command Station
 /////////////////////////////////////////////////////////////////////////////////////
 //
 // MAX_NUM_TCP_CLIENTS: If you on STM32 Ethernet (and only there) want more than
-// 10 TCP clients, enable this here **AND** follow the instructions in STM32lwiopts.h
+// 9 (*) TCP clients, change this number to for example 20 here **AND** in
+// STM32lwiopts.h and follow the instructions in STM32lwiopts.h
+//
+// (*) It would be 10 if there would not be a bug in LwIP by STM32duino.
 //
 //#define MAX_NUM_TCP_CLIENTS 20
 

defines.h

@@ -240,9 +240,15 @@
 #endif
 
 #if defined(ARDUINO_ARCH_STM32)
-// Allow override of MAX_NUM_TCP_CLIENTS but default is 10
+// The LwIP library for the STM32 wired ethernet has by default 10 TCP
+// clients defined but because of a bug in the library #11 is not
+// rejected but kicks out any old connection. By restricting our limit
+// to 9 the #10 will be rejected by our code so that the number can
+// never get to 11 which would kick an existing connection.
+// If you want to change this value, do that in
+// config.h AND in STM32lwipopts.h.
  #ifndef MAX_NUM_TCP_CLIENTS
-  #define MAX_NUM_TCP_CLIENTS 10
+  #define MAX_NUM_TCP_CLIENTS 9
  #endif
 #else
  #if defined(ARDUINO_ARCH_ESP32)