Stop accept()-ing connections if already at max connection limit

The constant CONFIG_LWIP_MAX_SOCKETS, if available, tells how many
sockets can be opened at once. If already at this limit, and an attempt
is made to accept() an additional incoming connection from a listening
socket, the accept() call will fail. Fix this by checking the socket
list and refusing to add the listening socket to the set of readable
sockets to check in the select() call. This will cause connections to
remain in the listening backlog until some other socket is closed.

Author: avillacis

src/AsyncTCP.cpp

@@ -92,12 +92,19 @@ void _asynctcpsock_task(void *)
         FD_ZERO(&sockSet_r); FD_ZERO(&sockSet_w);
         for (it = _socketBaseList.begin(); it != _socketBaseList.end(); it++) {
             if ((*it)->_socket != -1) {
-                FD_SET((*it)->_socket, &sockSet_r);
+#ifdef CONFIG_LWIP_MAX_SOCKETS
+                if (!(*it)->_isServer() || _socketBaseList.size() < CONFIG_LWIP_MAX_SOCKETS) {
+#endif
+                    FD_SET((*it)->_socket, &sockSet_r);
+                    if (max_sock <= (*it)->_socket) max_sock = (*it)->_socket + 1;
+#ifdef CONFIG_LWIP_MAX_SOCKETS
+                }
+#endif
                 if ((*it)->_pendingWrite()) {
                     FD_SET((*it)->_socket, &sockSet_w);
+                    if (max_sock <= (*it)->_socket) max_sock = (*it)->_socket + 1;
                 }
                 (*it)->_selected = true;
-                if (max_sock <= (*it)->_socket) max_sock = (*it)->_socket + 1;
             }
         }
 

src/AsyncTCP.h

@@ -71,6 +71,7 @@ class AsyncSocketBase
     virtual void _sockDelayedConnect(void) {} // Action to take on DNS-resolve finished
 
     virtual bool _pendingWrite(void) { return false; }  // Test if there is data pending to be written
+    virtual bool _isServer(void) { return false; }      // Will a read from this socket result in one more client?
 
 public:
     AsyncSocketBase(void);
@@ -239,6 +240,9 @@ class AsyncServer : public AsyncSocketBase
 
     // Listening socket is readable on incoming connection
     void _sockIsReadable(void);
+
+    // Mark this class as a server
+    bool _isServer(void) { return true; }
 };