tcp: fix dispatching closed connections to tcp readers

Under very heavy load it is possible that send2child() might try
to send an already closed connection/fd to a tcp reader.
This can happen only if the tcp connection is watched for read
(POLLIN) by tcp_main (and not by a tcp reader), the connection
becomes available for reading (either new data received, EOF or
RST) and tcp_main chooses a specific tcp reader to send the
connection to while in the same time the same tcp reader tries to
send on the same connection, fails for some reason (no more space
for buffering, EOF, RST a.s.o) and sends a close command back to
tcp_main. Because send2child() executes first any pending commands
from the choosen tcp_reader, this might lead to closing the fd
before attempting to send it to the tcp_reader.
Under normal circumstances the impact is only an extra syscall and
some log messages, however it is possible (but highly unlikely)
that after sending the close command the tcp_reader opens a new
connection for sending and sends its fd back to tcp_main. This new
fd might get the same number as the freshly closed fd and
send2child might send the wrong (fd, tcp connection) pair.

tcp_main.c

@@ -2891,6 +2891,7 @@ close_again:
 					su2a(&tcpconn->rcv.src_su, sizeof(tcpconn->rcv.src_su)),
 					fd, tcpconn->flags, strerror(errno), errno);
 	}
+	tcpconn->s=-1;
 }
 
 
@@ -3836,10 +3837,20 @@ inline static int send2child(struct tcp_connection* tcpconn)
 	 * send a release command, but the master fills its socket buffer
 	 * with new connection commands => deadlock) */
 	/* answer tcp_send requests first */
-	while(handle_ser_child(&pt[tcp_children[idx].proc_no], -1)>0);
+	while(unlikely((tcpconn->state != S_CONN_BAD) &&
+					(handle_ser_child(&pt[tcp_children[idx].proc_no], -1)>0)));
 	/* process tcp readers requests */
-	while(handle_tcp_child(&tcp_children[idx], -1)>0);
-		
+	while(unlikely((tcpconn->state != S_CONN_BAD &&
+					(handle_tcp_child(&tcp_children[idx], -1)>0))));
+	
+	/* the above possible pending requests might have included a
+	   command to close this tcpconn (e.g. CONN_ERROR, CONN_EOF).
+	   In this case the fd is already closed here (and possible
+	   even replaced by another one with the same number) so it
+	   must not be sent to a reader anymore */
+	if (unlikely(tcpconn->state == S_CONN_BAD ||
+					(tcpconn->flags & F_CONN_FD_CLOSED)))
+		return -1;
 #ifdef SEND_FD_QUEUE
 	/* if queue full, try to queue the io */
 	if (unlikely(send_fd(tcp_children[idx].unix_sock, &tcpconn,
@@ -3961,8 +3972,6 @@ static inline int handle_new_connect(struct socket_info* si)
 		DBG("handle_new_connect: new connection from %s: %p %d flags: %04x\n",
 			su2a(&su, sizeof(su)), tcpconn, tcpconn->s, tcpconn->flags);
 		if(unlikely(send2child(tcpconn)<0)){
-			LOG(L_ERR,"ERROR: handle_new_connect: no children "
-					"available\n");
 			tcpconn->flags&=~F_CONN_READER;
 			tcpconn_put(tcpconn);
 			tcpconn_try_unhash(tcpconn);
@@ -4142,7 +4151,6 @@ send_to_child:
 		tcpconn->flags&=~(F_CONN_MAIN_TIMER|F_CONN_READ_W|F_CONN_WANTS_RD);
 		tcpconn_ref(tcpconn); /* refcnt ++ */
 		if (unlikely(send2child(tcpconn)<0)){
-			LOG(L_ERR,"ERROR: handle_tcpconn_ev: no children available\n");
 			tcpconn->flags&=~F_CONN_READER;
 #ifdef TCP_ASYNC
 			if (tcpconn->flags & F_CONN_WRITE_W){