fabtests: Add backlog > 0 to listen call

Currently the listen() call has a 0 value for backlog.
This causes syn (synchronize) flooding on port 3000.
Port 3000 is used by fabtests for OOO (out of bounds) exchanges.

This call needs to have a non-zero positive value for backlog
to prevent this flooding. In this patch we also add an environment
variable to make this setting configurable.

Setting the backlog is necessary so that it is changeable by the user
and not hidden values that users must set at the kernel level.
The kernel variables are called net.ipv4.tcp_max_syn_backlog, and
net.core.somaxconn to fix the issue instead. However, it is encouraged
that a user make sure all of these values are set appropriately as well
as using an adequate size for backlog in the listen call.

Signed-off-by: Zach Dworkin <[email protected]>

fabtests/common/shared.c

@@ -3804,7 +3804,7 @@ int ft_sock_listen(char *node, char *service)
 		goto out;
 	}
 
-	ret = listen(listen_sock, 0);
+	ret = listen(listen_sock, 511);
 	if (ret)
 		perror("listen");
 

fabtests/component/sock_test.c

@@ -113,7 +113,7 @@ static int start_server(void)
 		goto close;
 	}
 
-	ret = listen(listen_sock, 0);
+	ret = listen(listen_sock, 511);
 	if (ret) {
 		FT_PRINTERR("listen", -errno);
 		goto close;