First pass at connection counting in nginx.conf

nginx.conf

@@ -36,6 +36,11 @@ stream {
         server 192.88.99.15:8023 max_conns=2;
     }
 
+    # Create a shared DICT (an in-memory database) to keep track of active
+    # users that have passed the challenge. This will be used to give a
+    # sensible response to connections made when all nodes are busy.
+    lua_shared_dict users 64k;
+
     # Define our actual listeners, math quiz and all.
     server {
         # Listen on port 23 (duh); the whole point of this exercise is to be
@@ -46,8 +51,27 @@ stream {
         # The real magic: A block of LUA code which takes over the
         # connection before it gets proxied to the BBS nodes.
         # See https://anduin.net/~ltning/bbs/bbs_math.lua ..
+        # We start by creating an Nginx variable, which will be used from
+        # the script:
+        set $bbs_challenge_passed 0;
+
+        # Then we call the script:
         preread_by_lua_file bbs_math.lua;
 
+        # Assuming the user has passed the challenge, we now have an active
+        # connection, and the counter has presumably been increased (see the
+        # bbs_math.lua file above). We need to decrease it again once the
+        # session ends.
+        # We register a lua function to be run at the end of the connection
+        # (during the "log" phase), which triggers only if the challenge
+        # was passed.
+        log_by_lua_block {
+            local users = ngx.shared.users
+            if ngx.var.bbs_challenge_passed == 1 then
+                users:incr(active, -1, 1)
+            end
+        }
+
         # Don't wait too long when we attempt to connect to the BBS. If it
         # doesn't want to talk to us (dead?), we drop the connection.
         proxy_connect_timeout 10s;