BizHawkClient: Add support for multiple concurrent instances (#2475)

This allows multiple client/connector pairs to run at the same time. It also includes a few other miscellaneous small changes that accumulated as I went. They can be split if desired

- Whatever the `client_socket:send` line (~440) was doing with that missing operator, it's no longer doing. Don't ask me how it was working before. Lua is witchcraft.
- Removed the `settimeout(2)` which causes the infamous emulator freeze (and replaced it with a `settimeout(0)` when the server socket is created). It appears to be unnecessary to set a timeout for discovering a client. Maybe at some point in time it was useful to keep the success rate for connecting high, but it seems to not be a problem if the timeout is 0 instead.
  - Also updated the Emerald setup to remove mention of the freezing.
- Connector script now picks the first port that's not in use in a range of 5 ports.
  - To summarize why I was previously under the impression that multiple running scripts would not detect when a port was in use:
    1. Calling `socket.bind` in the existing script will first create an ipv6 socket.
    2. A second concurrent script trying to bind to the same port would I think fail to create an ipv6 socket but then succeed in creating an ipv4 socket on the same port.
    3. That second socket could never communicate with a client; extra clients would just bounce off the first script.
    4. The third concurrent script will then fail on both and actually give an `address already in use` error.  
  - I'm not _really_ sure what's going on there. But forcing one or the other by calling `socket.tcp4()` or `socket.tcp6()` means that only one script will believe it has the port while any others will give `address already in use` as you'd expect.
  - As a side note, our `socket.lua` is much wonkier than I had previously thought. I understand some parts were added for LADX and when BizHawk 2.9 came out, but as far back as the file's history in this repo, it has provided a strange, modified interface as compared to the file it was originally derived from, to no benefit as far as I can tell.
- The connector script closes `server` once it finds a client and opens a new one if the connection drops. I'm not sure this ultimately has an effect, but it seems more proper.
- If the connector script's main function returns because of some error or refusal to proceed, the script no longer tries to resume the coroutine it was part of, which would flood the log with irrelevant errors.
- Creating `SyncError`s in `guarded_read` and `guarded_write` would raise its own error because the wrong variable was being used in its message.
- A call to `_bizhawk.connect` can take a while as the client tries the possible ports. There's a modification that will wait on either the `connect` or the exit event. And if the exit event fires while still looking for a connector script, this cancels the `connect` so the window can close.
  - Related: It takes 2-3 seconds for a call to `asyncio.open_connection` to come back with any sort of response on my machine, which can be significant now that we're trying multiple ports in sequence. I guess it could fire off 5 tasks at once. Might cause some weirdness if there exist multiple scripts and multiple clients looking for each other at the same time.
  - Also related: The first time a client attempts to connect to a script, they accept each other and start communicating as expected. The second client to try that port seems to believe it connects and will then time out on the first message. And then all subsequent attempts to connect to that port by any client will be refused (as expected) until the script shuts down or restarts. I haven't been able to explain this behavior. It adds more time to a client's search for a script, but doesn't ultimately cause problems.

data/lua/connector_bizhawk_generic.lua

@@ -249,6 +249,24 @@ Response:
     - `err` (`string`): A description of the problem
 ]]
 
+local bizhawk_version = client.getversion()
+local bizhawk_major, bizhawk_minor, bizhawk_patch = bizhawk_version:match("(%d+)%.(%d+)%.?(%d*)")
+bizhawk_major = tonumber(bizhawk_major)
+bizhawk_minor = tonumber(bizhawk_minor)
+if bizhawk_patch == "" then
+    bizhawk_patch = 0
+else
+    bizhawk_patch = tonumber(bizhawk_patch)
+end
+
+local lua_major, lua_minor = _VERSION:match("Lua (%d+)%.(%d+)")
+lua_major = tonumber(lua_major)
+lua_minor = tonumber(lua_minor)
+
+if lua_major > 5 or (lua_major == 5 and lua_minor >= 3) then
+    require("lua_5_3_compat")
+end
+
 local base64 = require("base64")
 local socket = require("socket")
 local json = require("json")
@@ -257,7 +275,9 @@ local json = require("json")
 -- Will cause lag due to large console output
 local DEBUG = false
 
-local SOCKET_PORT = 43055
+local SOCKET_PORT_FIRST = 43055
+local SOCKET_PORT_RANGE_SIZE = 5
+local SOCKET_PORT_LAST = SOCKET_PORT_FIRST + SOCKET_PORT_RANGE_SIZE
 
 local STATE_NOT_CONNECTED = 0
 local STATE_CONNECTED = 1
@@ -277,24 +297,6 @@ local locked = false
 
 local rom_hash = nil
 
-local lua_major, lua_minor = _VERSION:match("Lua (%d+)%.(%d+)")
-lua_major = tonumber(lua_major)
-lua_minor = tonumber(lua_minor)
-
-if lua_major > 5 or (lua_major == 5 and lua_minor >= 3) then
-    require("lua_5_3_compat")
-end
-
-local bizhawk_version = client.getversion()
-local bizhawk_major, bizhawk_minor, bizhawk_patch = bizhawk_version:match("(%d+)%.(%d+)%.?(%d*)")
-bizhawk_major = tonumber(bizhawk_major)
-bizhawk_minor = tonumber(bizhawk_minor)
-if bizhawk_patch == "" then
-    bizhawk_patch = 0
-else
-    bizhawk_patch = tonumber(bizhawk_patch)
-end
-
 function queue_push (self, value)
     self[self.right] = value
     self.right = self.right + 1
@@ -435,7 +437,7 @@ function send_receive ()
     end
 
     if message == "VERSION" then
-        local result, err client_socket:send(tostring(SCRIPT_VERSION).."\n")
+        client_socket:send(tostring(SCRIPT_VERSION).."\n")
     else
         local res = {}
         local data = json.decode(message)
@@ -463,14 +465,45 @@ function send_receive ()
     end
 end
 
-function main ()
-    server, err = socket.bind("localhost", SOCKET_PORT)
+function initialize_server ()
+    local err
+    local port = SOCKET_PORT_FIRST
+    local res = nil
+
+    server, err = socket.socket.tcp4()
+    while res == nil and port <= SOCKET_PORT_LAST do
+        res, err = server:bind("localhost", port)
+        if res == nil and err ~= "address already in use" then
+            print(err)
+            return
+        end
+
+        if res == nil then
+            port = port + 1
+        end
+    end
+
+    if port > SOCKET_PORT_LAST then
+        print("Too many instances of connector script already running. Exiting.")
+        return
+    end
+
+    res, err = server:listen(0)
+
     if err ~= nil then
         print(err)
         return
     end
 
+    server:settimeout(0)
+end
+
+function main ()
     while true do
+        if server == nil then
+            initialize_server()
+        end
+
         current_time = socket.socket.gettime()
         timeout_timer = timeout_timer - (current_time - prev_time)
         message_timer = message_timer - (current_time - prev_time)
@@ -482,16 +515,16 @@ function main ()
         end
 
         if current_state == STATE_NOT_CONNECTED then
-            if emu.framecount() % 60 == 0 then
-                server:settimeout(2)
+            if emu.framecount() % 30 == 0 then
+                print("Looking for client...")
                 local client, timeout = server:accept()
                 if timeout == nil then
                     print("Client connected")
                     current_state = STATE_CONNECTED
                     client_socket = client
+                    server:close()
+                    server = nil
                     client_socket:settimeout(0)
-                else
-                    print("No client found. Trying again...")
                 end
             end
         else
@@ -527,27 +560,27 @@ else
             emu.frameadvance()
         end
     end
-    
+
     rom_hash = gameinfo.getromhash()
 
-    print("Waiting for client to connect. Emulation will freeze intermittently until a client is found.\n")
+    print("Waiting for client to connect. This may take longer the more instances of this script you have open at once.\n")
 
     local co = coroutine.create(main)
     function tick ()
         local status, err = coroutine.resume(co)
-    
-        if not status then
+
+        if not status and err ~= "cannot resume dead coroutine" then
             print("\nERROR: "..err)
             print("Consider reporting this crash.\n")
     
             if server ~= nil then
                 server:close()
             end
-    
+
             co = coroutine.create(main)
         end
     end
-    
+
     -- Gambatte has a setting which can cause script execution to become
     -- misaligned, so for GB and GBC we explicitly set the callback on
     -- vblank instead.
@@ -557,7 +590,7 @@ else
     else
         event.onframeend(tick)
     end
-    
+
     while true do
         emu.frameadvance()
     end