mtp_config: parse default DC id from Telegram config; drop get_netloc_safe; ETS protected

- parse_config/1 now returns {DefaultDc, Downstreams} using a single
  lists:foldl pass; the 'default X;' line sets the fallback DC id
- update_ids/3 stores {default_dc, DcId} in ETS alongside dc_ids
- get_default_dc/0 reads default_dc from ETS (safe from any process)
- get_downstream_safe/2 fallback uses get_default_dc() instead of
  random_choice; errors immediately if default == requested (avoid loop)
- get_netloc_safe/1 removed: dead code since 2018, never called
- ETS table changed from public to protected (only mtp_config writes)
- doc/handler-downstream-flow.md: new sequence diagram + update note
  about pool resolution fallback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: seriyps

doc/handler-downstream-flow.md

@@ -0,0 +1,45 @@
+# Handler ↔ downstream lookup and handshake
+
+Shows how `mtp_handler` locates an `mtp_down_conn` for a new client connection
+and the steady-state data flow that follows.
+
+**Key actors:**
+- `mtp_handler` — one process per Telegram client TCP connection
+- `mtp_dc_pool` — manages a pool of downstream connections for one DC
+- `mtp_down_conn` — multiplexed TCP connection to a Telegram DC
+- `Telegram DC` — the upstream Telegram data-centre server
+
+```mermaid
+sequenceDiagram
+    participant Client as Telegram client
+    participant Handler as mtp_handler
+    participant Pool as mtp_dc_pool
+    participant Down as mtp_down_conn
+    participant TG as Telegram DC
+
+    Client->>Handler: TCP connect + Hello bytes
+
+    Note over Handler: decode protocol headers<br/>(fake-TLS / obfuscated / secure)<br/>stage: hello → tunnel
+
+    Note over Handler: resolve pool: whereis(dc_to_pool_name(DcId))<br/>(registered name lookup; falls back to default DC from mtp_config if not found)
+    Handler->>Pool: mtp_dc_pool:get(Pool, self(), Opts) [sync]
+    Pool-->>Down: upstream_new(Handler, Opts) [cast]
+    Pool->>Handler: Downstream pid
+
+    Note over Handler: down = Downstream<br/>stage = tunnel
+
+    loop steady-state data exchange
+        Client->>Handler: TCP data
+        Handler->>Down: mtp_down_conn:send(Down, Data) [sync]
+        Down->>TG: TCP data (RPC-framed)
+        TG->>Down: TCP data
+        Down->>Handler: ok
+        Down-->>Handler: {proxy_ans, Down, Data} [cast]
+        Handler->>Client: TCP data
+        Handler-->>Down: mtp_down_conn:ack(Down, Count, Size) [cast]
+    end
+
+    Client->>Handler: TCP close
+    Handler-->>Pool: mtp_dc_pool:return(Pool, self()) [cast]
+    Pool-->>Down: upstream_closed(Down, Handler) [cast]
+```

src/mtp_config.erl

@@ -18,8 +18,8 @@
 -export([get_downstream_safe/2,
          get_downstream_pool/1,
          get_netloc/1,
-         get_netloc_safe/1,
          get_secret/0,
+         get_default_dc/0,
          status/0,
          update/0]).
 
@@ -37,6 +37,7 @@
 -define(TAB, ?MODULE).
 -define(IPS_KEY(DcId), {id, DcId}).
 -define(IDS_KEY, dc_ids).
+-define(DEFAULT_DC_KEY, default_dc).
 -define(SECRET_URL, "https://core.telegram.org/getProxySecret").
 -define(CONFIG_URL, "https://core.telegram.org/getProxyConfig").
 
@@ -65,9 +66,11 @@ get_downstream_safe(DcId, Opts) ->
                     error({pool_empty, DcId, Pool})
             end;
         not_found ->
-            [{?IDS_KEY, L}] = ets:lookup(?TAB, ?IDS_KEY),
-            NewDcId = random_choice(L),
-            get_downstream_safe(NewDcId, Opts)
+            case get_default_dc() of
+                undefined  -> error({no_pool, DcId});
+                DcId       -> error({no_pool, DcId});   % default == requested, avoid loop
+                NewDcId    -> get_downstream_safe(NewDcId, Opts)
+            end
     end.
 
 get_downstream_pool(DcId) ->
@@ -78,17 +81,6 @@ get_downstream_pool(DcId) ->
             not_found
     end.
 
--spec get_netloc_safe(dc_id()) -> {dc_id(), netloc()}.
-get_netloc_safe(DcId) ->
-    case get_netloc(DcId) of
-        {ok, Addr} -> {DcId, Addr};
-        not_found ->
-            [{?IDS_KEY, L}] = ets:lookup(?TAB, ?IDS_KEY),
-            NewDcId = random_choice(L),
-            %% Get random DC; it might return 0 and recurse aggain
-            get_netloc_safe(NewDcId)
-    end.
-
 get_netloc(DcId) ->
     Key = ?IPS_KEY(DcId),
     case ets:lookup(?TAB, Key) of
@@ -107,6 +99,13 @@ get_secret() ->
     [{_, Key}] = ets:lookup(?TAB, key),
     Key.
 
+-spec get_default_dc() -> dc_id() | undefined.
+get_default_dc() ->
+    case ets:lookup(?TAB, ?DEFAULT_DC_KEY) of
+        [{?DEFAULT_DC_KEY, DcId}] -> DcId;
+        [] -> undefined
+    end.
+
 -spec status() -> [mtp_dc_pool:status()].
 status() ->
     [{?IDS_KEY, L}] = ets:lookup(?TAB, ?IDS_KEY),
@@ -130,7 +129,7 @@ init([]) ->
               #{timeout => {env, ?APP, conf_refresh_interval, 3600},
                 unit => second}),
     Tab = ets:new(?TAB, [set,
-                         public,
+                         protected,
                          named_table,
                          {read_concurrency, true}]),
     State = #state{tab = Tab,
@@ -189,17 +188,23 @@ update_key(Tab) ->
 update_config(Tab) ->
     Url = application:get_env(mtproto_proxy, proxy_config_url, ?CONFIG_URL),
     {ok, Body} = http_get(Url),
-    Downstreams = parse_config(Body),
+    {DefaultDc, Downstreams} = parse_config(Body),
     update_downstreams(Downstreams, Tab),
-    update_ids(Downstreams, Tab).
+    update_ids(Downstreams, DefaultDc, Tab).
 
 parse_config(Body) ->
     Lines = string:lexemes(Body, "\n"),
-    ProxyLines = lists:filter(
-                   fun("proxy_for " ++ _) -> true;
-                      (_) -> false
-                   end, Lines),
-    [parse_downstream(Line) || Line <- ProxyLines].
+    {DefaultDc, Downstreams} =
+        lists:foldl(
+          fun("default " ++ _ = Line, {_, Ds}) ->
+                  ["default", DcIdStr] = string:lexemes(Line, " ;"),
+                  {list_to_integer(DcIdStr), Ds};
+             ("proxy_for " ++ _ = Line, {Def, Ds}) ->
+                  {Def, [parse_downstream(Line) | Ds]};
+             (_, Acc) ->
+                  Acc
+          end, {undefined, []}, Lines),
+    {DefaultDc, lists:reverse(Downstreams)}.
 
 parse_downstream(Line) ->
     ["proxy_for",
@@ -231,9 +236,10 @@ update_downstreams(Downstreams, Tab) ->
       end,
       maps:keys(ByDc)).
 
-update_ids(Downstreams, Tab) ->
+update_ids(Downstreams, DefaultDc, Tab) ->
     Ids = lists:usort([DcId || {DcId, _, _} <- Downstreams]),
-    true = ets:insert(Tab, {?IDS_KEY, Ids}).
+    true = ets:insert(Tab, {?IDS_KEY, Ids}),
+    true = ets:insert(Tab, {?DEFAULT_DC_KEY, DefaultDc}).
 
 update_ip() ->
     case application:get_env(?APP, ip_lookup_services) of
@@ -283,15 +289,22 @@ random_choice(L) ->
 -include_lib("eunit/include/eunit.hrl").
 
 parse_test() ->
-    Config = ("# force_probability 1 10
-proxy_for 1 149.154.175.50:8888;
-proxy_for -1 149.154.175.50:8888;
-proxy_for 2 149.154.162.39:80;
-proxy_for 2 149.154.162.33:80;"),
+    Config = ("# force_probability 1 10\n"
+              "default 2;\n"
+              "proxy_for 1 149.154.175.50:8888;\n"
+              "proxy_for -1 149.154.175.50:8888;\n"
+              "proxy_for 2 149.154.162.39:80;\n"
+              "proxy_for 2 149.154.162.33:80;"),
     Expect = [{1, {149, 154, 175, 50}, 8888},
               {-1, {149, 154, 175, 50}, 8888},
               {2, {149, 154, 162, 39}, 80},
               {2, {149, 154, 162, 33},80}],
-    ?assertEqual(Expect, parse_config(Config)).
+    ?assertEqual({2, Expect}, parse_config(Config)).
+
+parse_no_default_test() ->
+    Config = ("proxy_for 1 149.154.175.50:8888;\n"
+              "proxy_for 2 149.154.162.39:80;"),
+    {DefaultDc, _} = parse_config(Config),
+    ?assertEqual(undefined, DefaultDc).
 
 -endif.