rendezvous: cap unauthenticated punches in flight (#16256)

A PunchHole needs no authentication: anyone who knows this id can ask hbbs
to have us open a punch, and each one hbbs sends is spawned without a limit.
Every punch pays per request. punch_udp_hole binds a socket of its own and
start_ipv6 another; the TCP punch opens a listener on the ephemeral port of
its own connection to hbbs, with a punch of its own in flight beside it -
`reuse` sets the socket flags, it shares no fd - and the LAN listen a
FetchLocalAddr opens is that listener again. All of them then wait up to
CONNECT_TIMEOUT for the peer, so a stream of requests holds as many sockets
as it likes for as long as it keeps sending.

Each takes a place in one of two pools of 32 - UDP over v4 and v6 in one,
the TCP punch and the LAN listen in the other - and gives it back the
moment the peer's session is up - the KCP accept, the TCP stream in hand -
or when the wait ends without one: a place stands for a socket waiting for
its peer and nothing past it, and from the session on the connection layer's
own limits apply, the same handoff the WebRTC answerer's slot makes at its
open data channel. At the limit an arrival is declined rather than an older
punch cut short, since the places turn over on their own, within
CONNECT_TIMEOUT and the few seconds the punch phases add to it, and cutting
one short would drop a socket that may be a moment from carrying a session. Two pools rather than one, so that neither
transport pays for the other's crowd: a connection costs a place in each,
the controller's preferred request punching UDP and its TCP fallback
request punching TCP, and the transports that lose the race hold theirs for
the whole wait, so one pool of 32 would be about ten connections setting up
at once and a crowd of UDP punches would decline a TCP punch that had
nothing to do with it. A place is taken only for a punch that will
be made: whether the v4 legs relay is decided first, since the relay branch
runs the whole session and a place held across it would let ordinary relay
traffic use the pool up. A declined LAN listen falls to the relay, as one
that fails for any other reason does.

Declined is the listen alone, never the reply. One PunchHoleSent carries the
WebRTC answer, the v6 address and the relay server along with the v4 punch,
and the controller sends one request for all of them and retries it three
times before it fails outright, so a reply withheld for want of a v4 place
would lose the transports that needed none. The reply goes out as before, on
a socket that then goes at once, resends and all - a declined request is not
worth a socket kept for its reply's sake, and the controller re-asks on its
own - and what the controller loses is its v4 attempt at a mapping that no
longer answers; its others go on.

A request that carries a v6 address costs two places, and the v4 one is
taken first: the v6 punch starts before the v4 one does, and taken in that
order the last place would go to the transport the peer may have no route
for and leave the one it can count on with none. A declined v6 punch leaves
the v4 path to carry the connection, as it already does wherever this
machine has no public IPv6 address. Each kind logs at most one line a
minute, carrying the number of requests it stands for, since the peer
decides how often it asks.

Tests: the places are the bound and come back on drop; a full UDP pool
leaves the TCP places alone; at the last place it
is the v6 punch that goes without; a declined UDP punch still sends its
PunchHoleSent, answer and v6 address intact, to a loopback stand-in for
hbbs; a listen whose peer never probes gives its place back.


Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

src/server.rs

@@ -182,6 +182,7 @@ async fn accept_connection_(
     socket: Stream,
     secure: bool,
     meta: ConnectionMeta,
+    slot: crate::rendezvous_mediator::PunchSlot,
 ) -> ResultType<()> {
     let local_addr = socket.local_addr();
     drop(socket);
@@ -191,6 +192,8 @@ async fn accept_connection_(
     let listener = new_listener(local_addr, true).await?;
     log::info!("Server listening on: {}", &listener.local_addr()?);
     if let Ok((stream, addr)) = timeout(CONNECT_TIMEOUT, listener.accept()).await? {
+        // The peer is in: the place goes back before the session runs, as every punch's does.
+        drop(slot);
         stream.set_nodelay(true).ok();
         let stream_addr = stream.local_addr()?;
         create_tcp_connection(
@@ -320,14 +323,15 @@ async fn identity_handshake(stream: &mut Stream, secure: bool) -> ResultType<()>
     Ok(())
 }
 
-pub async fn accept_connection(
+pub(crate) async fn accept_connection(
     server: ServerPtr,
     socket: Stream,
     peer_addr: SocketAddr,
     secure: bool,
     meta: ConnectionMeta,
+    slot: crate::rendezvous_mediator::PunchSlot,
 ) {
-    if let Err(err) = accept_connection_(server, socket, secure, meta).await {
+    if let Err(err) = accept_connection_(server, socket, secure, meta, slot).await {
         log::warn!("Failed to accept connection from {}: {}", peer_addr, err);
     }
 }