net: one socket per client - four-tuple demux, spawning accept, bounded close

A server could only ever hold ONE connection, which makes an ircd pointless:
listen() turned the listener into the connection, and net_find_tcp demuxed on
local port alone, so two clients on 6667 were indistinguishable.

- net_find_tcp now matches an established socket on the full four-tuple
  (local port + peer IP + peer port) and only falls back to a LISTENing
  socket when none matches.
- A SYN at a listener no longer consumes it: tcp_spawn_conn clones a new
  socket (local port + owning pid, so exit/kill still reclaims it) and the
  listener keeps listening. accept() returns that socket; closing it leaves
  the listener alone. A full table drops the SYN, and the client's retransmit
  is taken once a slot frees.
- tcp_peer_is_current preserves BC: net_find_tcp scans with the socket index
  there, and the compare needs BC for the rx buffer base. Without this the
  scan died at the first mismatching socket, so a second client's handshake
  ACK never reached its socket and it hung in SYNRCVD - tcpdump showed our
  SYN+ACK going out and the client's NICK retransmitted five times into
  silence.
- listen() only conflicts with another LISTENER on the port; connections
  share it by design now.
- MAX_SOCKS 4 -> 6 (listener + 3 clients + 2 spare; 537 bytes of WRAM0 still
  free), and SK_ACC marks a spawned connection as not yet accepted.

tcp_close's settle is also time-boxed to ~250 ms of real time. It counted
PUMPS (8 x 8000), which is ~30s of wall clock when the peer never answers -
invisible with well-behaved peers, but a daemon refusing a client that stayed
connected froze itself, and every other client with it, for half a minute.

httpd keeps its listener open across requests now, so the next client's SYN
is accepted immediately instead of being dropped between close and re-listen:
three parallel clients go 0.1/1.1/2.1s -> 0.4/1.0/0.1s.

1 files changed, 16 insertions, 15 deletions

diff --git a/usr/httpd.c b/usr/httpd.c
index 335e824..e704e8c 100644
--- a/usr/httpd.c
+++ b/usr/httpd.c
@@ -33,7 +33,8 @@
 #define REQSZ    120 /* the request line is all we parse */
 #define PATHSZ   48
 
-static unsigned char sock;
+static unsigned char lsock; /* the listener, held for our lifetime */
+static unsigned char sock;  /* the client being served right now   */
 static unsigned char rbuf[BUFSZ];
 static char obuf[OBUFSZ];
 static unsigned char olen;
@@ -256,22 +257,22 @@ void main(void) {
     puts("  (run it with & and kill it to stop)");
     nl();
 
+    lsock = net_socket(SOCK_TCP);
+    if (lsock == 0xFF) {
+        puts("httpd: no socket");
+        nl();
+        sexit(1);
+    }
+    if (net_listen(lsock, port) == 0xFF) {
+        puts("httpd: listen failed (port in use?)");
+        nl();
+        net_close(lsock);
+        sexit(1);
+    }
     for (;;) {
-        sock = net_socket(SOCK_TCP);
-        if (sock == 0xFF) {
-            puts("httpd: no socket");
-            nl();
-            sexit(1);
-        }
-        if (net_listen(sock, port) == 0xFF) {
-            puts("httpd: listen failed");
-            nl();
-            net_close(sock);
-            sexit(1);
-        }
-        while (net_accept(sock) != 0) /* one RX pump per call */
+        while ((sock = net_accept(lsock)) == 0xFE) /* one RX pump per call */
             msleep(20);
         handle();
-        net_close(sock); /* FIN: this is what ends the body */
+        net_close(sock); /* FIN: this is what ends the body; listener stays */
     }
 }