Implements TCP half-duplex close.  It is needed for Go and may be useful too.

Reference: /n/sources/patch/tcp-halfduplex-close
Date: Thu Feb  9 23:45:43 CET 2017
Signed-off-by: skip.tavakkolian@gmail.com

--- /sys/src/9/ip/tcp.c	Thu Feb  9 23:41:10 2017
+++ /sys/src/9/ip/tcp.c	Thu Feb  9 23:41:02 2017
@@ -432,6 +432,7 @@
 static	void	tcpsynackrtt(Conv*);
 static	void	tcptimeout(void*);
 static	int	tcptrim(Tcpctl*, Tcp*, Block**, ushort*);
+static	char*	tcphangup_rdwr(Conv*);
 
 static void
 tcpsetstate(Conv *s, uchar newstate)
@@ -1410,11 +1411,43 @@
 }
 
 /*
+ *  close the conversation or perform half-duplex close in the specified direction
+ *  called with s qlocked
+ */
+static char*
+tcphangup(Conv *s, char **f, int n)
+{
+	Tcpctl *tcb;
+
+	if(n == 1)
+		return tcphangup_rdwr(s);
+
+	tcb = (Tcpctl*)s->ptcl;
+	if(tcb->state != Established)
+		return "connection must be in Establised state";
+
+	if(strcmp(f[1], "read") == 0){
+		qhangup(s->rq, nil);
+		tcb->rcv.blocked = 1;
+		tcb->rcv.wnd = 0;
+	} else if(strcmp(f[1], "write") == 0){
+		qhangup(s->wq, nil);
+		tcb->flgcnt++;
+		tcb->snd.nxt++;
+		tcpsetstate(s, Finwait1);
+	} else
+		return "hangup needs read or write parameter";
+
+	tcpoutput(s);
+	return nil;
+}
+
+/*
  *  send a reset to the remote side and close the conversation
  *  called with s qlocked
  */
 static char*
-tcphangup(Conv *s)
+tcphangup_rdwr(Conv *s)
 {
 	Tcp seg;
 	Tcpctl *tcb;
@@ -3287,13 +3320,13 @@
 static char*
 tcpctl(Conv* c, char** f, int n)
 {
-	if(n == 1 && strcmp(f[0], "hangup") == 0)
-		return tcphangup(c);
+	if(n >= 1 && strcmp(f[0], "hangup") == 0)
+		return tcphangup(c, f, n);
 	if(n >= 1 && strcmp(f[0], "keepalive") == 0)
 		return tcpstartka(c, f, n);
-	if(n >= 1 && strcmp(f[0], "checksum") == 0)
+	if(n > 1 && strcmp(f[0], "checksum") == 0)
 		return tcpsetchecksum(c, f, n);
-	if(n >= 1 && strcmp(f[0], "tcpporthogdefense") == 0)
+	if(n > 1 && strcmp(f[0], "tcpporthogdefense") == 0)
 		return tcpporthogdefensectl(f[1]);
 	return "unknown control request";
 }
--- /sys/man/3/ip	Thu Feb  9 23:41:16 2017
+++ /sys/man/3/ip	Thu Feb  9 23:41:12 2017
@@ -720,8 +720,16 @@
 .TF "\fLkeepalive\fI n\fR"
 .PD
 .TP
-.B hangup
-close down this TCP connection
+.BI hangup \ [direction]
+close down this TCP connection. If specified, the
+.I direction
+can be used to perform a
+.I half-duplex
+close. Specifying
+.B read
+will close the incoming side of the connection; a
+.I write
+will close the outgoing side.
 .TP
 .BI keepalive \ n
 turn on keep alive messages.