telnet: Fix infinite loop if local output generates SIGPIPE.

Instead of catching SIGPIPE and jumping out of the signal handler with
longjmp, ignore it and handle write errors to the local output by exiting
from there. I have changed the error message to mention the local output
instead of NetBSD's wrong "Connection closed by foreign host". Write errors
to the network were already handled by exiting immediately and this now
applies to EPIPE too.

The code assumed that SIGPIPE could only be generated by the network
connection; if it was generated by the local output, it would longjmp out of
the signal handler and write an error message which caused another SIGPIPE.

PR:		19773
Obtained from:	NetBSD
MFC after:	1 week

contrib/telnet/telnet/commands.c

@@ -2491,8 +2491,7 @@ tn(int argc, char *argv[])
 	env_export("USER");
     }
     (void) call(status, "status", "notmuch", 0);
-    if (setjmp(peerdied) == 0)
-	telnet(user);
+    telnet(user); 
     (void) NetClose(net);
     ExitString("Connection closed by foreign host.\n",1);
     /*NOTREACHED*/

contrib/telnet/telnet/externs.h

@@ -233,7 +233,6 @@ extern void
     SetNetTrace(char *);	/* Function to change where debugging goes */
 
 extern jmp_buf
-    peerdied,
     toplevel;		/* For error conditions. */
 
 extern void

contrib/telnet/telnet/network.c

@@ -158,7 +158,7 @@ netflush(void)
 	    perror(hostname);
 	    (void)NetClose(net);
 	    ring_clear_mark(&netoring);
-	    longjmp(peerdied, -1);
+	    ExitString("Connection closed by foreign host.\n", 1);
 	    /*NOTREACHED*/
 	}
 	n = 0;

contrib/telnet/telnet/sys_bsd.c

@@ -808,14 +808,6 @@ NetNonblockingIO(int fd, int onoff)
  * Various signal handling routines.
  */
 
-/* ARGSUSED */
-static SIG_FUNC_RET
-deadpeer(int sig __unused)
-{
-	setcommandmode();
-	longjmp(peerdied, -1);
-}
-
 /* ARGSUSED */
 SIG_FUNC_RET
 intr(int sig __unused)
@@ -884,7 +876,7 @@ sys_telnet_init(void)
 {
     (void) signal(SIGINT, intr);
     (void) signal(SIGQUIT, intr2);
-    (void) signal(SIGPIPE, deadpeer);
+    (void) signal(SIGPIPE, SIG_IGN);
 #ifdef	SIGWINCH
     (void) signal(SIGWINCH, sendwin);
 #endif

contrib/telnet/telnet/telnet.c

@@ -146,7 +146,6 @@ unsigned char telopt_environ = TELOPT_NEW_ENVIRON;
 #endif
 
 jmp_buf	toplevel;
-jmp_buf	peerdied;
 
 int	flushline;
 int	linemode;

contrib/telnet/telnet/terminal.c

@@ -111,7 +111,8 @@ init_terminal(void)
 }
 
 /*
- *		Send as much data as possible to the terminal.
+ *		Send as much data as possible to the terminal, else exits if
+ *		it encounters a permanent failure when writing to the tty.
  *
  *		Return value:
  *			-1: No useful work done, data waiting to go out.
@@ -152,8 +153,19 @@ ttyflush(int drop)
 	}
 	ring_consumed(&ttyoring, n);
     }
-    if (n < 0)
+    if (n < 0) {
+	if (errno == EAGAIN || errno == EINTR) {
+	    return -1;
+	} else {
+	    ring_consumed(&ttyoring, ring_full_count(&ttyoring));
+	    setconnmode(0);
+	    setcommandmode();
+	    NetClose(net);
+	    fprintf(stderr, "Write error on local output.\n");
+	    exit(1);
+	}
 	return -1;
+    }
     if (n == n0) {
 	if (n0)
 	    return -1;