script input redirection / eof handling

script input redirection / eof handling

[Phillip Susi]

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I came across an old bug filed in debian about script being redirected
from a file misbehaving.  It seems that when the master ( input )
process hits eof, it exits immediately, orphaning the child ( output )
and user processes.

I believe the correct thing to do is signal the eof to the user
process and wait for the children to exit, but I'm not sure how to
make a pty signal eof.

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Re: script input redirection / eof handling

[Karel Zak]

On Thu, Oct 31, 2013 at 10:21:10AM -0400, Phillip Susi wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
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> 
> I came across an old bug filed in debian about script being redirected
> from a file misbehaving. 

Do you mean something like:

   echo "ps uf" | script

.. if yes, then it's pretty unexpected usage. The code expects that
stdin is terminal. (For example it copy the current terminal
attributes the new pseudo-terminal.)

Unfortunately it does not check tcgetatt() return code so the problem 
is not visible. I'll fix it.

> It seems that when the master ( input )
> process hits eof, it exits immediately, orphaning the child ( output )
> and user processes.

Yep, I think it would be enough to add 

       kill(0, SIGTERM);

before done() in doinput() to avoid orphans.

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

Re: script input redirection / eof handling

[Karel Zak]

On Mon, Dec 02, 2013 at 01:36:59PM +0100, Karel Zak wrote:
> On Thu, Oct 31, 2013 at 10:21:10AM -0400, Phillip Susi wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> > 
> > I came across an old bug filed in debian about script being redirected
> > from a file misbehaving. 
> 
> Do you mean something like:
> 
>    echo "ps uf" | script
> 
> .. if yes, then it's pretty unexpected usage. The code expects that
> stdin is terminal. (For example it copy the current terminal
> attributes the new pseudo-terminal.)
> 
> Unfortunately it does not check tcgetatt() return code so the problem 
> is not visible. I'll fix it.

Note, that if you want to support this scenario (stdin is not a
terminal) then send a patch ;-) 

I guess you have to initialize the terminal (see inlude/ttyutils.h, 
reset_virtual_console()) and use this dummy struct termios instead of
the current 'tt'.

> Yep, I think it would be enough to add 
> 
>        kill(0, SIGTERM);
> 
> before done() in doinput() to avoid orphans.

 This is probably not the best solution (because shell will print
 "Terminated" etc.), but it's definitely better than orphans.

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

Re: script input redirection / eof handling

[Phillip Susi]

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On 12/2/2013 7:54 AM, Karel Zak wrote:
> Note, that if you want to support this scenario (stdin is not a 
> terminal) then send a patch ;-)

It seems to work fine up until it hits eof, which can also happen on a
tty and you'll get the same bad behavior, so I think refusing to run
on a non tty isn't a proper fix.

>> Yep, I think it would be enough to add
>> 
>> kill(0, SIGTERM);
>> 
>> before done() in doinput() to avoid orphans.

You don't want to just make sure the children die, you want to make
sure that the child sees the eof and capture any final output it may have.


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Re: script input redirection / eof handling

[Karel Zak]

On Mon, Dec 02, 2013 at 09:45:19AM -0500, Phillip Susi wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
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> 
> On 12/2/2013 7:54 AM, Karel Zak wrote:
> > Note, that if you want to support this scenario (stdin is not a 
> > terminal) then send a patch ;-)
> 
> It seems to work fine up until it hits eof, which can also happen on a

 well, it's nasty that the code ignores return codes and it call's tty
 ioctls/functions for non-tty file descriptors.

> tty and you'll get the same bad behavior, so I think refusing to run
> on a non tty isn't a proper fix.
> 
> >> Yep, I think it would be enough to add
> >> 
> >> kill(0, SIGTERM);
> >> 
> >> before done() in doinput() to avoid orphans.
> 
> You don't want to just make sure the children die, you want to make
> sure that the child sees the eof and capture any final output it may have.

 never ending orphans suck...

 The possible solution (hack) to emulate end-of-file is by write "exit\n" to 
 the master tty. For example patch below works for me (it also make it
 more robust for non-ttys).

    Karel

diff --git a/term-utils/script.c b/term-utils/script.c
index c18274c..8437ec8 100644
--- a/term-utils/script.c
+++ b/term-utils/script.c
@@ -107,6 +107,7 @@ int	fflg = 0;
 int	qflg = 0;
 int	tflg = 0;
 int	forceflg = 0;
+int	isterm;
 
 int die;
 int resized;
@@ -230,9 +231,6 @@ main(int argc, char **argv) {
 		die_if_link(fname);
 	}
 
-	if (!isatty(STDIN_FILENO))
-		errx(EXIT_FAILURE, _("The stdin is not a terminal."));
-
 	if ((fscript = fopen(fname, aflg ? "a" : "w")) == NULL) {
 		warn(_("cannot open %s"), fname);
 		fail();
@@ -314,11 +312,21 @@ doinput(void) {
 		else if (cc < 0 && errno == EINTR && resized)
 		{
 			/* transmit window change information to the child */
-			ioctl(STDIN_FILENO, TIOCGWINSZ, (char *)&win);
-			ioctl(slave, TIOCSWINSZ, (char *)&win);
+			if (isterm) {
+				ioctl(STDIN_FILENO, TIOCGWINSZ, (char *)&win);
+				ioctl(slave, TIOCSWINSZ, (char *)&win);
+			}
 			resized = 0;
-		}
-		else
+		} else if (cc == 0 && errno == 0) {
+			/* end-of-file, probably "foo | script" */
+			strcpy(ibuf, "exit\n");
+			if (write(master, ibuf, 5) < 0) {
+				warn (_("write failed"));
+				fail();
+			}
+			pause();		/* wait for SIGCHLD */
+			break;
+		} else
 			break;
 	}
 
@@ -473,6 +481,9 @@ void
 fixtty(void) {
 	struct termios rtt;
 
+	if (!isterm)
+		return;
+
 	rtt = tt;
 	cfmakeraw(&rtt);
 	rtt.c_lflag &= ~ECHO;
@@ -503,7 +514,8 @@ done(void) {
 
 		master = -1;
 	} else {
-		tcsetattr(STDIN_FILENO, TCSADRAIN, &tt);
+		if (isterm)
+			tcsetattr(STDIN_FILENO, TCSADRAIN, &tt);
 		if (!qflg)
 			printf(_("Script done, file is %s\n"), fname);
 #ifdef HAVE_LIBUTEMPTER
@@ -524,9 +536,19 @@ done(void) {
 void
 getmaster(void) {
 #if defined(HAVE_LIBUTIL) && defined(HAVE_PTY_H)
-	tcgetattr(STDIN_FILENO, &tt);
-	ioctl(STDIN_FILENO, TIOCGWINSZ, (char *)&win);
-	if (openpty(&master, &slave, NULL, &tt, &win) < 0) {
+	int rc;
+
+	isterm = isatty(STDIN_FILENO);
+
+	if (isterm) {
+		if (tcgetattr(STDIN_FILENO, &tt) != 0)
+			err(EXIT_FAILURE, _("failed to get terminal attributes"));
+		ioctl(STDIN_FILENO, TIOCGWINSZ, (char *) &win);
+		rc = openpty(&master, &slave, NULL, &tt, &win);
+	} else
+		rc = openpty(&master, &slave, NULL, NULL, NULL);
+
+	if (rc < 0) {
 		warn(_("openpty failed"));
 		fail();
 	}
@@ -534,6 +556,8 @@ getmaster(void) {
 	char *pty, *bank, *cp;
 	struct stat stb;
 
+	isterm = isatty(STDIN_FILENO);
+
 	pty = &line[strlen("/dev/ptyp")];
 	for (bank = "pqrs"; *bank; bank++) {
 		line[strlen("/dev/pty")] = *bank;
@@ -552,9 +576,11 @@ getmaster(void) {
 				ok = access(line, R_OK|W_OK) == 0;
 				*tp = 'p';
 				if (ok) {
-					tcgetattr(STDIN_FILENO, &tt);
-					ioctl(STDIN_FILENO, TIOCGWINSZ,
-						(char *)&win);
+					if (isterm) {
+						tcgetattr(STDIN_FILENO, &tt);
+						ioctl(STDIN_FILENO, TIOCGWINSZ,
+								(char *)&win);
+					}
 					return;
 				}
 				close(master);
@@ -577,8 +603,10 @@ getslave(void) {
 		warn(_("cannot open %s"), line);
 		fail();
 	}
-	tcsetattr(slave, TCSANOW, &tt);
-	ioctl(slave, TIOCSWINSZ, (char *)&win);
+	if (isterm) {
+		tcsetattr(slave, TCSANOW, &tt);
+		ioctl(slave, TIOCSWINSZ, (char *)&win);
+	}
 #endif
 	setsid();
 	ioctl(slave, TIOCSCTTY, 0);

Re: script input redirection / eof handling

[Phillip Susi]

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On 12/2/2013 12:02 PM, Karel Zak wrote:
> never ending orphans suck...

Yes, but the idea is not to orphan them in the first place and wait
for them to terminate instead.

> The possible solution (hack) to emulate end-of-file is by write
> "exit\n" to the master tty. For example patch below works for me
> (it also make it more robust for non-ttys).

That only works if the program on the other end knows what "exit"
means, which is basically only true if it's a shell.  Surely there
must be a proper way to signal EOF over a pty?  Can you use shutdown()
on a pty?


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Re: script input redirection / eof handling

[Karel Zak]

On Mon, Dec 02, 2013 at 01:06:05PM -0500, Phillip Susi wrote:
> That only works if the program on the other end knows what "exit"

yep, fingers have been faster than brain.. (I forgot we have
--command).

> means, which is basically only true if it's a shell.  Surely there
> must be a proper way to signal EOF over a pty?  Can you use shutdown()
> on a pty?

I'm going to play with that tomorrow.

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

Re: script input redirection / eof handling

[Karel Zak]

On Mon, Dec 02, 2013 at 01:06:05PM -0500, Phillip Susi wrote:
> That only works if the program on the other end knows what "exit"
> means, which is basically only true if it's a shell.  Surely there
> must be a proper way to signal EOF over a pty?  Can you use shutdown()
> on a pty?

 Oh, now it reminds me... in include/ttyutils.h we have macros to
 generate tty control chars, for example ^C

			char eof = DEF_EOF;

			if (write(master, &eof, 1) < 0) {
				warn (_("write failed"));
				fail();
			}
            pause();
           
 is what we need. 

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

Re: script input redirection / eof handling

[Karel Zak]

On Tue, Dec 03, 2013 at 12:25:13AM +0100, Karel Zak wrote:
> On Mon, Dec 02, 2013 at 01:06:05PM -0500, Phillip Susi wrote:
> > That only works if the program on the other end knows what "exit"
> > means, which is basically only true if it's a shell.  Surely there
> > must be a proper way to signal EOF over a pty?  Can you use shutdown()
> > on a pty?
> 
>  Oh, now it reminds me... in include/ttyutils.h we have macros to
>  generate tty control chars, for example ^C
> 
> 			char eof = DEF_EOF;
> 
> 			if (write(master, &eof, 1) < 0) {
> 				warn (_("write failed"));
> 				fail();
> 			}
>             pause();
>            
>  is what we need. 

 It was more tricky than I have originally expected, but it seems it
 works. (It seems that it is not enough to send EOF, you have to be
 sure that someone already listens on slave side.)

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com