NetBSD Problem Report #58151

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Message-Id: <20240414142613.B097D1A9239@mollari.NetBSD.org>
Date: Sun, 14 Apr 2024 14:26:13 +0000 (UTC)
From: achowe@snert.com
Reply-To: achowe@snert.com
To: gnats-bugs@NetBSD.org
Subject: Should Curses getnstr() family handle erase and kill characters when input is not a tty?
X-Send-Pr-Version: www-1.0

>Number:         58151
>Category:       lib
>Synopsis:       Should Curses getnstr() family handle erase and kill characters when input is not a tty?
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    lib-bug-people
>State:          closed
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Apr 14 14:30:01 +0000 2024
>Closed-Date:    Mon Apr 22 22:19:11 +0000 2024
>Last-Modified:  Mon Apr 22 22:19:11 +0000 2024
>Originator:     Anthony Howe
>Release:        10.0
>Organization:
>Environment:
NetBSD elf.snert.com 10.0 NetBSD 10.0 (GENERIC) #0: Thu Mar 28 08:33:33 UTC 2024  mkrepro@mkrepro.NetBSD.org:/usr/src/sys/arch/amd64/compile/GENERIC amd64

>Description:
When input is from a TTY, the Curses getnstr() family of functions correctly handle erasechar() (killchar() is subject of bug #57860).

If I attempt to pipe input to a Curses program (see below) which contains erasechar() or killchar() or both, then those characters are not processed and appear as part of the input buffer.

I expect Curses to treat standard input from a TTY or pipe the same.  One use case for this is it allows creating application test cases where input is piped.

In the case of redirected input, a Curses application would use the TERM defined keys or fall back on ^H and ^U when undefined.


>How-To-Repeat:
```
#include <stdio.h>
#include <string.h>
#include <curses.h>
#include <unistd.h>

int
main(int argc, char **argv)
{
	int erase, kill;
	char buf[80];
	if (initscr() == NULL) {
		return 1;
	}
	echo();
	cbreak();
	addstr(" 123456789.123456789.123456789.1234567890\n");
	standout();
	addstr("[                                        ]\n");
	mvgetnstr(1, 1, buf, sizeof (buf));
	standend();
	erase = erasechar();
	kill = killchar();
	endwin();
	printf("%ld:\"%s\"\n", strlen(buf), buf);
	printf("erasechar=%d killchar=%d\n", erase, kill);
	return 0;
}

/*
Type in a string like "abcdefghi", ^U, 1234.
Observe how there is a gap between 1 and 2.
Appears kill char ^U not handled properly.
Backspace / erase char (^H) works just fine.

NetBSD 9.3 lib/57860
*/
```

$ gcc getnstr.c -lcurses
$ print "abcd\b\b\b123\n" | ./a.out | tee output
$ od -a output

Note:
1. the erasechar and killchar are both zero (no defaults).
2. the buffer length is 10, instead of 4, and contains the BS characters from the pipe.


>Fix:

>Release-Note:

>Audit-Trail:
From: David Holland <dholland-bugs@netbsd.org>
To: gnats-bugs@netbsd.org
Cc: 
Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 characters when input is not a tty?
Date: Tue, 16 Apr 2024 02:09:00 +0000

 On Sun, Apr 14, 2024 at 02:30:01PM +0000, achowe@snert.com wrote:
  > 	echo();
  > 	cbreak();

 Calling cbreak() instructs curses to tell the tty to _not_ handle
 special characters (other than those that generate signals, which
 doesn't include erase and kill). So I don't see a bug here. (Same
 for 57680.)

 One could argue that getnstr/mvgetnstr in cbreak mode should handle
 simple line editing, but it does not, and there are problems with
 doing so. (For example: how simple? Tty cooked mode editing is fairly
 straightforward, but also not particularly useful or adequate since
 it's no longer 1983, but anything more complicated gets into religious
 questions like whether you should move to the beginning of the line
 with ^A or esc-^-i or what.)

 One could _also_ make an argument that curses should fake tty cooked
 mode when the caller does _not_ put the terminal in cbreak mode and
 the input is not a tty, but (a) that's not the way things historically
 work, (b) very few curses applications use tty cooked mode and there
 are good reasons for that, (c) there's an easy workaround (open a
 pty), and (d) it's not clear that there's much if any practical
 application for such behavior anyhow.

 -- 
 David A. Holland
 dholland@netbsd.org

From: David Holland <dholland-bugs@netbsd.org>
To: gnats-bugs@netbsd.org
Cc: 
Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 characters when input is not a tty?
Date: Tue, 16 Apr 2024 02:36:31 +0000

 On Tue, Apr 16, 2024 at 02:10:02AM +0000, David Holland wrote:
  >  One could argue that getnstr/mvgetnstr in cbreak mode should handle
  >  simple line editing, but it does not, and there are problems with
  >  doing so. (For example: how simple? Tty cooked mode editing is fairly
  >  straightforward, but also not particularly useful or adequate since
  >  it's no longer 1983, but anything more complicated gets into religious
  >  questions like whether you should move to the beginning of the line
  >  with ^A or esc-^-i or what.)

 Hmm. I take it back, apparently it does. This behavior is
 undocumented, and that's a bug. It's also lacking support for werase.
 And EOF, and REPRINT. And what about STATUS? Or INTR/QUIT/SUSP?

 The whole thing seems like a can of worms and a mistake, if you ask
 me.

 Anyway, it appears that if curses thinks it's not on a tty, both
 erasechar() and killchar() return 0. What happens if you send 0 down
 the pipe? If that causes an erase, and it looks like it will, that
 seems like undesirable behavior and erasechar() ought to default to ^H
 or ^? when not on a tty. (But, which?)

 -- 
 David A. Holland
 dholland@netbsd.org

From: Anthony Howe <achowe@snert.com>
To: gnats-bugs@netbsd.org
Cc: 
Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 characters when input is not a tty?
Date: Tue, 16 Apr 2024 05:55:59 -0400

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 Message-ID: <471d51c0-0f37-4d8d-92e9-1d7be3d32e8d@snert.com>
 Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
  characters when input is not a tty?
 References: <pr-lib-58151@gnats.netbsd.org>
  <20240414142613.B097D1A9239@mollari.NetBSD.org>
  <20240416024001.CEF911A923A@mollari.NetBSD.org>
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From: Anthony Howe <achowe@snert.com>
To: gnats-bugs@netbsd.org
Cc: 
Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 characters when input is not a tty?
Date: Tue, 16 Apr 2024 06:05:08 -0400

 This is a multi-part message in MIME format.
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 On 2024-04-15 22:40, David Holland wrote:
 > The following reply was made to PR lib/58151; it has been noted by GNATS.
 > 
 > From: David Holland <dholland-bugs@netbsd.org>
 > To: gnats-bugs@netbsd.org
 > Cc:
 > Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 >   characters when input is not a tty?
 > Date: Tue, 16 Apr 2024 02:36:31 +0000
 > 
 >   On Tue, Apr 16, 2024 at 02:10:02AM +0000, David Holland wrote:
 >    >  One could argue that getnstr/mvgetnstr in cbreak mode should handle
 >    >  simple line editing, but it does not, and there are problems with
 >    >  doing so. (For example: how simple? Tty cooked mode editing is fairly
 >    >  straightforward, but also not particularly useful or adequate since
 >    >  it's no longer 1983, but anything more complicated gets into religious
 >    >  questions like whether you should move to the beginning of the line
 >    >  with ^A or esc-^-i or what.)
 >   
 >   Hmm. I take it back, apparently it does. This behavior is
 >   undocumented, and that's a bug. It's also lacking support for werase.
 >   And EOF, and REPRINT. And what about STATUS? Or INTR/QUIT/SUSP?

 Historically Curses does not handle werase.  There is no equivalent of 
 erasechar() or killchar() for werase.

 INTR/QUIT/SUSP are signals (not line editing) which are handled by cbreak() and 
 raw() family.

 Curses has never supported `readline` for vi/emacs line editing.  That would be 
 an extension to historical behaviour.

 >   The whole thing seems like a can of worms and a mistake, if you ask
 >   me.

 Nope.  Historical.

 >   Anyway, it appears that if curses thinks it's not on a tty, both
 >   erasechar() and killchar() return 0. What happens if you send 0 down
 >   the pipe? If that causes an erase, and it looks like it will, that
 >   seems like undesirable behavior and erasechar() ought to default to ^H
 >   or ^? when not on a tty. (But, which?)

 By definition BS = backspace = ^H.  And kill typically defaults to stty ^U.  The 
 problem when stdin is not a tty, is that termios does not apply so all the 
 special keys are zero.

 In theory, Curses could `open(ctermid(NULL), ...)` a separate tty to get the 
 user's stty settings then apply them to the redirected stdin.

 -- 
 Anthony C Howe
 achowe@snert.com                                   BarricadeMX & Milters
 http://nanozen.snert.com/                          http://software.snert.com/

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 --------------0YUG8VAdVE0WkSAil0E23vm1--

From: Anthony Howe <achowe@snert.com>
To: gnats-bugs@netbsd.org
Cc: 
Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 characters when input is not a tty?
Date: Tue, 16 Apr 2024 07:57:27 -0400

 On 2024-04-15 22:40, David Holland wrote:
 > The following reply was made to PR lib/58151; it has been noted by GNATS.
 > 
 > From: David Holland <dholland-bugs@netbsd.org>
 > To: gnats-bugs@netbsd.org
 > Cc:
 > Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 >   characters when input is not a tty?
 > Date: Tue, 16 Apr 2024 02:36:31 +0000
 > 
 >   On Tue, Apr 16, 2024 at 02:10:02AM +0000, David Holland wrote:
 >    >  One could argue that getnstr/mvgetnstr in cbreak mode should handle
 >    >  simple line editing, but it does not, and there are problems with
 >    >  doing so. (For example: how simple? Tty cooked mode editing is fairly
 >    >  straightforward, but also not particularly useful or adequate since
 >    >  it's no longer 1983, but anything more complicated gets into religious
 >    >  questions like whether you should move to the beginning of the line
 >    >  with ^A or esc-^-i or what.)
 >   
 >   Hmm. I take it back, apparently it does. This behavior is
 >   undocumented, and that's a bug. It's also lacking support for werase.
 >   And EOF, and REPRINT. And what about STATUS? Or INTR/QUIT/SUSP?

 Historically Curses does not handle werase.  There is no equivalent of 
 erasechar() or killchar() for werase.

 INTR/QUIT/SUSP are signals (not line editing) which are handled by cbreak() and 
 raw() family.

 Curses has never supported `readline` for vi/emacs line editing.  That would be 
 an extension to historical behaviour.

 >   The whole thing seems like a can of worms and a mistake, if you ask
 >   me.

 Nope.  Historical.

 >   Anyway, it appears that if curses thinks it's not on a tty, both
 >   erasechar() and killchar() return 0. What happens if you send 0 down
 >   the pipe? If that causes an erase, and it looks like it will, that
 >   seems like undesirable behavior and erasechar() ought to default to ^H
 >   or ^? when not on a tty. (But, which?)

 By definition BS = backspace = ^H.  And kill typically defaults to stty ^U.  The 
 problem when stdin is not a tty, is that termios does not apply so all the 
 special keys are zero.

 In theory, Curses could `open(ctermid(NULL), ...)` a separate tty to get the 
 user's stty settings then apply them to the redirected stdin.

 -- 
 Anthony C Howe
 achowe@snert.com                                   BarricadeMX & Milters
 http://nanozen.snert.com/                          http://software.snert.com/

From: David Holland <dholland-bugs@netbsd.org>
To: gnats-bugs@netbsd.org
Cc: 
Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 characters when input is not a tty?
Date: Wed, 17 Apr 2024 16:20:36 +0000

 On Tue, Apr 16, 2024 at 12:00:05PM +0000, Anthony Howe wrote:
  >  Historically Curses does not handle werase.  There is no equivalent of 
  >  erasechar() or killchar() for werase.

 Historically certain old and broken systems don't have werase in the
 tty driver either. But if we're going to argue that in this particular
 combination of circumstances curses should pretend to be tty cooked
 mode, it should be the same cooked mode as the tty driver, not the
 cooked mode from SVr2.

 In principle it ought to also support silly archaic modes like ECHOKE
 too, although since the tty driver doesn't support that one either
 it's not exactly critical.

 However, nothing in that code handles echo vs. noecho modes correctly.

  > INTR/QUIT/SUSP are signals (not line editing) which are handled by
  > cbreak() and raw() family.

 Yes, so if we're in cbreak mode and curses is pretending to be a tty
 because its input is a pipe, it ought to post SIGINT if it receives a
 ^C down that pipe.

  > Curses has never supported `readline` for vi/emacs line editing.
  > That would be an extension to historical behaviour.

 Yes, so? Today all serious programs that handle line input use
 readline or libedit (or implement it by hand) and that's the minimum
 expectation. Curses could behave like libedit here, and that would be
 more consistent with user expectations.

 (Except that curses apps that intend to meet user expectations don't
 use this pathway because it doesn't, either currently, historically,
 or portably, meet those expectations.)

  >> The whole thing seems like a can of worms and a mistake, if you ask
  >> me.
  >  
  >  Nope.  Historical.

 ECHOKE is historical too.

  >>   Anyway, it appears that if curses thinks it's not on a tty, both
  >>   erasechar() and killchar() return 0. What happens if you send 0 down
  >>   the pipe? If that causes an erase, and it looks like it will, that
  >>   seems like undesirable behavior and erasechar() ought to default to ^H
  >>   or ^? when not on a tty. (But, which?)
  >  
  > By definition BS = backspace = ^H.

 Historically erase is ^?, not ^H.

  > In theory, Curses could `open(ctermid(NULL), ...)` a separate tty
  > to get the user's stty settings then apply them to the redirected
  > stdin.

 That's just going to cause more problems.

 -- 
 David A. Holland
 dholland@netbsd.org

From: Brett Lymn <blymn@internode.on.net>
To: gnats-bugs@netbsd.org
Cc: lib-bug-people@netbsd.org, gnats-admin@netbsd.org, netbsd-bugs@netbsd.org,
        achowe@snert.com
Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 characters when input is not a tty?
Date: Thu, 18 Apr 2024 07:51:43 +0930

 Before I sort through the below, I will addres the original statement
 about the output and what is expected to happen.

 It looks like there is some expectation that the backspaces in the input
 will somehow be magically processed internally and only the resultant
 string will be printed.  This may or may not happen depending on what
 the optimisation decides needs to do to update the screen to match the
 internal image of the terminal.  There may be all sorts of characters
 emitted to update the "screen" image.

 Also, erase and kill character processing is not done in cbreak mode
 according to the curses standard.

 On Wed, Apr 17, 2024 at 04:25:01PM +0000, David Holland wrote:
 > The following reply was made to PR lib/58151; it has been noted by GNATS.
 > 
 > From: David Holland <dholland-bugs@netbsd.org>
 > To: gnats-bugs@netbsd.org
 > Cc: 
 > Subject: Re: lib/58151: Should Curses getnstr() family handle erase and kill
 >  characters when input is not a tty?
 > Date: Wed, 17 Apr 2024 16:20:36 +0000
 > 
 >  On Tue, Apr 16, 2024 at 12:00:05PM +0000, Anthony Howe wrote:
 >   >  Historically Curses does not handle werase.  There is no equivalent of 
 >   >  erasechar() or killchar() for werase.
 >  
 >  Historically certain old and broken systems don't have werase in the
 >  tty driver either. But if we're going to argue that in this particular
 >  combination of circumstances curses should pretend to be tty cooked
 >  mode, it should be the same cooked mode as the tty driver, not the
 >  cooked mode from SVr2.
 >  

 We follow the Opengroup SUSv2 curses specification.  Well, we try.

 >  Yes, so if we're in cbreak mode and curses is pretending to be a tty
 >  because its input is a pipe, it ought to post SIGINT if it receives a
 >  ^C down that pipe.
 >  

 This is the problem here, a pipe is not a terminal so you should not
 expect termios behaviour.  If you really want to do this properly you
 should communicate with the curses based application over a pty.

 If you want to see an example of how this is done, have a like at the
 libcurses automated test framework which is used to perform automated
 tests on curses functions.

 >  Yes, so? Today all serious programs that handle line input use
 >  readline or libedit (or implement it by hand) and that's the minimum
 >  expectation. Curses could behave like libedit here, and that would be
 >  more consistent with user expectations.
 >  

 No, it should comply to the documented specification.

 >   >>   erasechar() and killchar() return 0. What happens if you send 0 down
 >   >>   the pipe? If that causes an erase, and it looks like it will, that
 >   >>   seems like undesirable behavior and erasechar() ought to default to ^H
 >   >>   or ^? when not on a tty. (But, which?)
 >   >  
 >   > By definition BS = backspace = ^H.
 >  
 >  Historically erase is ^?, not ^H.
 >  

 Or, in reality, whatever the user sets using stty but also for curses
 the terminfo definitions apply too.

 >   > In theory, Curses could `open(ctermid(NULL), ...)` a separate tty
 >   > to get the user's stty settings then apply them to the redirected
 >   > stdin.
 >  
 >  That's just going to cause more problems.
 >  

 And solve nothing as there is no guarantee that the termios settings are
 the same.

 -- 
 Brett Lymn
 --
 Sent from my NetBSD device.

 "We are were wolves",
 "You mean werewolves?",
 "No we were wolves, now we are something else entirely",
 "Oh"

State-Changed-From-To: open->closed
State-Changed-By: blymn@NetBSD.org
State-Changed-When: Mon, 22 Apr 2024 22:19:11 +0000
State-Changed-Why:
Closed, will not fix.
The standard says backspace is not processed in cbreak mode.
Also, the expectation that curses only emits a processed string is
incorrect.  Screen optimisation may inject cursor motion into the
output.


>Unformatted:

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