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From: Eric Pruitt <eric.pruitt@gmail.com>
Newsgroups: gnu.bash.bug
Subject: Re: Option to disable VSUSP at prompt (feature request with proof of concept)
Date: Wed, 3 Oct 2018 00:14:09 -0700
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On Thu, Nov 24, 2016 at 12:05:20AM -0800, Eric Pruitt wrote:
> In my Bash configuration, I have things setup so Ctrl+Z is no longer
> translated into a signal at the Bash prompt so it can be remapped. Most
> recently, I decided to modify the Bash source to implement this change
> in the interpreter because the stty invocations introduced a perceptible
> amount of lag on a virtualized OpenBSD host I use. I think this feature
> would be a useful default since it usually does not make sense to send
> SIGTSTP to a prompt. Here's an accompanying snippet from my inputrc:
>
>     # Allows Ctrl+Z to be used to bring programs back into the
>     # foreground. The cursor is moved to the beginning of the line
>     # before typing so a specific job can be resumed by typing its
>     # identifier (e.g. a number) then hitting Ctrl+Z. This depends on
>     # Ctrl+Z being a literal sequence i.e. "stty susp undef".
>     "\C-z": "\C-afg \C-m"

This patch had a bug that I recently fixed, and I've attached an updated
version. Under certain conditions, ^Z for SIGTSTP would not be
re-enabled after executing a command. For example, if "cat /dev/zero >
/dev/null" was interrupted by pressing Ctrl+C, pressing Ctrl+Z would not
generate a literal ^Z control character at the next prompt. Some
debugging revealed that I did not fully understand the control flow of
the prompt handling when writing the original patch; control may be
resumed at the top of the "reader_loop" function by way of a long lump
which bypassed the vsusp toggling code. The bug has been fixed by moving
the vsusp toggle logic after the aforementioned long-jump destination.

I'm not requesting that this patch be accepted upstream, just posting an
updated version for anyone that might interested in this feature.

Eric

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Content-Disposition: attachment; filename="bash-4.4-automatic-susp-toggle.patch"

When running commands interactively, modify the terminal attributes so the
suspend character is interpreted as literal sequence at the prompt but produces
a SIGTSTP signal during command execution. This makes it possible to bind ^Z in
readline. For example, adding `"\C-z": "\C-afg \C-m"` to "~/.inputrc" would
make it possible to suspend a program using ^Z then pressing ^Z again to bring
the program back to the foreground.

--- eval.c	2016-06-02 15:49:27.000000000 -0700
+++ eval.c	2016-11-20 15:02:23.681680378 -0800
@@ -31,6 +31,7 @@
 #include <stdio.h>
 
 #include <signal.h>
+#include <termios.h>
 
 #include "bashintl.h"
 
@@ -69,6 +70,8 @@
 int
 reader_loop ()
 {
+  struct termios ttyattr;
+  cc_t vsusp = 0;
   int our_indirection_level;
   COMMAND * volatile current_command;
 
@@ -84,6 +87,13 @@
 
       code = setjmp_nosigs (top_level);
 
+      if (!vsusp && indirection_level == 1 && interactive &&
+       !tcgetattr(STDERR_FILENO, &ttyattr)) {
+	vsusp = ttyattr.c_cc[VSUSP];
+	ttyattr.c_cc[VSUSP] = 0;
+	tcsetattr(STDERR_FILENO, TCSADRAIN, &ttyattr);
+      }
+
 #if defined (PROCESS_SUBSTITUTION)
       unlink_fifo_list ();
 #endif /* PROCESS_SUBSTITUTION */
@@ -177,6 +187,13 @@
 		  free (ps0_string);
 		}
 
+	      if (vsusp) {
+		ttyattr.c_cc[VSUSP] = vsusp;
+		if (!tcsetattr(STDERR_FILENO, TCSADRAIN, &ttyattr)) {
+		  vsusp = 0;
+		}
+	      }
+
 	      execute_command (current_command);
 
 	    exec_done:
@@ -199,6 +216,11 @@
 	EOF_Reached = EOF;
     }
   indirection_level--;
+
+  if (vsusp) {
+    ttyattr.c_cc[VSUSP] = vsusp;
+    tcsetattr(STDERR_FILENO, TCSADRAIN, &ttyattr);
+  }
   return (last_command_exit_value);
 }
 

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