>From f740346aeffaa2b5c70727ef0fe83c8f1e2e0d33 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Sat, 11 Feb 2017 18:13:54 -0500
Subject: [PATCH v2 3/6] Escape control characters in backtraces (Bug#6991)

* src/print.c (syms_of_print): Add new variable,
print-escape-control-characters.
(print_object): Print control characters with octal escape codes when
print-escape-control-characters is true.
* lisp/subr.el (backtrace):
* lisp/emacs-lisp/debug.el (debugger-setup-buffer): Bind
`print-escape-control-characters' to t.
---
 lisp/emacs-lisp/debug.el |  1 +
 lisp/subr.el             |  3 ++-
 src/print.c              | 45 +++++++++++++++++++++++++++++++++------------
 3 files changed, 36 insertions(+), 13 deletions(-)

diff --git a/lisp/emacs-lisp/debug.el b/lisp/emacs-lisp/debug.el
index 0c8306d428..effe7f0cb3 100644
--- a/lisp/emacs-lisp/debug.el
+++ b/lisp/emacs-lisp/debug.el
@@ -362,6 +362,7 @@ (defun debugger-setup-buffer (args)
                    (if (eq (car args) 'debug) 3 1)
                    (backtrace-frames 'debug)))
           (print-escape-newlines t)
+          (print-escape-control-characters t)
           (print-level 8)
           (print-length 50))
       (when (eq (car args) 'exit)
diff --git a/lisp/subr.el b/lisp/subr.el
index 8d5d2a779c..c8036665db 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -4513,7 +4513,8 @@ (defun backtrace--print-frame (evald func args flags)
 (defun backtrace ()
   "Print a trace of Lisp function calls currently active.
 Output stream used is value of `standard-output'."
-  (let ((print-level (or print-level 8)))
+  (let ((print-level (or print-level 8))
+        (print-escape-control-characters t))
     (mapbacktrace #'backtrace--print-frame 'backtrace)))
 
 (defun backtrace-frames (&optional base)
diff --git a/src/print.c b/src/print.c
index 49408bbeb4..dd95507624 100644
--- a/src/print.c
+++ b/src/print.c
@@ -1839,21 +1839,36 @@ print_object (Lisp_Object obj, Lisp_Object printcharfun, bool escapeflag)
 		}
 	      else
 		{
+                  bool still_need_nonhex = false;
 		  /* If we just had a hex escape, and this character
 		     could be taken as part of it,
 		     output `\ ' to prevent that.  */
-		  if (need_nonhex && c_isxdigit (c))
-		    print_c_string ("\\ ", printcharfun);
-
-		  if (c == '\n' && print_escape_newlines
-		      ? (c = 'n', true)
-		      : c == '\f' && print_escape_newlines
-		      ? (c = 'f', true)
-		      : c == '\"' || c == '\\')
-		    printchar ('\\', printcharfun);
-
-		  printchar (c, printcharfun);
-		  need_nonhex = false;
+                  if (c_isxdigit (c))
+                    {
+                      if (need_nonhex)
+                        print_c_string ("\\ ", printcharfun);
+                      printchar (c, printcharfun);
+                    }
+                  else if (c == '\n' && print_escape_newlines
+                           ? (c = 'n', true)
+                           : c == '\f' && print_escape_newlines
+                           ? (c = 'f', true)
+                           : c == '\0' && print_escape_control_characters
+                           ? (c = '0', still_need_nonhex = true)
+                           : c == '\"' || c == '\\')
+                    {
+                      printchar ('\\', printcharfun);
+                      printchar (c, printcharfun);
+                    }
+                  else if (print_escape_control_characters && c_iscntrl (c))
+                    {
+                      char outbuf[1 + 3 + 1];
+                      int len = sprintf (outbuf, "\\%03o", c + 0u);
+                      strout (outbuf, len, len, printcharfun);
+                    }
+                  else
+                    printchar (c, printcharfun);
+		  need_nonhex = still_need_nonhex;
 		}
 	    }
 	  printchar ('\"', printcharfun);
@@ -2298,6 +2313,11 @@ syms_of_print (void)
 Also print formfeeds as `\\f'.  */);
   print_escape_newlines = 0;
 
+  DEFVAR_BOOL ("print-escape-control-characters", print_escape_control_characters,
+	       doc: /* Non-nil means print control characters in strings as `\\OOO'.
+\(OOO is the octal representation of the character code.)*/);
+  print_escape_control_characters = 0;
+
   DEFVAR_BOOL ("print-escape-nonascii", print_escape_nonascii,
 	       doc: /* Non-nil means print unibyte non-ASCII chars in strings as \\OOO.
 \(OOO is the octal representation of the character code.)
@@ -2387,6 +2407,7 @@ representation) and `#N#' in place of each subsequent occurrence,
   DEFSYM (Qprint_escape_newlines, "print-escape-newlines");
   DEFSYM (Qprint_escape_multibyte, "print-escape-multibyte");
   DEFSYM (Qprint_escape_nonascii, "print-escape-nonascii");
+  DEFSYM (Qprint_escape_control_characters, "print-escape-control-characters");
 
   print_prune_charset_plist = Qnil;
   staticpro (&print_prune_charset_plist);
-- 
2.11.1