Fix emacs backslash escaping behavior (#179)

This commit fixes the following:

1. Emacs mode ignores --nobackslashctrl (re: 24598fed) when in
   reverse search.

2. When entering more than one backslash, emacs reverse search mode
   deletes multiple backslashes after pressing backspace once.
   Reproducer:
   $ set --emacs --nobackslashctrl
   $ <Ctrl+R> \\\\<Backspace>

3. Except when in reverse search, the backslash fails to escape a
   subsequent interrupt character (^C). Reproducer:
   $ set --emacs --backslashctrl
   $ teststring \<Ctrl+C>

src/cmd/ksh93/edit/emacs.c:
- Disable escaping backslashes in emacs reverse search if
  'nobackslashctrl' is enabled.
- Fix the buggy behavior of backslashes in emacs reverse
  search by processing backslashes in a loop.

src/cmd/ksh93/tests/pty.sh:
- Add regression tests.

src/cmd/ksh93/sh.1:
- Fix a minor documentation error (^C is the usual interrupt
  character, not ^?).

Co-authored-by: Martijn Dekker <martijn@inlv.org>

NEWS

@@ -3,6 +3,13 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-02-17:
+
+- Emacs mode fixes:
+  1. Erasing a backslash while doing a reverse search (^R) no longer deletes
+     extra characters.
+  2. The backslash now escapes a subsequent interrupt (^C) as documented.
+
 2021-02-15:
 
 - Fixed a regression introduced by ksh93 (was not in ksh88): an empty 'case'

src/cmd/ksh93/edit/edit.c

@@ -1068,7 +1068,7 @@ int ed_getchar(register Edit_t *ep,int mode)
 	{
 		ed_flush(ep);
 		ep->e_inmacro = 0;
-		/* The while is necessary for reads of partial multbyte chars */
+		/* The while is necessary for reads of partial multibyte chars */
 		*ep->e_vi_insert = (mode==-2);
 		if((n=ed_read(ep,ep->e_fd,readin,-LOOKAHEAD,0)) > 0)
 			n = putstack(ep,readin,n,1);

src/cmd/ksh93/edit/emacs.c

@@ -281,10 +281,11 @@ int ed_emacsread(void *context, int fd,char *buff,int scend, int reedit)
 #endif /* ESH_NFIRST */
 	}
 	ep->CntrlO = 0;
-	while ((c = ed_getchar(ep->ed,0)) != (-1))
+	while ((c = ed_getchar(ep->ed,backslash)) != (-1))
 	{
 		if (backslash)
 		{
+			if(c!='\\')
 				backslash = 0;
 			if (c==usrerase||c==usrkill||(!print(c) &&
 				(c!='\r'&&c!='\n')))
@@ -1298,20 +1299,35 @@ static void search(Emacs_t* ep,genchar *out,int direction)
 			beep();
 			goto restore;
 		}
-		if (i == '\\')
+		if (!sh_isoption(SH_NOBACKSLCTRL))
 		{
+			while (i == '\\')
+			{
+				/*
+				 * Append the backslash to the command line, then escape
+				 * the next control character. Repeat the loop if the
+				 * next input is another backslash (otherwise every
+				 * second backslash is skipped).
+				 */
 				string[sl++] = '\\';
 				string[sl] = '\0';
 				cur = sl;
 				draw(ep,APPEND);
 				i = ed_getchar(ep->ed,1);
+
+				/* Backslashes don't affect newlines */
+				if (i == '\n' || i == '\r')
+					goto skip;
+				else if (i == usrerase || !print(i))
 					string[--sl] = '\0';
 			}
+		}
 		string[sl++] = i;
 		string[sl] = '\0';
 		cur = sl;
 		draw(ep,APPEND);
 	}
+	skip:
 	i = genlen(string);
 	
 	if(ep->prevdirection == -2 && i!=2 || direction!=1)

src/cmd/ksh93/include/version.h

@@ -20,7 +20,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-02-15"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-02-17"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh.1

@@ -4969,7 +4969,7 @@ If the
 shell option is on (which is the default setting), this escapes the next character.
 Editing characters, the user's erase, kill and
 interrupt (normally
-.BR ^? )
+.BR ^C )
 characters
 may be entered
 in a command line or in a search string if preceded by a

src/cmd/ksh93/tests/pty.sh

@@ -640,5 +640,35 @@ r ^/dev/null\r\n$
 r ^:test-2:
 !
 
+# err_exit #
+((SHOPT_ESH)) && [[ -o ?backslashctrl ]] && tst $LINENO <<"!"
+L nobackslashctrl in emacs
+
+d 10
+w set -o emacs --nobackslashctrl
+
+# --nobackslashctrl shouldn't be ignored by reverse search
+p :test-2:
+w \cR\\\cH\cH
+r ^:test-2: \r\n$
+!
+
+# err_exit #
+((SHOPT_ESH)) && tst $LINENO <<"!"
+L emacs backslash escaping
+
+d 10
+w set -o emacs
+
+# Test for too many backslash deletions in reverse-search mode
+p :test-2:
+w \cRset\\\\\\\\\cH\cH\cH\cH\cH
+r ^:test-2: set -o emacs$
+
+# \ should escape the interrupt character (usually Ctrl+C)
+w true \\\cC
+r true \^C
+!
+
 # ======
 exit $((Errors<125?Errors:125))